Image summary: This is a photograph. The image depicts a young boy holding a glass of milk while clutching his stomach with a distressed facial expression. The boy appears to be experiencing physical discomfort or nausea after consuming the beverage, suggesting a negative reaction such as lactose intolerance or an upset stomach.

The Digestive System

The Digestive System and Homeostasis

The digestive system contributes to homeostasis by breaking down food into forms that can be absorbed and used by body cells. It also absorbs water, vitamins, and minerals, and it eliminates wastes from the body.

The food we eat contains a variety of nutrients, which are used for building new body tissues and repairing damaged tissues. Food is also vital to life because it is our only source of chemical energy. However, most of the food we eat consists of molecules that are too large to be used by body cells. Therefore, foods must be broken down into molecules that are small enough to enter body cells for their use. This is accomplished by the digestive system, which forms an extensive surface area in contact with the external environment, and is closely associated with the cardiovascular system. The combination of extensive environmental exposure and close association with blood vessels is essential for processing the food that we eat.

24.1 Overview of the Digestive System

Objectives

• Identify the organs of the digestive system.

• Describe the basic processes performed by the digestive system.

The digestive system (dis = apart; gerere = to carry) consists of a group of organs that break down the food we eat into smaller molecules that can be used by body cells. Two groups of organs compose the digestive system (Figure 24.1): the digestive canal and the accessory digestive organs. The digestive canal or gastrointestinal G.I tract is a continuous tube that extends through the thoracic and abdominal cavities from the esophagus to the anus through the thoracic and abdominal opelvic cavities. Organs of the digestive canal include the esophagus, stomach, small intestine, large intestine, and anal canal. The length of the digestive canal is about 5 to 7 meters (16.5 to 23 ft) in a living person when the muscles along the wall of the digestive canal organs are in a state of tonus (sustained contraction). It is longer in a cadaver (about 7 to 9 meters or 23 to 29.5 ft) because of the loss of muscle tone after death.

Figure 24.1 summary: This figure consists of two anatomical diagrams. The first is an illustrative diagram showing the human digestive system within a human body, and the second is a photograph of a dissected anterior view of the abdominal cavity. The illustrations identify the various organs and structures of the gastrointestinal tract, starting from the mouth and salivary glands, moving through the pharynx, esophagus, stomach, and small intestine, and ending at the large intestine and anus. Accessory organs such as the liver, gallbladder, and pancreas are also labeled. The dissection image provides a realistic view of the internal arrangement of these organs, highlighting the liver, stomach, and the complex folding of the small and large intestines. From these images, it can be inferred that the digestive system is a continuous tube with specialized sections for different stages of digestion. The liver and gallbladder are positioned superior to the intestines, while the small intestine occupies a significant portion of the central abdominal cavity, indicating its role in extensive nutrient absorption.

The accessory digestive organs include the mouth teeth, tongue, salivary glands, pharynx, liver, gallbladder, and pancreas. Teeth aid in the physical breakdown of food, and the tongue assists in chewing and swallowing and the pharynx is the initial transport pathway for food. The other accessory digestive organs, however, never come into direct contact with food.

They produce or store secretions that flow into the digestive canal through ducts; the secretions aid in the chemical breakdown of food.

The digestive canal contains food from the time it is eaten until it is digested and absorbed or eliminated. Muscular contractions in the wall of the digestive canal physically break down the food by churning it and propel the food along the canal from the esophagus to the anus. The contractions also help to dissolve foods by mixing them with fluids secreted into the digestive canal. Enzymes secreted by accessory digestive organs and cells that line digestive canal break down the food chemically.

Figure 24.1 Organs of the Digestive System Necessary Organs Are Indicated in Red.

Organs of the digestive canal are the esophagus, stomach, small intestine, large intestine, and anal canal. Accessory digestive organs include the mouth, teeth, tongue, salivary glands, pharynx, liver, gallbladder, and pancreas and are indicated in red.

Functions of the Digestive System

1. Ingestion: taking food into mouth.

2. Secretion: release of water, acid, buffers, and enzymes into lumen of digestive canal.

3. Mixing and propulsion: churning and movement of food through digestive canal.

4. Digestion: mechanical and chemical breakdown of food.

5. Absorption: passage of digested products from digestive canal into blood plasma and lymph plasma.

6. Defecation: elimination of feces from digestive canal.

The digestive system performs six basic processes: ingestion, secretion, motility, digestion, absorption, and defecation.

Q What is absorption?

Overall, the digestive system performs six basic processes (Figure 24.2):

Figure 24.2 summary: This figure is a schematic diagram. It illustrates the six basic processes of the digestive system, tracing the path of food from the mouth to the anus. The diagram depicts the ingestion of food, the secretion of digestive enzymes from glands, the motility involving mixing and propulsion, the digestion of large food particles into small molecules via enzymes, the absorption of these molecules into blood or lymphatic vessels, and finally, the defecation of feces. The figure demonstrates that digestion is a sequential process where food is progressively broken down and nutrients are extracted before waste is eliminated from the body.

1. Ingestion. This process involves taking foods and liquids into the mouth (eating).

2. Secretion. Each day, cells within the walls of the digestive canal and accessory digestive organs secrete a total of about 7 liters of water, acid, buffers, and enzymes into the lumen (interior space) of the digestive canal.

3. Motility. Alternating contractions and relaxations of smooth muscle in the walls of the digestive canal mix food and secretions and move them toward the anus. This capability of the digestive canal to mix and move material along its length is called motility motility.

4. Digestion. Digestion is the process of breaking down ingested food into small molecules that can be used by body cells. In mechanical digestion the teeth cut and grind food before it is swallowed, and then smooth muscles of the stomach and small intestine churn the food to further assist the process. As a result, food molecules become dissolved and thoroughly mixed with digestive enzymes. In chemical digestion the large carbohydrate, lipid, protein, and nucleic acid molecules in food are split into smaller molecules by hydrolysis (see Figure 2.15). Digestive enzymes produced by the salivary glands, tongue, stomach, pancreas, and small intestine catalyze these catabolic reactions.

5. Absorption. The movement of the products of digestion from the lumen of the digestive canal into blood plasma or lymph plasma is called absorption absorption. Once absorbed, these substances circulate to cells throughout the body. A few substances in food can be absorbed without undergoing digestion. These include vitamins, ions, cholesterol, and water.

6. Defecation. Wastes, indigestible substances, bacteria, cells sloughed from the lining of the digestive canal, and

digested materials that were not absorbed in their journey through the digestive canal leave the body through the anus in a process called defecation (def-e-KÃ-shun). The eliminated material is termed feces (FÊ-sez) or stool.

Checkpoint

2. Which organs of the digestive system come in contact with food, and what are some of their digestive functions?

3. Which kinds of food molecules undergo chemical digestion, and which do not?

24.2 Layers of the Digestive Canal

Objective

- Describe the structure and function of the layers that form the wall of the digestive canal.

The wall of the digestive canal from the lower esophagus to the anal canal has the same basic, four-layered arrangement of tissues. The four layers of the digestive canal, from deep to superficial, are the mucosa, submucosa, muscular layer, and serosa/adventitia (Figure 24.3).

Figure 24.3 summary: This is an anatomical diagram. The figure illustrates the histological layers of the digestive canal wall, detailing the lumen, mucosa, submucosa, muscular layer, and serosa. It identifies specific components such as the epithelium, lamina propria, muscularis mucosae, submucosal and myenteric neural plexuses, as well as associated blood vessels, nerves, and glands. The diagram demonstrates that the digestive tract is organized into concentric layers with specialized tissues and neural networks distributed throughout the wall to regulate function. The presence of lymphatic tissue and various glands indicates that the wall is equipped for immune defense and secretion, while the dual layers of muscle suggest a capacity for complex motility.

Figure 24.3 Layers of the digestive canal. Variations in this basic plan may be seen in the esophagus (Figure 24.10), stomach (Figure 24.13), small intestine (Figure 24.20), and large intestine (Figure 24.25).

Figure 24.10 summary: This figure consists of an anatomical diagram and two microscopic images. The content illustrates the structure of the esophagus, showing a transverse plane cut through the neck, a light micrograph of the tissue layers, and a scanning electron micrograph of a cross-section. The images identify the lumen and the surrounding layers including the mucosa, submucosa, muscular layers, and adventitia. It can be inferred that the esophagus is composed of multiple concentric layers of specialized tissues that provide structural support and facilitate the movement of food toward the stomach.

Figure 24.13 summary: This figure consists of a series of anatomical diagrams and micrographs. The content illustrates the histological structure of the stomach wall, detailing the various layers including the mucosa, submucosa, muscular layer, and serosa. It specifically highlights the composition of the gastric pits and glands, identifying specialized cell types such as surface mucous cells, mucous neck cells, parietal cells, chief cells, and G cells, while also showing the underlying blood vessels, lymphatic vessels, and muscle layers. From this detailed anatomy, it can be inferred that gastric juice is a complex mixture produced by the coordinated secretions of multiple cell types, including mucous, parietal, and chief cells, which are strategically positioned within the gastric glands to facilitate digestion within the stomach lumen.

Figure 24.20 summary: This figure consists of a series of anatomical diagrams, histological illustrations, an endoscopic image, and a scanning electron micrograph. The content details the hierarchical structural organization of the small intestine, moving from the macroscopic level of circular folds to the microscopic level of intestinal villi, and further down to the cellular level including microvilli and various specialized cell types such as absorptive cells, goblet cells, enteroendocrine cells, and Paneth cells. It also illustrates the underlying tissue layers, including the mucosa, submucosa, muscular layer, and serosa, as well as the associated vascular and lymphatic networks. The figure demonstrates that the small intestine employs a multi-level folding strategy to vastly increase its internal surface area. This structural adaptation is essential for maximizing the efficiency of nutrient digestion and absorption by providing more contact area between the intestinal lining and the food passing through the lumen.

Figure 24.25 summary: This figure is an anatomical diagram. It provides a three-dimensional cross-sectional view of the wall of the large intestine, detailing the various histological layers and specialized cellular components. The diagram labels the outermost serosa, the muscular layer consisting of longitudinal and circular muscle fibers with an intervening myenteric neural plexus, the submucosa containing blood and lymphatic vessels, and the innermost mucosa. Within the mucosa, it identifies the muscularis mucosae, lamina propria, and intestinal glands containing absorptive cells with microvilli and mucus-secreting goblet cells, as well as lymphoid nodules. The figure demonstrates that the large intestine is organized into distinct concentric layers, each with specialized structures that support its functions of water absorption, mucus production, and immune defense.

The four layers of the digestive canal, from deep to superficial, are the mucosa, submucosa, muscular layer, and serosa.

Mucosa

The mucosa, or inner lining of the digestive canal, is a mucous membrane. It is composed of (1) a layer of epithelium in direct contact with the contents of the digestive canal, (2) a layer of connective tissue called the lamina propria, and a thin layer of smooth muscle (muscularis mucosae).

1. The epithelium in the mouth, pharynx, esophagus, and anal canal is mainly nonkeratinized stratified squamous epithelium that serves a protective function. Simple columnar epithelium, which functions in secretion and absorption, lines the stomach and intestines. The tight junctions that firmly seal neighboring simple columnar epithelial cells to one another restrict leakage between the cells. The rate of renewal of digestive canal epithelial cells is rapid: Every 5 to 7 days they slough off and are replaced by new cells. Located among the epithelial cells are exocrine cells that secrete mucus and fluid into the lumen of the digestive canal, and several types of endocrine cells, collectively called enteroendocrine cells enteroendocrine, which secrete hormones.

2. The lamina propria (lamina = thin, flat plate; propria = one's own) is areolar connective tissue containing many blood and lymphatic vessels, which are the routes by which nutrients absorbed into the digestive canal reach the other tissues of the body. This layer supports the epithelium and binds it to the muscularis mucosae (discussed next). The lamina propria also contains the majority of the cells of the mucosa-associated lymphoid tissue (malt). These prominent lymphoid nodules contain immune system cells that protect against disease (see Chapter 22). malt is present all along the digestive canal, especially in the tonsils, small intestine, appendix, and large intestine.

3. A thin layer of smooth muscle fibers called the muscularis mucosae (mu-Ko-se) throws the mucous membrane of the stomach and small intestine into many small folds, which increase the surface area for digestion and absorption. Movements of the muscularis mucosae ensure that all absorptive cells are fully exposed to the contents of the digestive canal.

Submucosa

The submucosa consists of areolar connective tissue that binds the mucosa to the muscularis. It contains many blood and lymphatic vessels that receive absorbed food molecules. Also located in the submucosa is an extensive network of neurons known as the submucosal neural plexus (to be described shortly). The submucosa may also contain glands and lymphatic tissue.

Muscular Layer

The muscular layer of the mouth, pharynx, and superior and middle parts of the esophagus contains skeletal muscle that produces voluntary swallowing. Skeletal muscle also forms the external anal sphincter, which permits voluntary control of defecation. Throughout the rest of the digestive canal, the muscular layer consists of smooth muscle that is generally found in two sheets: an inner sheet of circular fibers and an outer sheet of longitudinal fibers. Involuntary contractions of the smooth muscle help break down food, mix it with digestive secretions, and propel it along the digestive canal. Between the layers of the muscular layer is a second plexus of neurons—the myenteric neural plexus (to be described shortly).

Serosa

Those portions of the digestive canal that are suspended in the abdominal cavity have a superficial layer called the serosa. As its name implies, the serosa is a serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium). The serosa is also called the visceral peritoneum because it forms a portion of the peritoneum, which we examine in detail shortly. The esophagus lacks a serosa; instead, only a single layer of areolar connective tissue called the adventitio forms the superficial layer of this organ.

Checkpoint

4. Where along the digestive canal is the muscular layer composed of skeletal muscle? Is control of this skeletal muscle voluntary or involuntary?

5. Name the four layers of the digestive canal, and describe their functions.

24.3 Neural Innervation of the Digestive Canal

• Describe the nerve supply of the digestive canal.

The digestive canal is regulated by an intrinsic set of nerves known as the enteric nervous system and by an extrinsic set of nerves that are part of the autonomic nervous system.

Enteric Nervous System

We first introduced you to the enteric nervous system (E.N.S), the "brain of the gut," in Chapter 12. It consists of about 100 million neurons that extend from the esophagus to the anus. The neurons of the E.N.S are arranged into two neural plexuses: the myenteric neural plexus and submucosal neural plexus (see Figure 24.3). The myenteric neural plexus (myo-= muscle), or plexus of Auerbach owerbak, is located between the longitudinal and circular smooth muscle layers of the muscular layer. The submucosal neural plexus, or plexus of Meissner mizner, is found within the submucosa.

The plexuses of the E.N.S consist of motor neurons, interneurons, and sensory neurons (Figure 24.4). Because the motor neurons of the myenteric neural plexus supply the longitudinal and circular smooth muscle layers of the muscular layer, this neural plexus mostly controls digestive canal motility (movement), particularly the frequency and strength of contraction of the muscular layer. The motor neurons of the submucosal neural plexus supply the secretory cells of the mucosal epithelium, controlling the secretions of the organs of the digestive canal. The interneurons of the E.N.S interconnect the neurons of the myenteric and submucosal neural plexuses.

Figure 24.4 summary: This is a schematic diagram illustrating the organization of the enteric nervous system. The figure depicts the connectivity between the myenteric neural plexus and the submucosal neural plexus, showing the roles of interneurons, motor neurons, and sensory neurons. It traces the pathways from the mucosal epithelium through sensory neurons to the plexuses, and from the plexuses via motor neurons to the longitudinal and circular smooth muscle layers. The diagram also shows a connection leading to the autonomic and central nervous systems. The figure demonstrates that the enteric nervous system operates as a complex network capable of local reflex arcs while maintaining communication with the broader nervous system. It concludes that the system integrates sensory input from the gut lining to coordinate motor responses in the muscular layers of the gastrointestinal tract.

The sensory neurons of the E.N.S supply the mucosal epithelium and contain receptors that detect stimuli in the lumen of the digestive canal. The wall of the digestive canal contains two major types of sensory receptors: chemoreceptors,

Figure 24.4 Organization of the Enteric Nervous System.

The enteric nervous system consists of neurons arranged into the myenteric and submucosal neural plexuses.

Q What are the functions of the myenteric and submucosal neural plexuses of the enteric nervous system? which respond to certain chemicals in the food present in the lumen, and mechanoreceptors, such as stretch receptors, that are activated when food distends (stretches) the wall of a digestive canal organ.

Autonomic Nervous System

Although the neurons of the E.N.S can function independently, they are subject to regulation by the neurons of the autonomic nervous system. The vagus (10) nerves supply parasympathetic fibers to most parts of the digestive canal, with the exception of the last half of the large intestine, which is supplied with parasympathetic fibers from the sacral spinal cord. The parasympathetic nerves that supply the digestive canal form neural connections with the E.N.S. Parasympathetic preganglionic neurons of the vagus or pelvic splanchnic nerves synapse with parasympathetic postganglionic neurons located in the myenteric and submucosal neural plexuses.

Some of the parasympathetic postganglionic neurons in turn synapse with neurons in the E.N.S; others directly innervate smooth muscle and glands within the wall of the digestive canal. In general, stimulation of the parasympathetic nerves that innervate the digestive canal causes an increase in digestive canal secretion and motility by increasing the activity of E.N.S neurons.

Sympathetic nerves that supply the digestive canal arise from the thoracic and upper lumbar regions of the spinal cord. Like the parasympathetic nerves, these sympathetic nerves form neural connections with the E.N.S. Sympathetic postganglionic neurons synapse with neurons located in the myenteric neural plexus and the submucosal neural plexus. In general, the sympathetic nerves that supply the digestive canal cause a decrease in digestive canal secretion and motility by inhibiting the neurons of the E.N.S. Emotions such as anger, fear, and anxiety may slow digestion because they stimulate the sympathetic nerves that supply the digestive canal.

Digestive Canal Reflex Pathways

Many neurons of the E.N.S are components of digestive canal reflex pathways that regulate digestive canal secretion and motility in response to stimuli present in the lumen of the digestive canal. The initial components of a typical digestive canal reflex pathway are sensory receptors (such as chemoreceptors and stretch receptors) that are associated with the sensory neurons of the E.N.S. The axons of these sensory neurons can synapse with other neurons located in the E.N.S, C.N.S, or A.N.S, informing these regions about the nature of the contents and the degree of distension (stretching) of the digestive canal. The neurons of the E.N.S, C.N.S, or A.N.S subsequently activate or inhibit digestive canal glands and smooth muscle, altering digestive canal secretion and motility.

Checkpoint

6. How is the enteric nervous system regulated by the autonomic nervous system?

7. What is a digestive canal reflex pathway?

24.4

Peritoneum

Objective

• Describe the peritoneum and its folds.

The peritoneum (per'-i-to-Ne-um; peri-= around) is the largest serous membrane of the body; it consists of a layer of simple squamous epithelium (mesothelium) with an underlying supporting layer of areolar connective tissue. The peritoneum is divided into the parietal peritoneum, which lines the wall of the abdominal cavity, and the visceral peritoneum, which covers some of the organs in the cavity and is their serosa (Figure 24.5a). The slim space containing lubricating serous fluid that is between the parietal and visceral portions of the peritoneum is called the peritoneal cavity. In certain diseases, the peritoneal cavity may become distended by the accumulation of several liters of fluid, a condition called ascites (a-Si-tez).

As you will see shortly, some organs lie on the posterior abdominal wall and are covered by peritoneum only on their anterior surfaces; they are not in the peritoneal cavity. Such organs, including the kidneys, ascending and descending colons of the large intestine, duodenum of the small intestine, and pancreas, are said to be retroperitoneal (retro-= behind).

Unlike the pericardium and pleurae, which smoothly cover the heart and lungs, the peritoneum contains large folds that weave between the viscera. The folds bind the organs to one another and to the walls of the abdominal cavity. They also contain blood vessels, lymphatic vessels, and nerves that supply the abdominal organs. There are five major peritoneal folds: the greater omentum, falciform ligament, lesser omentum, mesentery, and mesocolon:

1. The greater omentum omentum = fat skin), the longest peritoneal fold, drapes over the transverse colon and coils of the small intestine like a "fatty apron" (Figure 24.5a-d). The greater omentum is a double sheet that folds back on itself, giving it a total of four layers. From attachments along the stomach and duodenum, the greater omentum extends downward anterior to the small intestine, then turns and extends upward and attaches to the transverse colon. The greater omentum normally contains a considerable amount of adipose tissue. Its adipose tissue content can greatly expand with weight gain, contributing to the characteristic "beer belly" seen in some overweight individuals. The many lymph nodes of the greater omentum contribute macrophages and antibody-producing plasmocytes that help combat and contain infections of the digestive canal.

2. The falciform ligament falsiform; falc-= sickle-shaped) attaches the liver to the anterior abdominal wall and diaphragm (Figure 24.5a, b). The liver is the only digestive organ that is attached to the anterior abdominal wall.

3. The lesser omentum arises as an anterior fold in the serosa of the stomach and duodenum, and it connects the

stomach and duodenum to the liver (Figure 24.5a, b). It is the pathway for blood vessels entering the liver and contains the hepatic portal vein, common hepatic artery, and bile duct, along with some lymph nodes.

4. A fan-shaped fold of the peritoneum, called the mesentery mesentery; mes-= middle), binds the jejunum and ileum of the small intestine to the posterior abdominal wall (Figure 24.5a, c). This is the most massive peritoneal

Figure 24.5 summary: This figure consists of a series of anatomical diagrams and a cadaveric photograph. The images illustrate the arrangement of the peritoneum and its associated folds in relation to the organs of the digestive system, featuring a sagittal section, multiple anterior views with varying levels of organ displacement, and a real anatomical specimen. The diagrams identify key structures including the liver, stomach, intestines, and the various peritoneal membranes such as the falciform ligament, lesser omentum, greater omentum, mesocolon, and mesentery. From these views, it can be inferred that the peritoneum forms a complex, continuous lining that supports abdominal organs and creates a potential space known as the peritoneal cavity. The images demonstrate how the greater omentum hangs over the intestines and how the mesentery anchors the small intestine to the posterior abdominal wall, highlighting the structural organization and spatial relationships of the abdominal viscera.

Figure 24.5 Continued

fold, is typically laden with fat, and contributes extensively to the large abdomen in obese individuals. It extends from the posterior abdominal wall to wrap around the small intestine and then returns to its origin, forming a double-layered structure. Between the two layers are blood and lymphatic vessels and lymph nodes.

5. Two separate folds of peritoneum, called the mesocolon (mez'-o-Ko-lon), bind the transverse colon (transverse mesocolon) and sigmoid colon (sigmoid mesocolon) of the large intestine to the posterior abdominal wall (Figure 24.5a, c). It also carries blood and lymphatic vessels to the intestines. Together, the mesentery and mesocolon hold the intestines loosely in place, allowing movement as muscular contractions mix and move the luminal contents along the digestive canal.

Clinical Connection

Peritonitis

A common cause of peritonitis (per'-i-tô-Nî-tis), an acute inflammation of the peritoneum, is contamination of the peritoneum by infectious microbes, which can result from accidental or surgical wounds in the abdominal wall, or from perforation or rupture of microbe-containing abdominal organs. If, for example, bacteria gain access to the peritoneal cavity through an intestinal perforation or rupture of the appendix, they can produce an acute, life-threatening form of peritonitis. A less serious (but still painful) form of peritonitis can result from the rubbing together of inflamed peritoneal surfaces. The increased risk of peritonitis is of particular concern to those who rely on peritoneal dialysis, a procedure in which the peritoneum is used to filter the blood when the kidneys do not function properly (see Clinical Connection: Dialysis in Section 26.9).

8. Where are the visceral peritoneum and parietal peritoneum located?

9. Describe the attachment sites and functions of the mesentery, mesocolon, falciform ligament, lesser omentum, and greater omentum.

Mouth

Objectives

• Identify the locations of the salivary glands, and describe the functions of their secretions.

• Describe the structure and functions of the tongue.

• Identify the parts of a typical tooth, and compare deciduous and permanent dentitions.

The mouth, is formed by the lips, cheeks, hard and soft palates, oral cavity, teeth, salivary glands, and tongue (Figure 24.6). The cheeks form the lateral walls of the mouth. They are covered externally by skin and internally by a mucous membrane, which consists of nonkeratinized stratified squamous epithelium. Buccinator muscles and connective tissue lie between the skin and mucous membranes of the cheeks. The anterior portions of the cheeks end at the lips.

The lips or labia (= fleshy borders) are fleshy folds surrounding the opening of the mouth. They contain the orbicularis oris muscle and are covered externally by skin and internally by a mucous membrane. The inner surface of each lip is attached to its corresponding gum by a midline fold of mucous membrane called the labial frenulum (LÃ-bê-al frenulum; frenulum = small bridle).

During chewing, contraction of the buccinator muscles in the cheeks and orbicularis oris muscle in the lips helps keep food between the upper and lower teeth. These muscles also assist in speech.

The oral cavity is the space that extends from the lips and teeth to the fauces and is divided into an oral vestibule and an oral cavity proper. The oral vestibule (= entrance to a canal) of the oral cavity is the space bounded externally by the cheeks and lips and internally by the gums and teeth. The oral cavity proper is the space that extends from the gums and teeth to the fauces fauces = passages), the opening between the oral cavity proper and the oropharynx (throat).

The palate is a wall or septum that separates the oral cavity from the nasal cavity, and forms the roof of the mouth. This important structure makes it possible to chew and breathe at the same time. The hard palate—the anterior portion of the roof of the mouth—is formed by the maxillae and palatine bones and is covered by a mucous membrane; it forms a bony partition between the oral and nasal cavities. The soft palate, which forms the posterior portion of the roof of the mouth, is The mouth is formed by the lips, cheeks, hard and soft palates, oral cavity, teeth, salivary glands, and tongue.

Q What is the function of the uvula?

an arch-shaped muscular partition between the oropharynx and nasopharynx that is lined with mucous membrane.

Hanging from the free border of the soft palate is a fingerlike muscular structure called the uvula (Ü-vü-la = little grape). During swallowing, the soft palate and uvula are drawn superiorly, closing off the nasopharynx and preventing swallowed foods and liquids from entering the nasal cavity. Lateral to the base of the uvula are two muscular folds that run down the lateral sides of the soft palate: Anteriorly, the palatoglossal arch palatoglossal extends to the side of the base of the tongue; posteriorly, the palatopharyngeal arch palatopharyngeal extends to the side of the pharynx.

The palatine tonsils are situated between the arches, and the lingual tonsils are situated at the base of the tongue. At the posterior border of the soft palate, the mouth opens into the oropharynx through the fauces (Figure 24.6).

Salivary Glands

A salivary gland salivary is a gland that releases a secretion called saliva into the oral cavity. Ordinarily, just enough saliva is secreted to keep the mucous membranes of the mouth and pharynx moist and to cleanse the mouth and teeth. When food enters the mouth, however, secretion of saliva increases, and it lubricates, dissolves, and begins the chemical break- The mucosa and submucosa of the mouth and tongue contains about 800 to 1,000 small salivary glands that open directly, or indirectly via short ducts, to the oral cavity. These glands include labial, buccal, and palatal glands in the lips, cheeks, and palate, respectively, and lingual glands in the tongue, all of which make a small contribution to saliva. Together, these small salivary glands are called minor salivary glands.

However, most saliva is secreted by the major salivary glands, which lie beyond the oral mucosa, into ducts that lead to the oral cavity. There are three pairs of major salivary glands: the parotid, submandibular, and sublingual glands (Figure 24.7a). The parotid glands parotid; par-= near; oto-= ear) are located inferior and anterior to the ears, between the skin and the masseter muscle. Each secretes saliva into the oral cavity via a parotid (Stensen's) duct that pierces the buccinator muscle to open into the vestibule opposite the second maxillary (upper) molar tooth.

The submandibular glands (sub'-mandib-ü-lar) are found in the floor of the oral cavity proper; they are medial and partly inferior to the body of the mandible. Their ducts, the submandibular (Wharton's) ducts, run under the mucosa on either side of the midline of the floor of the mouth and enter the oral cavity proper lateral to the lingual frenulum. The sublingual glands sublingual are beneath the tongue and superior to the submandibular glands.

Their ducts, the major and minor sublingual ducts (ducts of Rivinius), open into the floor of the mouth in the oral cavity proper.

Figure 24.7 The three major salivary glands—parotid, sublingual, and submandibular. The submandibular glands, shown in the light micrograph (b), consist mostly of serous acini (serous fluid-secreting portions of gland) and a few mucous acini (mucus-secreting portions of gland); the parotid glands consist of serous acini only; and the sublingual glands consist of mostly mucous acini and a few serous acini.

Figure 24.7 summary: This figure consists of an anatomical diagram and a corresponding histological micrograph. The diagram illustrates the location and connectivity of the major salivary glands in the human head, specifically labeling the parotid, submandibular, and sublingual glands along with their respective ducts, surrounding muscles, and dental landmarks. The micrograph provides a detailed view of the glandular tissue, distinguishing between different types of secretory units. Based on the figure, it can be inferred that the salivary system is composed of multiple glands distributed across the oral cavity that utilize a network of ducts to deliver secretions. The histological detail indicates that these glands are composed of a mixture of serous and mucous acini, suggesting a combined functional role in producing different types of salivary components.

Saliva lubricates and dissolves foods and begins the chemical breakdown of carbohydrates and lipids.

Composition and Functions of Saliva Chemically, saliva is 99.5% water and 0.5% solutes. Among the solutes are ions, including sodium, potassium, chloride, bicarbonate, and phosphate. Also present are some dissolved gases and various organic substances, including urea and uric acid, mucus, immunoglobulin A, the bacteriolytic enzyme lysozyme, and salivary amylase, a digestive enzyme that acts on starch.

Not all major salivary glands supply the same ingredients. The parotid glands secrete a watery (serous) liquid containing salivary amylase. Because the submandibular glands contain cells similar to those found in the parotid glands, plus some mucous cells, they secrete a fluid that contains amylase but is thickened with mucus. The sublingual glands contain mostly mucous cells, so they secrete a much thicker fluid that contributes only a small amount of salivary amylase.

The water in saliva provides a medium for dissolving foods so that they can be tasted by gustatory receptors and so that digestive reactions can begin. Chloride ions in the saliva activate salivary amylase amylase, an enzyme that starts the breakdown of starch in the mouth into maltose, maltotriose, and α-dextrin. Bicarbonate and phosphate ions buffer acidic foods that enter the mouth, so saliva is only slightly acidic (pH 6.35 to 6.85). Salivary glands (like the sweat glands of the skin) help remove waste molecules from the body, which accounts for the presence of urea and uric acid in saliva. Mucus lubricates food so it can be moved around easily in the mouth, formed into a ball, and swallowed. Immunoglobulin A (IgA) prevents attachment of microbes so they cannot penetrate the epithelium, and the enzyme lysozyme kills microbes; however, these substances are not present in large enough quantities to eliminate all oral bacteria.

Salivation The secretion of saliva, called salivation (sal-i-Vâ-shun), is controlled by the autonomic nervous system. Amounts of saliva secreted daily vary considerably but average 1000 to 1500 mL (1 to 1.6 qt). Normally, parasympathetic stimulation promotes continuous secretion of a moderate amount of saliva, which keeps the mucous membranes moist and lubricates the movements of the tongue and lips during speech. The saliva is then swallowed and helps moisten the esophagus.

Eventually, most components of saliva are reabsorbed, which prevents fluid loss. Sympathetic stimulation dominates during stress, resulting in dryness of the mouth. If the body becomes dehydrated, the salivary glands stop secreting saliva to conserve water; the resulting dryness in the mouth contributes to the sensation of thirst. Drinking not only restores the homeostasis of body water but also moistens the mouth.

Clinical Connection

Mumps

Although any of the major salivary glands may be the target of a nasopharyngeal infection, the mumps virus (paramyxovirus) typically attacks the parotid glands. Mumps is an inflammation and enlargement of the parotid glands accompanied by moderate fever, malaise (general discomfort), and extreme pain in the throat, especially when swallowing sour foods or acidic juices. Swelling occurs on one or both sides of the face, just anterior to the ramus of the mandible.

In about 30% of males past puberty, the testes may also become inflamed; sterility rarely occurs because testicular involvement is usually unilateral (one testis only). Since a vaccine became available for mumps in 1967, the incidence of the disease has declined dramatically.

Image summary: This is a portrait photograph. The image shows a close up view of a young man's face, featuring freckles across the nose and cheeks and dark, slightly messy hair. The subject has a neutral facial expression and is looking slightly upward and away from the camera. Based on the composition and the presence of text in the background, the image appears to be an identification photo or a clinical record.

The feel and taste of food also are potent stimulators of salivary gland secretions. Chemicals in the food stimulate receptors in taste buds on the tongue, and impulses are conveyed from the taste buds to two salivary nuclei in the brain stem (superior and inferior salivatory nuclei). Returning parasympathetic impulses in fibers of the facial (7) and glossopharyngeal (9) nerves stimulate the secretion of saliva. Saliva continues to be secreted heavily for some time after food is swallowed; this flow of saliva washes out the mouth and dilutes and buffers the remnants of irritating chemicals such as that tasty (but hot!) salsa. The smell, sight, sound, or thought of food may also stimulate secretion of saliva.

Tongue

The tongue is an accessory digestive organ composed of skeletal muscle covered with mucous membrane. Together with its associated muscles, it forms the floor of the oral cavity proper. The tongue is divided into symmetrical lateral halves by a median septum that extends its entire length, and it is attached inferiorly to the hyoid bone, styloid process of the temporal bone, and mandible. Each half of the tongue consists of an identical complement of extrinsic and intrinsic muscles.

The extrinsic muscles of the tongue, which originate outside the tongue (attach to bones in the area) and insert into connective tissues in the tongue, include the hyoglossus, genioglossus, and styloglossus muscles (see Figure 11.7). The extrinsic muscles move the tongue from side to side and in and out to maneuver food for chewing, shape the food into a rounded mass, and force the food to the back of the mouth for swallowing. They also form the floor of the mouth and hold the tongue in position. The intrinsic muscles of the tongue originate in and insert into connective tissue within the tongue. They alter the shape and size of the tongue for speech and swallowing.

The intrinsic muscles include the superior longitudinal lingual, inferior longitudinal lingual, transversus linguae, and verticalis linguae muscles. The lingual frenulum (lingua = the tongue), a fold of mucous membrane in the midline of the undersurface of the tongue, is attached to the floor of the mouth and aids in limiting the movement of the tongue posteriorly (see Figures 24.6 and 24.7). If a person's lingual frenulum is abnormally short or rigid—a condition called ankyloglossia ankyloglossia—the person is said to be "tongue-tied" because of the resulting impairment to speech. It can be corrected surgically.

The dorsum (upper surface) and lateral surfaces of the tongue are covered with lingual papillae papillae = nipple-shaped projections), projections of the lamina propria covered with stratified squamous epithelium (see Figure 17.3). Many lingual papillae contain gustatory epithelial cells, the receptors for gustation (taste), in taste buds. Some lingual papillae lack taste buds, but they contain receptors for touch and increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity proper. The different types of taste buds are described in detail in Section 17.2. Lingual glands (minor salivary glands) in the lamina propria of the tongue secrete both mucus and a watery serous fluid that

Figure 24.8 A Typical Tooth and Surrounding Structures.

Teeth are anchored in dental alveoli of the alveolar processes of the mandible and maxillae.

Clinical Connection

Root Canal Therapy

Root canal therapy is a multistep procedure in which all traces of dental pulp tissue are removed from the pulp cavity and root canals of a badly diseased tooth. After a hole is made in the tooth, the root canals are filed out and irrigated to remove bacteria. Then, the canals are treated with medication and sealed tightly. The damaged crown is then repaired.

What type of tissue is the main component of teeth? contains the enzyme lingual lipase (Ll-pãs), which acts on as much as 30% of dietary triglycerides (fats and oils) and converts them to simpler fatty acids and diglycerides.

Teeth

The teeth, or dentes (Figure 24.8), are accessory digestive organs located in dental alveoli (sockets) of the alveolar processes of the mandible and maxillae. The alveolar processes (thickened ridges) are covered by the gingivae gingivae, or Pulp cavity contains dental pulp (connective tissue containing nerves and blood vessels).

Figure 24.8 summary: This figure is an anatomical diagram. It illustrates the internal and external structure of a human tooth, divided into the crown, neck, and root sections. The diagram labels various tissues and components, including the protective enamel, the underlying dentin, the gingiva, the pulp cavity, the root canal, and the supporting alveolar process and periodontal ligament, while also showing the nerve and blood supply entering through the apical foramen. The figure demonstrates that a tooth consists of multiple specialized layers and tissues that provide structural integrity and protection, while the internal pulp and root canal provide the necessary biological support through vascular and neural connections to the rest of the body.

Periodontal ligament helps anchor the tooth to the underlying bone Apical foramen is an opening at the base of a root canal through which blood vessels, lymphatic vessels, and nerves enter a tooth Nerve gums, which extend slightly into each dental alveolus. The dental alveoli are lined by the periodontium periodontium; odont-= tooth), which consists of all structures that attach a tooth to the dental alveolus of the alveolar process of the mandible and maxillae. One part of the periodontium is the periodontal ligament, which consists of strips of dense connective tissue that attach the cement of the root of the tooth (described shortly) to the dental alveolus of the alveolar process. The periodontal ligament also acts as a shock absorber during chewing.

A typical tooth has three major external regions: the crown, root, and neck. The crown is the visible portion above the level of the gums. Embedded in the dental alveolus are one to three roots. The neck is the constricted junction of the crown and root near the gum line.

Internally, dentin forms the majority of the tooth. Dentin consists of a calcified connective tissue that gives the tooth its basic shape and rigidity. It is harder than bone because of its higher content of hydroxyapatite (70% versus 55% of dry weight).

Dentin contains dentinal tubules, parallel microscopic tubules that radiate through the dentin from the pulp cavity. Within the tubules are processes from odontoblasts, cells that produce the dentin, and fluid derived as a filtrate of blood vessels in the pulp cavity. If the dental tubules are exposed and open from erosion of the enamel, the fluid in the tubules moves inward and outward and this fluid shift activates pain receptors.

This results in dentin hypersensitivity, a sharp pain which may be triggered by stimuli such as cold, heat, touch, and chemicals (for example, sugar).

The dentin of the crown is covered by enamel, which consists primarily of calcium phosphate and calcium carbonate. Enamel is also harder than bone because of its even higher content of calcium salts (about 95% of dry weight). In fact, enamel is the hardest substance in the body.

It serves to protect the tooth from the wear and tear of chewing. It also protects against acids that can easily dissolve dentin. The dentin of the root is covered by cement, another bone-like substance, which attaches the root to the periodontal ligament.

The dentin of a tooth encloses a space. The enlarged part of the space, the pulp cavity, lies within the crown and is filled with dental pulp, a connective tissue containing blood vessels, nerves, and lymphatic vessels. Narrow extensions of the pulp cavity, called root canals, run through the root of the tooth. Each root canal has an opening at its base, the apical foramen, through which blood vessels, lymphatic vessels, and nerves enter a tooth. The blood vessels bring nourishment, the lymphatic vessels offer protection, and the nerves provide sensation.

The branch of dentistry that is concerned with the prevention, diagnosis, and treatment of diseases that affect the pulp, root, periodontium, and alveolar bone is known as endodontics endodontics; endo-= within). Orthodontics orthodontics; ortho-= straight) is a branch of dentistry that is concerned with the prevention and correction of abnormally aligned teeth; periodontics periodontics is a branch of dentistry concerned with the treatment of abnormal conditions of the tissues immediately surrounding the teeth, such as gingivitis (gum disease).

Humans have two dentitions, or sets of teeth: deciduous and permanent. The first of these—the deciduous teeth (decidu-= falling out), also called primary teeth, milk teeth, or baby teeth—begin to erupt at about 6 months of age, and approximately two teeth appear each month thereafter, until all 20 are present (Figure 24.9a). The incisors, which are closest to the midline, are chisel-shaped and adapted for cutting into food. They are referred to as either central or lateral. incisors based on their position. Next to the incisors, moving posteriorly, are the canines, which have a pointed surface called a cusp. Canines are used to tear and shred food. Incisors and canines have only one root apiece.

Posterior to the canines lie the first and second deciduous molars, which have four cusps. Maxillary (upper) molars have three roots; mandibular (lower) molars have two roots. The molars crush and grind food to prepare it for swallowing.

All of the deciduous teeth are lost—generally between ages 6 and 12 years—and are replaced by the permanent (secondary) teeth (Figure 24.9b). The permanent dentition contains 32 teeth that erupt between age 6 and adulthood. The pattern resembles the deciduous dentition, with the following exceptions. The deciduous molars are replaced by the first and second premolars (bicuspids), which have two cusps and one root and are used for crushing and grinding. The permanent molars, which erupt into the mouth posterior to the premolars, do not replace any deciduous teeth and erupt as the jaw grows to accommodate them—the first permanent molars at age 6 (six-year molars), the second permanent molars at age 12 (twelve-year molars), and the third permanent molars (wisdom teeth) after age 17 or not at all.

Often the human jaw does not have enough room posterior to the second molars to accommodate the eruption of the third molars. In this case, the third molars remain embedded in the alveolar bone and are said to be impacted. They often cause pressure and pain and must be removed surgically. In some people, third molars may be dwarfed in size or may not develop at all.

Mechanical and Chemical Digestion in the Mouth

Mechanical digestion in the mouth results from chewing, or mastication (mas'-ti-KÃ-shun = to chew), in which food is manipulated by the tongue, ground by the teeth, and mixed with saliva. As a result, the food is reduced to a soft, flexible, easily swallowed mass called a bolus (= lump). Food molecules begin to dissolve in the water in saliva, an important activity because enzymes can react with food molecules in a liquid medium only.

Two enzymes, salivary amylase and lingual lipase, contribute to chemical digestion in the mouth. Salivary amylase, which is secreted by the salivary glands, initiates the breakdown of starch. Dietary carbohydrates are either monosaccharide and disaccharide sugars or complex polysaccharides such as starches. Most of the carbohydrates we eat are starches, but only monosaccharides can be absorbed into the bloodstream. Thus, ingested disaccharides and starches must be broken down into monosaccharides.

The function of salivary amylase is to begin starch digestion by breaking down starch into smaller molecules such as the disaccharide maltose, the trisaccharide maltotriose, and short-chain glucose polymers called alpha -dextrins. Even though food is usually swallowed too quickly for all starches to be broken down in the mouth, Figure 24.9 Dentitions and times of eruption. A designated letter (deciduous teeth) or number (permanent teeth) uniquely identifies each tooth. Deciduous teeth begin to erupt at 6 months of age, and approximately two teeth appear each month thereafter, until all 20 are present. Times of eruption are indicated in parentheses.

Figure 24.9 summary: This figure consists of anatomical diagrams. The diagrams illustrate the arrangement and eruption timing of both primary deciduous teeth and permanent teeth in the upper and lower jaws. Based on the comparison between the deciduous and permanent dentition, it can be inferred that while most permanent teeth replace their primary predecessors, the permanent molars, including the first, second, and third molars, emerge in spaces behind the primary teeth rather than replacing them.

There are 20 teeth in a complete deciduous set and 32 teeth in a complete permanent set.

salivary amylase in the swallowed food continues to act on the starches for about another hour, at which time stomach acids inactivate it. Saliva also contains lingual lipase, which is secreted by lingual glands in the tongue. This enzyme becomes activated in the acidic environment of the stomach and thus starts to work after food is swallowed. It breaks down dietary triglycerides (fats and oils) into fatty acids and diglycerides. A diglyceride consists of a glycerol molecule that is attached to two fatty acids.

Table 24.1 summarizes the digestive activities in the mouth.

Table 24.1 summary: This table outlines the various anatomical structures of the mouth and their corresponding roles in the initial stages of digestion. It details how mechanical actions from the cheeks, lips, tongue, and teeth work together to manipulate and break down food into smaller particles. Additionally, it describes the chemical contributions of the salivary and lingual glands, which lubricate the mouth and initiate the enzymatic breakdown of carbohydrates and fats.

Table summary: The table consists of a series of review questions focusing on the anatomy and physiology of the oral cavity, including the structures of the mouth, the location of salivary glands, the regulation of saliva, and the specific functions of different types of teeth.

24.6 Pharynx

Objective

• Describe the location and function of the pharynx.

When food is first swallowed, it passes from the mouth into the pharynx (= throat) or throat, a funnel-shaped tube that extends from the choanae to the esophagus posteriorly and to the larynx anteriorly (see Figure 23.2). The pharynx is composed of skeletal muscle and lined by mucous membrane, and is divided into three parts: the nasopharynx, the oropharynx, and the laryngopharynx. The nasopharynx functions only in respiration, but both the oropharynx and laryngopharynx have digestive as well as respiratory functions. Swallowed food passes from the mouth into the oropharynx and laryngopharynx; the muscular contractions of these areas help propel food into the esophagus and then into the stomach.

Checkpoint 14. To which two organ systems does the pharynx belong?

24.7 Esophagus

• Describe the location, anatomy, histology, and functions of the esophagus.

The esophagus esophagus = eating gullet) is a collapsible muscular tube, about 25 centimeters (10 in.) long, that lies posterior to the trachea. The esophagus begins at the inferior end of the laryngopharynx, passes through the inferior aspect of the neck, and enters the mediastinum anterior to the vertebral column. Then it pierces the diaphragm through an opening called the esophageal hiatus (e-sof-a-JÉ-al hi-Â-tus), and ends in the superior portion of the stomach (see Figure 24.1). Sometimes, part of the stomach protrudes above the diaphragm through the esophageal hiatus. This condition, termed a hiatus hernia hernia, is described in the Medical Terminology section at the end of the chapter.

Histology of the Esophagus

The mucosa of the esophagus consists of nonkeratinized stratified squamous epithelium, lamina propria (areolar connective tissue), and a muscularis mucosae (smooth muscle) (Figure 24.10). Near the stomach, the mucosa of the esophagus also contains mucous glands. The stratified squamous epithelium associated with the lips, mouth, tongue, oropharynx, laryngopharynx, and esophagus affords considerable protection against abrasion and wear and tear from food particles that are chewed, mixed with secretions, and swallowed. The submucosa contains areolar connective tissue, blood vessels, and mucous glands.

The muscular layer of the superior third of the esophagus is skeletal muscle, the intermediate third is skeletal and smooth muscle, and the inferior third is smooth muscle. At each end of the esophagus, the muscular layer becomes slightly more prominent and forms two sphincters—the upper esophageal sphincter U.E.S (e-sof'-a-JÊ-al), which consists of skeletal muscle, and the lower esophageal (cardiac) sphincter L.E.S, which consists of smooth muscle and is near the heart. The upper esophageal sphincter regulates the movement of food from the pharynx into the esophagus; the lower esophageal sphincter regulates the movement of food from the

Figure 24.10 Histology of the Esophagus. A Higher-

magnification view of nonkeratinized stratified squamous epithelium is shown in Table 4.1.G

Q In which layers of the esophagus are the glands that secrete lubricating mucus located? esophagus into the stomach. The superficial layer of the esophagus is known as the adventitia adventitia, rather than the serosa as in the stomach and intestines, because the areolar connective tissue of this layer is not covered by mesothelium and because the connective tissue merges with the connective tissue of surrounding structures of the mediastinum through which it passes. The adventitia attaches the esophagus to surrounding structures.

Physiology of the Esophagus

The esophagus secretes mucus and transports food into the stomach. It does not produce digestive enzymes, and it does not carry out absorption.

Checkpoint

15. Describe the location and histology of the esophagus. What is its role in digestion?

16. What are the functions of the upper and lower esophageal sphincters?

24.8

Deglutition

Objective

• Describe the three phases of deglutition.

The movement of food from the mouth into the stomach is achieved by the act of deglutition deglutition or swallowing (Figure 24.11). Deglutition is facilitated by the secretion of saliva and mucus and involves the mouth, pharynx, and esophagus. Swallowing occurs in three stages: (1) the voluntary stage, in which the bolus is passed into the oropharynx; (2) the pharyngeal stage, the involuntary passage of the bolus through the pharynx into the esophagus; and (3) the esophageal stage, the involuntary passage of the bolus through the esophagus into the stomach.

Swallowing starts when the bolus is forced to the back of the oral cavity and into the oropharynx by the movement of the tongue upward and backward against the palate; these actions constitute the voluntary stage of swallowing. With the passage of the bolus into the oropharynx, the involuntary pharyngeal stage of swallowing begins (Figure 24.11b). The bolus stimulates receptors in the oropharynx, which send impulses to the deglutition center in the medulla oblongata and lower pons of the brain stem. The returning impulses cause the soft palate and uvula to move upward to close off the nasopharynx, which prevents swallowed foods and liquids from entering the nasal cavity. In addition, the epiglottis closes off the opening to the larynx, which prevents the bolus from entering the rest of the respiratory tract.

The bolus moves through the oropharynx and the laryngopharynx. Once the upper esophageal sphincter relaxes, the bolus moves into the esophagus.

The esophageal stage of swallowing begins once the bolus enters the esophagus. During this phase, peristalsis Figure 24.11 Deglutition (swallowing). During the pharyngeal stage (b) the tongue rises against the palate, the nasopharynx is closed off, the larynx rises, the epiglottis seals off the larynx, and the bolus is passed into the esophagus. During the esophageal stage (c), food moves through the esophagus into the stomach via peristalsis.

peristalsis; stalsis = constriction), a progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscular layer, pushes the bolus onward (Figure 24.11c). (Peristalsis occurs in other tubular structures, including other parts of the digestive canal to the anus and the ureters, bile ducts, and uterine tubes; in the esophagus it is controlled by the medulla oblongata.)

1 In the section of the esophagus just superior to the bolus, the circular muscle fibers contract, constricting the esophageal wall and squeezing the bolus toward the stomach. ② Longitudinal muscle fibers inferior to the bolus also contract, which shortens this inferior section and pushes its • The tongue rises against the palate and closes the nasopharynx. walls outward so it can receive the bolus. The contractions are repeated in waves that push the food toward the stomach. Steps one and two repeat until the bolus reaches the lower esophageal sphincter muscles.

3 The lower esophageal sphincter relaxes and the bolus moves into the stomach.

Mucus secreted by esophageal glands lubricates the bolus and reduces friction. The passage of solid or semisolid food from the mouth to the stomach takes 4 to 8 seconds; very soft foods and liquids pass through in about 1 second.

Table 24.2 summarizes the digestive activities of the pharynx and esophagus.

Table 24.2 summary: This table outlines the functional roles of the pharynx and esophagus during the process of swallowing. It describes how the pharynx coordinates the movement of food while protecting the airway, and how the esophagus utilizes sphincter relaxation, peristaltic contractions, and mucus secretion to transport the bolus efficiently into the stomach.

Clinical Connection

Gastroesophageal Reflux Disease

If the lower esophageal sphincter fails to close adequately after food has entered the stomach, the stomach contents can reflux (back up) into the inferior portion of the esophagus. This condition is known as gastroesophageal reflux disease (gerd) (gas'-trö-e-sof-a-Jf-al). Hydrochloric acid H.C.L from the stomach contents can irritate the esophageal wall, resulting in a burning sensation that is called heartburn because it is experienced in a region very near the heart; it is unrelated to any cardiac problem. Drinking alcohol and smoking can cause the sphincter to relax, worsening the problem. The symptoms of gerd often can be controlled by avoiding foods that strongly stimulate stomach acid secretion (coffee, chocolate, tomatoes, fatty foods, orange juice, peppermint, spearmint, and onions). Other acid-reducing strategies include taking over-the-counter histamine-2 (H₂) blockers such as Tagamet H.B or Pepcid A.C 30 to 60 minutes before eating to block acid secretion, and neutralizing acid that has already been secreted with antacids such as Tums® or Maalox®. Symptoms are less likely to occur if food is eaten in smaller amounts and if the person does not lie down immediately after a meal. gerd may be associated with cancer of the esophagus.

Image summary: This figure is an anatomical diagram. It illustrates the relationship between the stomach and the esophagus, specifically highlighting the lower esophageal sphincter and the movement of stomach contents. The diagram shows stomach contents flowing backward into the esophagus, which is labeled as dilated and irritated. This indicates that when the esophageal sphincter fails to close properly, gastric contents reflux into the esophagus, leading to tissue irritation and enlargement of the esophageal canal.

24.9 Stomach

The stomach is a J-shaped enlargement of the digestive canal directly inferior to the diaphragm in the abdomen. The stomach connects the esophagus to the duodenum, the first part of the small intestine (Figure 24.12). Because a meal can be eaten much more quickly than the intestines can digest and absorb it, one of the functions of the stomach is to serve as a mixing chamber and holding reservoir. At appropriate intervals after food is ingested, the stomach forces a small quantity of material into the first portion of the small intestine. The position and size of the stomach vary continually; the diaphragm pushes it inferiorly with each The four regions of the stomach are the cardia, fundus, body, and pyloric part.

Figure 24.12 summary: This figure is a collection of anatomical diagrams and a medical image. The content illustrates the anatomy of the human stomach and its connections, labeling key structures such as the esophagus, lower esophageal sphincter, fundus, cardia, body, pyloric sphincter, and duodenum, while also detailing the muscular layers and internal mucosal folds. The figures demonstrate that the stomach is a complex organ with specialized sphincters at both the entrance and exit to regulate the flow of food from the esophagus and into the small intestine, and that the internal surface is characterized by folds to increase surface area for digestive processes.

Functions of the Stomach

1. Mixes saliva, food, and gastric juice to form a soupy liquid called chyme.

2. Serves as reservoir for food before release into small intestine.

3. Secretes gastric juice, which contains H.C.L (kills bacteria and denatures proteins), pepsin (begins the digestion of proteins), intrinsic factor (aids absorption of vitamin B 12 ), and gastric lipase (aids digestion of triglycerides).

4. Secretes gastrin into blood.

inhalation and pulls it superiorly with each exhalation. Empty, it is about the size of a large sausage, but it is the most distensible part of the digestive canal and can accommodate a large quantity of food. In the stomach, digestion of starch and triglycerides continues, digestion of proteins begins, the semisolid bolus is converted to a liquid called chyme, and certain substances are absorbed. The medical specialty that deals with the structure, function, diagnosis, and treatment of diseases of the stomach and intestines is called gastroenterology gastroenterology; gastro-= stomach; -entero-= intestines; -logy = study of).

Anatomy of the Stomach

The stomach has four main regions: the cardia, fundus, body, and pyloric part (Figure 24.12). The cardia cardia surrounds the opening of the esophagus into the stomach. The rounded portion superior to and to the left of the cardia is the fundus fundus. Inferior to the fundus is the large central portion of the stomach, the body. The pyloric part is divisible into three regions.

The first region, the pyloric antrum, connects to the body of the stomach. The second region, the pyloric canal, leads to the third region, the pylorus pylorus; pyl-= gate; -orus = guard), which in turn connects to the duodenum. When the stomach is empty, the mucosa lies in large gastric folds (rugae), that can be seen with the unaided eye. The pylorus communicates with the duodenum of the small intestine via a smooth

Figure 24.13 Histology of the Stomach.

muscle sphincter called the pyloric sphincter (valve). The concave medial border of the stomach is called the lesser curvature; the convex lateral border is called the greater curvature.

Pylorospasm and Pyloric Stenosis

Two abnormalities of the pyloric sphincter can occur in infants. In pylorospasm pylorospasm, the smooth muscle fibers of the sphincter fail to relax normally, so food does not pass easily from the stomach to the small intestine, the stomach becomes overly full, and the infant vomits often to relieve the pressure. Pylorospasm is treated by drugs that relax the muscle fibers of the pyloric sphincter. Pyloric stenosis (ste-No-sis) is a narrowing of the pyloric sphincter that must be corrected surgically. The hallmark symptom is projectile vomiting—the spraying of liquid vomitus some distance from the infant.

Histology of the Stomach

The stomach wall is composed of the same basic layers as the rest of the digestive canal, with certain modifications. The surface of the mucosa is a layer of simple columnar epithelial cells called surface mucous cells (Figure 24.13). The mucosa contains a lamina propria (areolar connective tissue) and a muscularis mucosae (smooth muscle) (Figure 24.13). Epithelial cells extend down into the lamina propria, where they form columns of secretory cells called gastric glands. Several gastric glands open into the bottom of narrow channels called gastric pits. Secretions from several gastric glands flow into each gastric pit and then into the lumen of the stomach.

The gastric glands contain three types of exocrine gland cells that secrete their products into the stomach lumen: mucous neck cells, chief cells, and parietal cells. Both surface mucous cells and mucous neck cells secrete mucus (Figure 24.13b). Parietal cells produce intrinsic factor (needed for absorption of vitamin B 12 ) and hydrochloric acid. The chief (zymogenic) cells secrete pepsinogen and gastric lipase.

The secretions of the mucous, parietal, and chief cells form gastric juice, which totals 2000 to 3000 mL (roughly 2 to 3 qt) per day. In addition, gastric glands include a type of enteroendocrine cell, the G cell, which is located mainly in the pyloric antrum and secretes the hormone gastrin into the bloodstream. As we will see shortly, this hormone stimulates several aspects of gastric activity.

Three additional layers lie deep to the mucosa. The submucosa of the stomach is composed of areolar connective tissue. The muscular layer has three layers of smooth muscle (rather than the two layers found in the esophagus and small and large intestines): an outer longitudinal layer, a middle circular layer, and an inner oblique layer.

The oblique layer is limited primarily to the body of the stomach. The serosa is composed of simple squamous epithelium (mesothelium) and areolar connective tissue; the portion of the serosa covering the stomach is part of the visceral peritoneum. At the lesser curvature of the stomach, the visceral peritoneum extends upward to the liver as the lesser omentum.

At the greater curvature of the stomach, the visceral peritoneum continues downward as the greater omentum and drapes over the intestines.

H.C.L secretion by parietal cells can be stimulated by several sources: acetylcholine A.C.h, gastrin, and histamine. proteins in the chief cells that produce it. Pepsinogen is not converted into active pepsin until it comes in contact with hydrochloric acid secreted by parietal cells or active pepsin molecules. Second, the stomach epithelial cells are protected from gastric juices by a layer 1 to 3 millimeters thick of alkaline mucus secreted by surface mucous cells and mucous neck cells.

Another enzyme of the stomach is gastric lipase, which splits triglycerides (fats and oils) in fat molecules (such as those found in milk) into fatty acids and monoglycerides. A monoglyceride consists of a glycerol molecule that is attached to one fatty acid molecule. This enzyme, which has a limited role in the adult stomach, operates best at a pH of 5 to 6. More important than either lingual lipase or gastric lipase is pancreatic lipase, an enzyme secreted by the pancreas into the small intestine.

Only a small amount of nutrients are absorbed in the stomach because its epithelial cells are impermeable to most materials. However, mucous cells of the stomach absorb some water, ions, and short-chain fatty acids, as well as certain drugs (especially aspirin) and alcohol.

Within 2 to 4 hours after eating a meal, the stomach has emptied its contents into the duodenum. Foods rich in carbohydrate spend the least time in the stomach; high-protein foods remain somewhat longer, and emptying is slowest after a fat-laden meal containing large amounts of triglycerides.

Table 24.3 summarizes the digestive activities of the stomach.

Table 24.3 summary: This table outlines the specialized functions of various stomach structures, detailing how different cell types and muscle layers contribute to digestion. It highlights the protective role of mucus, the chemical breakdown of proteins and fats via secretions from parietal and chief cells, and the hormonal regulation provided by G cells. Additionally, it describes the mechanical processes of the muscular layer and the regulatory function of the pyloric sphincter in managing the flow of chyme into the duodenum.

Clinical Connection

Vomiting

Vomiting or emesis is the forcible expulsion of the contents of the upper digestive canal (stomach and sometimes duodenum) through the mouth. The strongest stimuli for vomiting are irritation and distension of the stomach; other stimuli include unpleasant sights, general anesthesia, dizziness, and certain drugs such as morphine and derivatives of digitalis. Nerve impulses are transmitted to the vomiting center in the medulla oblongata, and returning impulses propagate to the upper digestive canal organs, diaphragm, and abdominal muscles. Vomiting involves squeezing the stomach between the diaphragm and abdominal muscles and expelling the contents through open esophageal sphincters. Prolonged vomiting, especially in infants and elderly people, can be serious because the loss of acidic gastric juice can lead to alkalosis (higher than normal blood pH), dehydration, and damage to the esophagus and teeth.

20. Compare the epithelium of the esophagus with that of the stomach. How is each adapted to the function of the organ?

21. What is the importance of gastric folds, surface mucous cells, mucous neck cells, chief cells, parietal cells, and G cells in the stomach?

22. What is the role of pepsin? Why is it secreted in an inactive form?

23. What are the functions of gastric lipase and lingual lipase in the stomach?

24.10 Pancreas

Objective

- Describe the location, anatomy, histology, and function of the pancreas.

From the stomach, chyme passes into the small intestine. Because chemical digestion in the small intestine depends on activities of the pancreas, liver, and gallbladder, we first consider the activities of these accessory digestive organs and their contributions to digestion in the small intestine.

Anatomy of the Pancreas

The pancreas (pan-= all; -creas = flesh), a retroperitoneal gland that is about 12 to 15 centimeters (5 to 6 in.) long and 2.5 centimeters (1 in.) thick, lies posterior to the greater curvature of the stomach. The pancreas consists of a head, neck, body, and tail and is usually connected to the duodenum of the small intestine by two ducts (Figure 24.16a). The head is the expanded portion of the organ near the curve of the duodenum; superior to and to the left of the head are the narrowing neck, the central body and the tapering tail.

Pancreatic juices are secreted by exocrine cells into small ducts that ultimately unite to form two larger ducts, the pancreatic duct and the accessory duct. These in turn convey the secretions into the small intestine. The pancreatic duct, or duct of Wirsung (Vër-sung), is the larger of the two ducts.

In most people, the pancreatic duct joins the bile duct from the liver and gallbladder and enters the duodenum as a dilated common duct called the hepatopancreatic ampulla (hep'-a-to-pan-kre-A-tik), or ampulla of Vater father. The ampulla opens on an elevation of the duodenal mucosa known as the major duodenal papilla, which lies about 10 centimeters (4 in.) inferior to the pyloric sphincter of the stomach. The passage of pancreatic juice and Figure 24.16 Relationship of the pancreas to the liver, gallbladder, and duodenum. The inset (b) shows details of the bile duct and pancreatic duct forming the hepatopancreatic ampulla and emptying into the duodenum.

Figure 24.16 summary: This figure consists of anatomical diagrams, a flow chart, and cadaveric photographs. The content illustrates the anatomy of the biliary system and pancreas, detailing the liver lobes, gallbladder, various hepatic and bile ducts, the pancreas and its duct, and their connection to the duodenum via the hepatopancreatic ampulla. The figure demonstrates that bile is produced in the liver and stored in the gallbladder before being transported through a network of ducts to merge with pancreatic secretions. It can be inferred that the common bile duct and pancreatic duct converge to deliver digestive enzymes and bile into the small intestine through a regulated opening, ensuring that these substances enter the duodenum for the digestion of nutrients.

Pancreatic enzymes digest starches (polysaccharides), proteins, triglycerides, and nucleic acids. bile through the hepatopancreatic ampulla into the duodenum of the small intestine is regulated by a mass of smooth muscle surrounding the ampulla known as the sphincter of the hepatopancreatic ampulla, or sphincter of Oddi ode. The other major duct of the pancreas, the accessory duct (duct of Santorini), leads from the pancreas and empties into the duodenum about 2.5 centimeters (1 in.) superior to the hepatopancreatic ampulla.

Histology of the Pancreas

The pancreas is made up of small clusters of glandular epithelial cells. About 99% of the clusters, called pancreatic acini asini, constitute the exocrine portion of the organ (see Figure 18.17b, c). The cells within pancreatic acini secrete a mixture of fluid and digestive enzymes called pancreatic juice. The remaining 1% of the clusters, called pancreatic islets (islets of Langerhans) (I-lets), form the endocrine portion of the pancreas.

These cells secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide. The functions of these hormones are discussed in Chapter 18.

Composition and Functions of Pancreatic Juice

Each day the pancreas produces 1200 to 1500 mL (about 1.2 to 1.5 qt) of pancreatic juice, a clear, colorless liquid consisting mostly of water, some salts, sodium bicarbonate, and several enzymes. The sodium bicarbonate gives pancreatic juice a slightly alkaline pH (7.1 to 8.2) that buffers acidic gastric juice in chyme, stops the action of pepsin from the stomach, and creates the proper pH for the action of digestive enzymes in the small intestine. The enzymes in pancreatic juice include a starch-digesting enzyme called pancreatic amylase; several enzymes that digest proteins into peptides called trypsin tripsin, chymotrypsin chimotrypsin, carboxypeptidase carboxypeptidase, and elastase elastase; the principal triglyceride-digesting enzyme in adults, called pancreatic lipase; and nucleic acid-digesting enzymes called ribonuclease ribonuclease and deoxyribonuclease deoxyribonuclease that digest ribonucleic acid (R.N.A) and deoxyribonucleic acid (D.N.A) into nucleotides.

The protein-digesting enzymes of the pancreas are produced in an inactive form, just as pepsin is produced in the stomach as pepsinogen. Because they are inactive, the enzymes do not digest cells of the pancreas itself. Trypsin is secreted in an inactive form called trypsinogen trypsinogen. Pancreatic acinar cells also secrete a protein called trypsin inhibitor that combines with any trypsin formed accidentally in the pancreas or in pancreatic juice and blocks its enzymatic activity. When trypsinogen reaches the lumen of the small intestine, it encounters an activating brush-border enzyme called enterokinase (en'-ter-ö-Kl-näs), which splits off part of the trypsinogen molecule to form trypsin. In turn, trypsin acts on the inactive precursors (called chymotrypsinogen, procarboxypeptidase, and proelastase) to produce chymotrypsin, carboxypeptidase, and elastase, respectively.

Clinical Connection

Pancreatitis and Pancreatic Cancer

Inflammation of the pancreas, as may occur in association with alcohol abuse or chronic gallstones, is called pancreatitis (pan'krê-a-Tî-tis). In a more severe condition known as acute pancreatitis, which is associated with heavy alcohol intake or biliary tract obstruction, the pancreatic cells may release either trypsin instead of trypsinogen or insufficient amounts of trypsin inhibitor, and the trypsin begins to digest the pancreatic cells. Patients with acute pancreatitis usually respond to treatment, but recurrent attacks are the rule. In some people pancreatitis is idiopathic, meaning that the cause is unknown. Other causes of pancreatitis include cystic fibrosis, high levels of calcium in the blood (hypercalcemia), high levels of blood fats (hyperlipidemia or hypertriglyceridemia), some drugs (such as estrogens, sulfa drugs, and corticosteroids), and certain autoimmune conditions. However, in roughly 70% of adults with pancreatitis, the cause is alcoholism. Often the first episode happens between ages 30 and 40.

Pancreatic cancer usually affects people over 50 years of age and occurs more frequently in males. Typically, there are few symptoms until the disorder reaches an advanced stage and often not until it has metastasized to other parts of the body such as the lymph nodes, liver, or lungs. The disease is nearly always fatal and is the fourth most common cause of death from cancer in the United States. Pancreatic cancer has been linked to fatty foods, high alcohol consumption, genetic factors, smoking, and chronic pancreatitis.

24.11 Liver and Gallbladder

Objective

• Describe the location, anatomy, histology, and functions of the liver and gallbladder.

The liver is the heaviest gland of the body, weighing about 1.4 kilograms (about 3 pounds) in an average adult. Of all of the organs of the body, it is second only to the skin in size. The liver is inferior to the diaphragm and occupies most of the right hypochondriac and part of the epigastric regions of the abdominopelvic cavity (see Figure 1.13b).

The gallbladder (gall-= bile) is a pear-shaped sac that is located in a depression of the posterior surface of the liver. It is 7 to 10 centimeters (3 to 4 in.) long and typically hangs from the anterior inferior margin of the liver (Figure 24.16a).

Anatomy of the Liver and Gallbladder

The liver is almost completely covered by visceral peritoneum and is completely covered by a dense irregular connective tissue layer (fibrous capsule) that lies deep to the peritoneum. The liver is divided into two principal lobes—a large right lobe and a smaller left lobe—by the falciform ligament, a fold of the mesentery (Figure 24.16a). Although the right lobe is considered by many anatomists to include an inferior quadrate lobe (kwadrAT) and a posterior caudate lobe kawdat, based on internal morphology (primarily the distribution of blood vessels), the quadrate and caudate lobes more appropriately belong to the left lobe. The falciform ligament extends from the undersurface of the diaphragm between the two principal lobes of the liver to the superior surface of the liver, helping to suspend the liver in the abdominal cavity.

In the free border of the falciform ligament is the ligamentum teres (round ligament), a remnant of the umbilical vein of the fetus (see Figure 21.31a, b); this fibrous cord extends from the liver to the umbilicus. The right and left coronary ligaments are narrow extensions of the parietal peritoneum that suspend the liver from the diaphragm.

The parts of the gallbladder include the broad fundus, which projects inferiorly beyond the inferior border of the liver; the body, the central portion; and the neck, the tapered portion. The body and neck project superiorly.

Histology of the Liver and Gallbladder

Histologically, the liver is composed of several components (Figure 24.17a–c):

1. Hepatocytes. Hepatocytes (hepat-= liver; -cytes = cells) are the major functional cells of the liver and perform a wide array of metabolic, secretory, and endocrine functions. These are specialized epithelial cells with 5 to 12 sides that make up about 80% of the volume of the liver.

Hepatocytes form complex three-dimensional arrangements called hepatic laminae lamine. The hepatic laminae are plates of hepatocytes one cell thick bordered on either side by the endothelial-lined vascular spaces called hepatic sinusoids. The hepatic laminae are highly branched, irregular structures. Grooves in the cell membranes between neighboring hepatocytes provide spaces for bile canaliculi (described next) into which the hepatocytes secrete bile. Bile, a yellow, brownish, or olive-green liquid secreted by hepatocytes, serves as both an excretory product and a digestive secretion.

2. Bile canaliculi. Bile canaliculi kanalikuli = small canals) are small ducts between hepatocytes that collect bile produced by the hepatocytes. From bile canaliculi, bile passes into bile ductules and then bile ducts. The bile ducts merge and eventually form the larger right and left hepatic ducts, which unite and exit the liver as the common hepatic duct (see Figure 24.16). The common hepatic duct joins the cystic duct (cystic = bladder) from the gallbladder to form the bile duct. From here, bile enters the duodenum of the small intestine to participate in digestion.

3. Hepatic sinusoids. Hepatic sinusoids are highly permeable blood capillaries between rows of hepatocytes that receive oxygenated blood from branches of the hepatic artery and nutrient-rich deoxygenated blood from branches of the hepatic portal vein. Recall that the hepatic portal vein brings venous blood from the digestive canal organs and spleen into the liver. Hepatic sinusoids converge and deliver blood into a central vein.

From central veins the blood flows into the hepatic veins, which drain into the inferior vena cava (see Figure 21.29). In contrast to blood, which flows toward a central vein, bile flows in the opposite direction. Also present in the hepatic sinusoids are fixed phagocytes called stellate reticuloendothelial cells stelat retikulendothelial or hepatic macrophages, or Kupffer kupfer cells which destroy worn-out white and red blood cells, bacteria, and other foreign matter in the venous blood draining from the digestive canal.

Together, a bile duct, branch of the hepatic artery, and branch of the hepatic vein are referred to as a portal triad (tri-= three).

The hepatocytes, bile duct system, and hepatic sinusoids can be organized into an anatomical and functional unit called the hepatic acinus asinus. Each hepatic acinus is an approximately oval mass that includes portions of two neighboring hepatic lobules. The short axis of the hepatic acinus is defined by branches of the portal triad—branches of the hepatic artery, vein, and bile ducts—that run along the border of the hepatic lobules. The long axis of the acinus is defined by two imaginary curved lines, which connect the two central veins closest to the short axis (Figure 24.17d, bottom). Hepatocytes in the hepatic acinus are arranged in three zones around the short axis, with no sharp boundaries between them (Figure 24.17d). Cells in zone 1 are closest to the branches of the portal triad and the first to receive incoming oxygen, nutrients, and toxins from incoming blood.

These cells are the first ones to take up glucose and store it as glycogen after a meal and break down glycogen to glucose during fasting. They are also the first to show morphological changes following bile duct obstruction or exposure to toxic substances. Zone 1 cells are the last ones to die if circulation is impaired and the first ones to regenerate. Cells in zone 3 are farthest from branches of the portal triad and are the last to show the effects of bile obstruction or exposure to toxins, the first ones to show the effects of impaired circulation, and the last ones to regenerate. Zone 3 cells also are the first to show evidence of fat accumulation.

Cells in zone 2 have structural and functional characteristics intermediate between the cells in zones 1 and 3.

The hepatic acinus is the smallest structural and functional unit of the liver. It provides a logical description and interpretation of (1) patterns of glycogen storage and release and toxic effects, degeneration, and regeneration relative to the proximity of the acinar zones to branches of the portal triad.

The mucosa of the gallbladder consists of simple columnar epithelium arranged in mucosal folds (rugae) resembling those of the stomach. The wall of the gallbladder lacks a submucosa. The middle, muscular layer consists of smooth muscle fibers. Contraction of the smooth muscle fibers ejects the contents of the gallbladder into the cystic duct.

The gallbladder's outer coat is the visceral peritoneum. The functions of the gallbladder are to store and concentrate the bile produced by the liver (up to tenfold) until it is needed in the duodenum. In the concentration process, water and ions are absorbed by the gallbladder mucosa. Bile aids in the digestion and absorption of fats.

Figure 24.17 summary: This figure is a composite anatomical illustration consisting of diagrams, light microscopy photomicrographs, and schematic models. The content provides a multi-scale overview of liver histology, ranging from the organ level down to the cellular and functional units. It illustrates the arrangement of hepatocytes in laminae, the structure of the portal triad containing the bile duct and branches of the hepatic artery and portal vein, and the positioning of the central vein. The figure further details the hepatic acinus, dividing it into distinct zones and showing the flow of blood through hepatic sinusoids. From this content, it can be inferred that the liver is organized into highly structured repeating units that facilitate the exchange of materials between the blood and hepatocytes. The spatial organization of the hepatic acinus suggests a functional gradient across its zones, where blood flows from the portal triad toward the central vein, ensuring efficient filtration and metabolic processing.

Blood Supply of the Liver

The liver receives blood from two sources (Figure 24.18). From the hepatic artery it obtains oxygenated blood, and from the hepatic portal vein it receives deoxygenated blood containing newly absorbed nutrients, drugs, and possibly microbes and toxins from the digestive canal (see Figure 21.29). Branches of both the hepatic artery and the hepatic portal vein carry blood into hepatic sinusoids, where oxygen, most of the nutrients, and certain toxic substances are taken up by the hepatocytes. Products manufactured by the hepatocytes and nutrients needed by other cells are secreted back into the blood, which

Image summary: This figure is an illustration. It depicts a single snake coiled around a vertical staff. The image represents the Rod of Asclepius, which is a widely recognized symbol of medicine and healing.

Figure 24.18 summary: This figure is a flow chart. It illustrates the pathway of blood flow through the liver, starting from the hepatic artery and hepatic portal vein, moving through the hepatic sinusoids, central vein, hepatic vein, and inferior vena cava, before finally reaching the right atrium of the heart. The chart demonstrates that the liver receives dual blood supplies consisting of oxygenated blood and nutrient-rich deoxygenated blood, which then merge and travel through a series of venous structures to return to the heart.

Clinical Connection

Jaundice

Jaundice jaundis = yellowed) is a yellowish coloration of the sclerae (whites of the eyes), skin, and mucous membranes due to a buildup of a yellow compound called bilirubin. After bilirubin is formed from the breakdown of the heme pigment in aged red blood cells, it is transported to the liver, where it is processed and eventually excreted into bile. The three main categories of jaundice are prehepatic jaundice, due to excess production of bilirubin; hepatic jaundice, due to congenital liver disease, cirrhosis of the liver, or hepatitis; and extrahepatic jaundice, due to blockage of bile drainage by gallstones or cancer of the bowel or the pancreas.

Because the liver of a newborn functions poorly for the first week or so, many babies experience a mild form of jaundice called neonatal (physiological) jaundice that disappears as the liver matures. Usually, it is treated by exposing the infant to blue light, which converts bilirubin into substances the kidneys can excrete.

Figure 24.18 Hepatic Blood Flow: Sources, Path Through the Liver, and Return to the Heart.

then drains into the central vein and eventually passes into a hepatic vein. Because blood from the digestive canal passes through the liver as part of the hepatic portal circulation, the liver is often a site for metastasis of cancer that originates in the digestive canal.

Clinical Connection

Liver Function Tests

Liver function tests are blood tests designed to determine the presence of certain chemicals released by liver cells. These include albumin globulinase, alanine aminotransferase A.L.T, aspartate aminotransferase A.S.T, alkaline phosphatase A.L.P, gamma-glutamyl-transpeptidase G.G.T, and bilirubin. The tests are used to evaluate and monitor liver disease or damage. Common causes of elevated liver enzymes include nonsteroidal anti-inflammatory drugs, cholesterol-lowering medications, some antibiotics, alcohol, diabetes, infections (viral hepatitis and mononucleosis), gallstones, tumors of the liver, and excessive use of herbal supplements such as kava, comfrey, pennyroyal, dandelion root, skullcap, and ephedra.

Functions of the Liver and Gallbladder

Each day, hepatocytes secrete 800 to 1000 mL (about 1 qt) of bile, a yellow, brownish, or olive-green liquid. It has a pH of 7.6 to 8.6 and consists mostly of water, bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions.

The principal bile pigment is bilirubin bilirubin. The phagocytosis of aged red blood cells liberates iron, globin, and bilirubin (derived from heme) (see Figure 19.5). The iron and globin are recycled; the bilirubin is secreted into the bile and is eventually broken down in the intestine. One of its breakdown products—stercobilin (ster-ko-Bi-lin)—gives feces their normal brown color.

Bile is partially an excretory product and partially a digestive secretion. Bile salts, which are sodium salts and potassium salts of bile acids (mostly chenodeoxycholic acid and cholic acid), play a role in emulsification (e-mul'-si-fi-KÅ-shun), the breakdown of large lipid globules into a suspension of small lipid globules. The small lipid globules present a very large surface area that allows pancreatic lipase to more rapidly accomplish digestion of triglycerides. Bile salts also aid in the absorption of lipids following their digestion.

Although hepatocytes continually release bile, they increase production and secretion when the portal blood contains more bile acids; thus, as digestion and absorption continue in the small intestine, bile release increases. Between meals, after most absorption has occurred, bile flows into the gallbladder for storage because the sphincter of the hepatopancreatic ampulla (see Figure 24.16) closes off the entrance to the duodenum. The sphincter surrounds the hepatopancreatic ampulla.

In addition to secreting bile, which is needed for absorption of dietary fats, the liver performs many other vital functions:

• Carbohydrate metabolism. The liver is especially important in maintaining a normal blood glucose level. When blood glucose is low, the liver can break down glycogen to glucose and release the glucose into the bloodstream. The liver can also convert certain amino acids and lactic acid to glucose, and it can convert other sugars, such as fructose and galactose, into glucose. When blood glucose is high, as occurs just after eating a meal, the liver converts glucose to glycogen and triglycerides for storage.

• Lipid metabolism. Hepatocytes store some triglycerides; break down fatty acids to generate A.T.P; synthesize lipoproteins, which transport fatty acids, triglycerides, and cholesterol to and from body cells; synthesize cholesterol; and use cholesterol to make bile salts.

• Protein metabolism. Hepatocytes deaminate (remove the amino group, N.H 2 , from) amino acids so that the amino acids can be used for A.T.P production or converted to

Clinical Connection

Gallstones

If bile contains either insufficient bile salts or lecithin or excessive cholesterol, the cholesterol may crystallize to form gallstones. As they grow in size (from a grain of sand to a golf ball) and number, gallstones may cause minimal, intermittent, or complete obstruction to the flow of bile from the gallbladder into the duodenum. Treatment consists of using gallstone-dissolving drugs, lithotripsy (shock-wave therapy), or surgery. For people with a history of gallstones or for whom drugs or lithotripsy are not options, cholecystectomy colecystectomy —the removal of the gallbladder and its contents—is necessary. More than half a million cholecystectomies are performed each year in the United States. To prevent side effects resulting from a loss of the gallbladder, patients should make lifestyle and dietary changes, including the following: (1) limiting the intake of saturated fat; avoiding the consumption of alcoholic beverages; eating smaller amounts of food during a meal and eating five to six smaller meals per day instead of two to three larger meals; and (4) taking vitamin and mineral supplements. carbohydrates or fats. The resulting toxic ammonia (N.H 3) is then converted into the much less toxic urea, which is excreted in urine. Hepatocytes also synthesize most blood plasma proteins, such as alpha and beta globulins, albumin, prothrombin, and fibrinogen.

Image summary: This is a medical photograph. The image displays a dissected gallbladder containing several gallstones, with labels identifying both the organ and the stones. The presence of multiple solid masses within the gallbladder indicates a condition of cholelithiasis, where bile has crystallized into stones.

• Processing of drugs and hormones. The liver can detoxify substances such as alcohol and excrete drugs such as penicillin, erythromycin, and sulfonamides into bile. It can also chemically alter or excrete thyroid hormones and steroid hormones such as estrogens and aldosterone.

• Excretion of bilirubin. As previously noted, bilirubin, derived from the heme of aged red blood cells, is absorbed by the liver from the blood and secreted into bile. Most of the bilirubin in bile is metabolized in the small intestine by bacteria and eliminated in feces.

• Synthesis of bile salts. Bile salts are used in the small intestine for the emulsification and absorption of lipids.

• Storage. In addition to glycogen, the liver is a prime storage site for certain vitamins (A, B _{12} , D, E, and K) and minerals (iron and copper), which are released from the liver when needed elsewhere in the body.

• Phagocytosis. The stellate reticuloendothelial cells of the liver phagocytize aged red blood cells, white blood cells, and some bacteria.

• Activation of vitamin D. The skin, liver, and kidneys participate in synthesizing the active form of vitamin D.

The liver functions related to metabolism are discussed more fully in Chapter 25.

Checkpoint

27. Draw and label a diagram of the cell zones of an hepatic acinus.

28. Describe the pathways of blood flow into, through, and out of the liver.

29. How are the liver and gallbladder connected to the duodenum?

30. Once bile has been formed by the liver, how is it collected and transported to the gallbladder for storage?

31. Describe the major functions of the liver and gallbladder.

24.12 Small Intestine

Objectives

• Describe the location and structure of the small intestine.

• Identify the functions of the small intestine.

Most digestion and absorption of nutrients occur in a long tube of the digestive canal called the small intestine. Because of this, its structure is specially adapted for these functions. Its length alone provides a large surface area for digestion and absorption, and that area is further increased by circular folds, intestinal villi, and microvilli. The small intestine begins at the pyloric sphincter of the stomach, coils through the central and inferior part of the abdominal cavity, and eventually opens into the large intestine. It averages 2.5 centimeters (1 in.) in diameter; its length is about 5 m (16 ft) in a living person and about 6.5 m (21 ft) in a cadaver due to the loss of smooth muscle tone after death.

Anatomy of the Small Intestine

The small intestine is divided into three regions (Figure 24.19). The first part of the small intestine is the duodenum (doo'-òDÊ-num or duodenum, the shortest region, and is retroperitoneal. It starts at the pyloric sphincter of the stomach and is in the form of a C-shaped tube that extends about 25 centimeters (10 in.) until it merges with the jejunum. Duodenum means "12"; it is so named because it is about as long as the width of 12 fingers. The jejunum jejunum is the next portion and is about 1 m Figure 24.19 Anatomy of the small intestine. Regions of the small intestine are the duodenum, jejunum, and ileum.

Figure 24.19 summary: This figure is an anatomical diagram. It illustrates the human digestive system within the abdominal cavity, specifically labeling the stomach, the large intestine, and the three segments of the small intestine: the duodenum, jejunum, and ileum. The diagram shows the spatial relationship and connectivity between these organs, demonstrating how the stomach leads into the duodenum, which then continues into the jejunum and finally the ileum, all while being partially surrounded by the large intestine.

Most digestion and absorption occur in the small intestine.

Functions of the Small Intestine

1. Segmentations mix chyme with digestive juices and bring food into contact with mucosa for absorption; peristalsis propels chyme through small intestine.

2. Completes digestion of carbohydrates, proteins, and lipids; begins and completes digestion of nucleic acids.

3. Absorbs about 90% of nutrients and water that pass through digestive system.

(3 ft) long and extends to the ileum. Jejunum means “empty,” which is how it is found at death. The final and longest region of the small intestine, the ileum ileum = twisted), measures about 2 m (6 ft) and joins the large intestine at a smooth muscle sphincter called the ileal orifice.

Figure 24.20 Histology of the Small Intestine.

Histology of the Small Intestine

The wall of the small intestine is composed of the same four layers that make up most of the digestive canal: mucosa, submucosa, muscular layer, and serosa (Figure 24.20b). The Circular folds, intestinal villi, and microvilli increase the surface area of the small intestine for digestion and absorption. mucosa is composed of a layer of epithelium, lamina propria, and muscularis mucosae. The epithelial layer of the small intestinal mucosa consists of simple columnar epithelium that contains many types of cells (Figure 24.20c). Absorptive cells of the epithelium contain enzymes that digest food and possess intestinal microvilli that absorb nutrients in small intestinal chyme. Also present in the epithelium are goblet cells, which secrete mucus.

The small intestinal mucosa contains many deep crevices lined with glandular epithelium. Cells lining the crevices form the intestinal glands, or crypts of Lieberkühn (LÉ-ber-kün), and secrete intestinal juice (to be discussed shortly). Besides absorptive cells and goblet cells, the intestinal glands also contain paneth cells and enteroendocrine cells. Paneth cells secrete lysozyme, a bactericidal enzyme, and are capable of phagocytosis. Paneth cells may have a role in regulating the microbial population in the small intestine. Three types of enteroendocrine cells are found in the intestinal glands of the small intestine: S cells, C.C.K cells, and K cells, which secrete the hormones secretin sekr {E} -tin), cholecystokinin (C.C.K) (k {O} -l {E} -sis'-t {O} -K {I} N-in), and glucose-dependent insulinotropic peptide (G.I.P) insulinotr {O} -pik), respectively.

The lamina propria of the small intestinal mucosa contains areolar connective tissue and has an abundance of mucosa-associated lymphoid tissue (malt). Solitary lymphoid nodules are most numerous in the distal part of the ileum (see Figure 24.21c). Groups of lymphatic nodules referred to as aggregated lymphoid nodules, or Peyer's patches piers, are also present in the ileum. The muscularis mucosae of the small intestinal mucosa consists of smooth muscle.

The submucosa of the duodenum contains duodenal glands, also called Brunner'sglands bruners (Figure 24.21a), which secrete an alkaline mucus that helps neutralize gastric acid in the chyme. Sometimes the lymphatic tissue of the lamina propria extends through the muscularis mucosae into the submucosa. The muscular layer of the small intestine consists of two layers of smooth muscle.

The outer, thinner layer contains longitudinal fibers; the inner, thicker layer contains circular fibers. Except for a major portion of the duodenum, which is retroperitoneal, the serosa (or visceral peritoneum) completely surrounds the small intestine.

Even though the wall of the small intestine is composed of the same four basic layers as the rest of the digestive canal, special structural features of the small intestine facilitate the process of digestion and absorption. These structural features include circular folds, intestinal villi, and microvilli. Circular folds or plicae circulares plise serrulares are folds of the mucosa and submucosa (see Figures 24.19b and 24.20a). These permanent ridges, which are about 10 millimeters (0.4 in.) long, begin near the proximal portion of the duodenum and end at about the midportion of the ileum.

Some extend all the way around the circumference of the intestine; others extend only part of the way around. Circular folds enhance absorption by increasing surface area and causing the chyme to spiral, rather than move in a straight line, as it passes through the small intestine.

Also present in the small intestine are intestinal villi (= tufts of hair), which are fingerlike projections of the mucosa that are 0.5 to 1 millimeters long (see Figure 24.20b, c). The large number of intestinal villi (20 to 40 per square millimeter) vastly increases the surface area of the epithelium available for absorption and digestion and gives the intestinal mucosa a velvety appearance. Each intestinal villus (singular form) is covered by epithelium and has a core of lamina propria; embedded in the connective tissue of the lamina propria are an arteriole, a venule, a blood capillary network, and a lymphatic capillary or lacteal lacteal = milky) (see Figure 24.20c). Nutrients absorbed by the epithelial cells covering the intestinal villus pass through the wall of a blood capillary or a lymphatic capillary to enter blood plasma or lymph plasma, respectively.

Besides circular folds and intestinal villi, the small intestine also has microvilli microvilli; micro-= small), which are projections of the apical (free) membrane of the absorptive cells. Each microvillus is a 1-μm-long cylindrical, membrane-covered projection that contains a bundle of 20 to 30 actin filaments. When viewed through a light microscope, the microvilli are too small to be seen individually; instead they form a fuzzy line, called the microvillous (brush) border, extending into the lumen of the small intestine (Figure 24.21d). There are an estimated 200 million microvilli per square millimeter of small intestine. Because the microvilli greatly increase the surface area of the plasma membrane, larger amounts of digested nutrients can diffuse into absorptive cells in a given period. The microvillous border also contains several microvillous border enzymes that have digestive functions (discussed shortly).

Role of Intestinal Juice and Microvillous-Border Enzymes

About 1 to 2 liters (1 to 2 qt) of intestinal juice, a clear yellow fluid, is secreted each day. Intestinal juice contains water and mucus and is slightly alkaline (pH 7.6). The alkaline pH of intestinal juice is due to its high concentration of bicarbonate ions ( H.C.O 3 superscript minus ). Together, pancreatic and intestinal juices provide a liquid medium that aids the absorption of substances from chyme in the small intestine. The absorptive cells of the small intestine synthesize several digestive enzymes, called microvillous-border enzymes, and insert them in the plasma membrane of the microvilli.

Thus, some enzymatic digestion occurs at the surface of the absorptive cells that line the intestinal villi, rather than in the lumen exclusively, as occurs in other parts of the digestive canal. Among the microvillous-border enzymes are four carbohydrate-digesting enzymes called alpha -dextrinase, maltase, sucrase, and lactase; protein-digesting enzymes called peptidases (aminopeptidase and dipeptidase); and two types of nucleotide-digesting enzymes, nucleosidases and phosphatases. Also, as absorptive cells slough off into the lumen of the small intestine, they break apart and release enzymes that help digest nutrients in the chyme.

Mechanical Digestion in the Small Intestine

The two types of movements of the small intestine—segmentations and a type of peristalsis called migrating motility complexes—are governed mainly by the myenteric neural plexus. Segmentations are localized, mixing contractions that occur in portions of intestine distended by a large volume of chyme. Segmentations mix chyme with the digestive juices and bring the particles of food into contact with the mucosa for absorption; they do not push the intestinal contents along the digestive canal. A segmentation starts with the contractions of circular muscle fibers in a portion of the small intestine, an action that constricts the intestine into segments.

Next, muscle fibers that encircle the middle of each segment also contract, dividing each segment again. Finally, the muscle fibers that first contracted relax, and each small segment unites with an adjoining small segment so that large segments are formed again. As this sequence of events repeats, the chyme sloshes back and forth.

Segmentations occur most rapidly in the duodenum, about 12 times per minute, and progressively slow to about 8 times per minute in the ileum. This movement is similar to alternately squeezing the middle and then the ends of a capped tube of toothpaste.

After most of a meal has been absorbed, which lessens distension of the wall of the small intestine, segmentation stops and peristalsis begins. The type of peristalsis that occurs in the small intestine, termed a migrating motility complex (M.M.C), begins in the lower portion of the stomach and pushes chyme Microvilli in the small intestine contain several microvillous border enzymes that help digest nutrients. forward along a short stretch of small intestine before dying out. The M.M.C slowly migrates down the small intestine, reaching the end of the ileum in 90 to 120 minutes. Then another M.M.C begins in the stomach. Altogether, chyme remains in the small intestine for 3 to 5 hours.

Figure 24.21 summary: This figure consists of a series of micrographs, including light microscopy, scanning electron microscopy, and transmission electron microscopy images. The images display various sections and magnifications of the small intestine, specifically focusing on the duodenum and ileum. The content includes the overall wall structure of the duodenum showing the mucosa, submucosa, muscular layer, and serosa; detailed views of intestinal villi with their simple columnar epithelium, goblet cells, and absorptive cells; the presence of solitary lymphoid nodules in the ileum; and high-magnification views of the microvillous border. From these images, it can be inferred that the small intestine is organized into a complex hierarchy of folding, from large-scale villi down to microscopic microvilli, which significantly increases the surface area for absorption. Additionally, the presence of specialized lymphoid nodules indicates an integrated immune function within the intestinal wall.

Chemical Digestion in the Small Intestine

In the mouth, salivary amylase converts starch (a polysaccharide) to maltose (a disaccharide), maltotriose (a trisaccharide), and α-dextrins (short-chain, branched fragments of starch with 5 to 10 glucose units). In the stomach, pepsin converts proteins to peptides (small fragments of proteins), and lingual and gastric lipases convert some triglycerides into fatty acids, diglycerides, and monoglycerides. Thus, chyme entering the small intestine contains partially digested carbohydrates, proteins, and lipids. The completion of the digestion of carbohydrates, proteins, and lipids is a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine.

Digestion of Carbohydrates Even though the action of salivary amylase may continue in the stomach for a while, the acidic pH of the stomach destroys salivary amylase and ends its activity. Thus, only a few starches are broken down by the time chyme leaves the stomach. Those starches not already broken down into maltose, maltotriose, and alpha -dextrins are cleaved by pancreatic amylase, an enzyme in pancreatic juice that acts in the small intestine.

Although pancreatic amylase acts on both glycogen and starches, it has no effect on another polysaccharide called cellulose, an indigestible plant fiber that is commonly referred to as “roughage” as it moves through the digestive system. After amylase (either salivary or pancreatic) has split starch into smaller fragments, a brush-border enzyme called alpha -dextrinase acts on the resulting alpha -dextrins, clipping off one glucose unit at a time.

Ingested molecules of sucrose, lactose, and maltose—three disaccharides—are not acted on until they reach the small intestine. Three microvillous-border enzymes digest the disaccharides into monosaccharides. Sucrase breaks sucrose into a molecule of glucose and a molecule of fructose; lactase digests lactose into a molecule of glucose and a molecule of galactose; and maltase splits maltose and maltotriose into two or three molecules of glucose, respectively. Digestion of carbohydrates ends with the production of monosaccharides, which the digestive system is able to absorb.

Digestion of Proteins Recall that protein digestion starts in the stomach, where proteins are fragmented into peptides by the action of pepsin. Enzymes in pancreatic juice—trypsin, chymotrypsin, carboxypeptidase, and elastase—continue to break down proteins into peptides. Although all of these enzymes convert whole proteins into peptides, their actions differ somewhat because each splits peptide bonds between different amino acids. Trypsin, chymotrypsin, and elastase all cleave the peptide bond between a specific amino acid.

Lactose Intolerance

As you learned previously, absorptive cells of the small intestine produce the enzyme lactase. This enzyme breaks down the disaccharide lactose, a sugar found in milk and milk products, into the monosaccharides glucose and galactose. These breakdown products are then absorbed into the bloodstream through the small intestinal wall.

Individuals who lack the enzyme lactase have a condition called lactose intolerance, a disorder that affects about 30 to 50 million people in the United States over age 20. Undigested lactose in the chyme draws fluid into the small intestine, ultimately resulting in diarrhea. The undigested lactose in the small intestine then passes into the large intestine, where it is fermented by bacteria resulting in gas, bloating, abdominal cramps, and nausea. Signs and symptoms usually appear from 30 minutes to 2 hours after consuming milk or milk products and their severity depends on the amount of lactose that a person can tolerate.

The most common type of lactose intolerance is related to age. The decline in lactase production typically begins after age 2. Other types result from damage or diseases of the small intestine or premature births. African Americans, Hispanics/Latinos, Native Americans, and Asian Americans are more likely to develop lactose intolerance than Americans of European descent.

Lactose intolerance is diagnosed on the basis of a medical history, review of signs and symptoms, and a hydrogen breath test. When undigested lactose is fermented by bacteria in the large intestine, hydrogen is produced. The gas passes into the bloodstream to the lungs, where it is exhaled and measured.

There are several ways to control lactose intolerance. One is to adjust the diet by limiting or eliminating lactose consumption. Another is to incorporate lactose-free and lactose-reduced milk and milk products. Some individuals can also take lactose-digesting enzyme (lactase) tablets when they eat or drink milk or milk products.

Due to dietary restrictions of lactose-intolerant individuals, there is concern that calcium and vitamin D intake might not be sufficient. This can be overcome by taking calcium and vitamin D support and by including a diet that contains fish with salmon), dark green vegetable and pinto acid and its neighbor; carboxypeptidase splits off the amino acid at the carboxyl end of a peptide. Protein digestion is completed by two peptidases in the brush border: aminopeptidase and dipeptidase. Aminopeptidase cleaves off the amino acid at the amino end of a peptide. Dipeptidase splits dipeptides (two amino acids joined by a peptide bond) into single amino acids.

Digestion of Lipids The most abundant lipids in the diet are triglycerides, which consist of a molecule of glycerol bonded to three fatty acid molecules (see Figure 2.17). Enzymes that split triglycerides and phospholipids are called lipases. Recall that there are three types of lipases that can participate in lipid digestion: lingual lipase, gastric lipase, and pancreatic lipase. Although some lipid digestion occurs in the stomach through the action of lingual and gastric lipases, most occurs in the small intestine through the action of pancreatic lipase. Triglycerides are broken down by pancreatic lipase into fatty acids and monoglycerides. The liberated fatty acids can be either short-chain fatty acids (with fewer than 10 to 12 carbons) or long-chain fatty acids.

Before a large lipid globule containing triglycerides can be digested in the small intestine, it must first undergo emulsification—a process in which the large lipid globule is broken down into several small lipid globules. Recall that bile contains bile salts, the sodium salts and potassium salts of bile acids (mainly chenodeoxycholic acid and cholic acid). Bile salts are amphipathic amfapathik, which means that each bile salt has a hydrophobic (nonpolar) region and a hydrophilic (polar) region.

The amphipathic nature of bile salts allows them to emulsify a large lipid globule: The hydrophobic regions of bile salts interact with the large lipid globule, while the hydrophilic regions of bile salts interact with the watery intestinal chyme. Consequently, the large lipid globule is broken apart into several small lipid globules, each about 1 μm in diameter. The small lipid globules formed from emulsification provide a large surface area that allows pancreatic lipase to function more effectively.

Digestion of Nucleic Acids Pancreatic juice contains two nucleases: ribonuclease, which digests R.N.A, and deoxyribonuclease, which digests D.N.A. The nucleotides that result from the action of the two nucleases are further digested by microvillous-border enzymes called nucleosidases (noo'-klë-o-SÎ-das-ez) and phosphatases fosfatases into pentoses, phosphates, and nitrogenous bases. These products are absorbed via active transport.

Absorption in the Small Intestine

All of the chemical and mechanical phases of digestion from the mouth through the small intestine are directed toward changing food into forms that can pass through the absorptive epithelial cells lining the mucosa and into the underlying blood and lymphatic vessels. These forms are monosaccharides (glucose, fructose, and galactose) from carbohydrates; single amino acids, dipeptides, and tripeptides from proteins; and fatty acids, glycerol, and monoglycerides from triglycerides. Passage of these digested nutrients from the digestive canal into the blood plasma or lymph plasma is called absorption.

Absorption of materials occurs via diffusion, facilitated diffusion, osmosis, and active transport. About 90% of all absorption of nutrients occurs in the small intestine; the other 10% occurs in the stomach and large intestine. Any undigested or unabsorbed material left in the small intestine passes on to the large intestine.

Absorption of Monosaccharides All carbohydrates are absorbed as monosaccharides. The capacity of the small intestine to absorb monosaccharides is huge—an estimated 120 grams per hour. As a result, all dietary carbohydrates that are digested normally are absorbed, leaving only indigestible cellulose and fibers in the feces. Monosaccharides pass from the lumen through the apical membrane via facilitated diffusion or active transport. Fructose, a monosaccharide found in fruits, is transported via facilitated diffusion; glucose and galactose are transported into absorptive cells of the intestinal villi via secondary active transport that is coupled to the active transport of Na⁺ (Figure 24.22a). The transporter has binding sites for one glucose molecule and two sodium ions; unless all three sites are filled, neither substance is transported. Galactose competes with glucose to ride the same transporter. (Because both Na⁺ and glucose or galactose move in the same direction, this is a symporter.) Monosaccharides then move out of the absorptive cells through their basolateral surfaces via facilitated diffusion and enter the blood capillaries of the intestinal villi (Figure 24.22b).

Absorption of Amino Acids, Dipeptides, and Tripeptides Most proteins are absorbed as amino acids via active transport processes that occur mainly in the duodenum and jejunum. About half of the absorbed amino acids are present in food; the other half come from the body itself as proteins in digestive juices and dead cells that slough off the mucosal surface! Normally, 95 to 98% of the protein present in the small intestine is digested and absorbed.

Different transporters carry different types of amino acids. Some amino acids enter absorptive cells of the intestinal villi via Na ^{+} -dependent secondary active transport processes that are similar to the glucose transporter; other amino acids are actively transported by themselves. At least one symporter brings in dipeptides and tripeptides together with H ^{+} ; the peptides then are hydrolyzed to single amino acids inside the absorptive cells. Amino acids move out of the absorptive cells via diffusion and enter capillaries of the intestinal villus (Figure 24.22). Both monosaccharides and amino acids are transported in the blood capillaries to the liver by way of the hepatic portal system. If not removed by hepatocytes, they enter the general circulation.

Figure 24.22 summary: This figure consists of a detailed anatomical diagram and a corresponding schematic flowchart. The content illustrates the physiological processes by which various nutrients are absorbed from the lumen of the small intestine, through the absorptive epithelial cells of the intestinal villi, and into the circulatory systems. It details the specific transport mechanisms for monosaccharides, amino acids, and lipids, showing their paths toward either the blood capillaries or the lymphatic capillaries. The figure demonstrates that water-soluble nutrients like sugars and amino acids enter the blood capillary and travel via the hepatic portal vein to the liver, while larger lipid-based products are packaged into chylomicrons and transported through the lymphatic system toward the subclavian vein. From this, it can be inferred that the body utilizes distinct transport pathways based on the chemical properties and size of the nutrients to ensure efficient delivery to the liver and systemic circulation.

Absorption of Lipids and Bile Salts All dietary lipids are absorbed via simple diffusion. Adults absorb about 95% of the lipids present in the small intestine; due to their lower production of bile, newborn infants absorb only about 85% of lipids. As a result of their emulsification and digestion, triglycerides are mainly broken down into monoglycerides and fatty acids, which can be either short-chain fatty acids or long-chain fatty acids. Small short-chain fatty acids are hydrophobic, contain less than 10 to 12 carbon atoms, and are more water-soluble. Thus, they can dissolve in the watery intestinal chyme, pass through the absorptive cells via simple diffusion, and follow the same route taken by monosaccharides and amino acids into a blood capillary of an intestinal villus (Figure 24.22a).

Large short-chain fatty acids (with more than 10 to 12 carbon atoms), long-chain fatty acids, and monoglycerides are larger and hydrophobic, and since they are not water-soluble, Figure 24.22 Absorption of digested nutrients in the small intestine. For simplicity, all digested foods are shown in the lumen of the small intestine, even though some nutrients are digested by microvillous border enzymes.

Long-chain fatty acids and monoglycerides are absorbed into lymphatic capillaries; other products of digestion enter blood capillaries.

Q A monoglyceride may be larger than an amino acid. Why can monoglycerides be absorbed by simple diffusion, but amino acids cannot? they have difficulty being suspended in the watery environment of the intestinal chyme. Besides their role in emulsification, bile salts also help to make these large short-chain fatty acids, long-chain fatty acids, and monoglycerides more soluble. The bile salts in intestinal chyme surround them, forming tiny spheres called micelles miselz = small morsels), each of which is 2 to 10 nm in diameter and includes 20 to 50 bile salt molecules (Figure 24.22a). Micelles are formed due to the amphipathic nature of bile salts: The hydrophobic regions of bile salts interact with the large short-chain fatty acids, long-chain fatty acids, and monoglycerides, and the hydrophilic regions of bile salts interact with the watery intestinal chyme.

Once formed, the micelles move from the interior of the small intestinal lumen to the microvillous-border of the absorptive cells. At that point, the large short-chain fatty acids, long-chain fatty acids, and monoglycerides diffuse out of the micelles into the absorptive cells, leaving the micelles behind in the chyme. The micelles continually repeat this ferrying function as they move from the microvillous-border back through the chyme to the interior of the small intestinal lumen to pick up more of the large short-chain fatty acids, long-chain fatty acids, and monoglycerides.

Micelles also solubilize other large hydrophobic molecules such as fat-soluble vitamins (A, D, E, and K) and cholesterol that may be present in intestinal chyme, and aid in their absorption. These fat-soluble vitamins and cholesterol molecules are packed in the micelles along with the long-chain fatty acids and monoglycerides.

Once inside the absorptive cells, long-chain fatty acids and monoglycerides are recombined to form triglycerides, which aggregate into globules along with phospholipids and cholesterol and become coated with proteins. These large spherical masses, about 80 nm in diameter, are called chylomicrons (ki-lo-Mi-kronz). Chylomicrons leave the absorptive cell via exocytosis. Because they are so large and bulky, chylomicrons cannot enter blood capillaries—the pores in the walls of blood capillaries are too small.

Instead, chylomicrons enter lymphatic capillaries, which have much larger pores than blood capillaries. From lymphatic capillaries, chylomicrons are transported by way of lymphatic vessels to the thoracic duct and enter the blood at the junction of the left internal jugular and left subclavian veins (Figure 24.22b). The hydrophilic protein coat that surrounds each chylomicron keeps the chylomicrons suspended in blood and prevents them from sticking to each other.

Within 10 minutes after absorption, about half of the chylomicrons have already been removed from the blood as they pass through blood capillaries in the liver and adipose tissue. This removal is accomplished by an enzyme attached to the apical surface of capillary endothelial cells, called lipoprotein lipase, that breaks down triglycerides in chylomicrons and other lipoproteins into fatty acids and glycerol. The fatty acids diffuse into hepatocytes and adipose cells and combine with glycerol during resynthesis of triglycerides. Two or three hours after a meal, few chylomicrons remain in the blood.

After participating in the emulsification and absorption of lipids, most of the bile salts are reabsorbed by active transport in the final segment of the small intestine (ileum) and returned by the blood to the liver through the hepatic portal system for recycling. This cycle of bile salt secretion by hepatocytes into bile, reabsorption by the ileum, and resecretion into bile is called the enterohepatic circulation enterohepatik. Insufficient bile salts, due either to obstruction of the bile ducts or removal of the gallbladder, can result in the loss of up to 40% of dietary lipids in feces due to diminished lipid absorption. There are several benefits to including some healthy fats in the diet.

For example, fats delay gastric emptying, which helps a person feel full. Fats also enhance the feeling of fullness by triggering the release of a hormone called cholecystokinin. Finally, fats are necessary for the absorption of fat-soluble vitamins.

Absorption of Electrolytes Many of the electrolytes absorbed by the small intestine come from digestive secretions, and some are part of ingested foods and liquids. Recall that electrolytes are compounds that separate into ions in water and conduct electricity. Sodium ions are actively transported out of absorptive cells by basolateral sodium-potassium pumps (Na⁺–K⁺ A.T.P.ase's after they have moved into absorptive cells via diffusion and secondary active transport.

Thus, most of the sodium ions (Na⁺) in digestive canal secretions are reclaimed and not lost in the feces. Negatively charged bicarbonate, chloride, iodide, and nitrate ions can passively follow Na⁺ or be actively transported. Calcium ions also are absorbed actively in a process stimulated by calcitriol. Other electrolytes such as iron, potassium, magnesium, and phosphate ions also are absorbed via active transport mechanisms.

Absorption of Vitamins As you have just learned, the fat-soluble vitamins A, D, E, and K are included with ingested dietary lipids in micelles and are absorbed via simple diffusion. Most water-soluble vitamins, such as most B vitamins and vitamin C, also are absorbed via simple diffusion. Vitamin B _{12} , however, combines with intrinsic factor produced by the stomach, and the combination is absorbed in the ileum via an active transport mechanism.

Absorption of Water The total volume of fluid that enters the small intestine each day—about 9.3 liters (9.8 qt)—comes from ingestion of liquids (about 2.3 liters) and from various gastrointestinal secretions (about 7.0 liters). Figure 24.23 depicts the amounts of fluid ingested, secreted, absorbed, and excreted by the digestive canal. The small intestine absorbs about 8.3 liters of the fluid; the remainder passes into the large intestine, where most of the rest of it—about 0.9 liter—is also absorbed. Only 0.1 liter (100 mL) of water is excreted in the feces each day.

Figure 24.23 summary: This figure is an anatomical diagram illustrating the flow of fluids through the human digestive system. The content details the various sources of fluid entering the canal, including ingested liquids and secretions such as saliva, gastric juice, bile, pancreatic juice, and intestinal juice, while also showing the volumes absorbed by the small and large intestines and the amount excreted in feces. It can be inferred that the vast majority of all ingested and secreted fluids are reabsorbed by the body, with the small intestine performing the bulk of this absorption. Consequently, only a very small fraction of the total fluid processed by the digestive canal is ultimately eliminated from the body.

All water absorption in the digestive canal occurs via osmosis from the lumen of the intestines through absorptive cells and into blood capillaries. Because water can move across the intestinal mucosa in both directions, the absorption of water from the small intestine depends on the absorption of electrolytes and nutrients to maintain an osmotic balance with Which two organs of the digestive system secrete the most fluid?

Absorption of Alcohol

The intoxicating and incapacitating effects of alcohol depend on the blood alcohol level. Because it is lipid-soluble, alcohol begins to be absorbed in the stomach. However, the surface area available for absorption is much greater in the small intestine than in the stomach, so when alcohol passes into the duodenum, it is absorbed more rapidly.

Thus, the longer the alcohol remains in the stomach, the more slowly blood alcohol level rises. Because fatty acids in chyme slow gastric emptying, blood alcohol level will rise more slowly when fat-rich foods, such as pizza, hamburgers, or nachos, are consumed with alcoholic beverages. Also, the enzyme alcohol dehydrogenase, which is present in gastric mucosa cells, breaks down some of the alcohol to acetaldehyde, which is not intoxicating.

When the rate of gastric emptying is slower, proportionally more alcohol will be absorbed and converted to acetaldehyde in the stomach, and thus less alcohol will reach the bloodstream. Given identical consumption of alcohol, females often develop higher blood alcohol levels (and therefore experience greater intoxication) than males of comparable size because the activity of gastric alcohol dehydrogenase is up to 60% lower in females than in males. Asian males may also have lower levels of this gastric enzyme. the blood. The absorbed electrolytes, monosaccharides, and amino acids establish a concentration gradient for water that promotes water absorption via osmosis.

Table 24.4 summarizes the digestive activities of the pancreas, liver, gallbladder, and small intestine and Table 24.5 summarizes the digestive enzymes and their functions in the digestive system.

Table 24.4 summary: This table outlines the functional roles of various organs and tissues involved in digestion and absorption. It details how the pancreas, liver, and gallbladder provide essential secretions like pancreatic juice and bile to the duodenum, while the small intestine serves as the primary location for nutrient and water absorption. The table further specifies the specialized roles of various small intestinal cells and structures, highlighting how folds, villi, and microvilli maximize surface area for efficiency. Additionally, it describes the muscular activities of segmentation and the migrating motility complex that mix chyme and propel it through the digestive tract.

Table 24.5 summary: This table provides a comprehensive overview of the digestive enzymes categorized by their site of secretion, including saliva, gastric juice, pancreatic juice, and the microvillous border of the small intestine. It details the specific source of each enzyme and describes the biochemical transformation of substrates, such as polysaccharides, proteins, lipids, and nucleic acids, into simpler absorbable products like glucose, amino acids, fatty acids, and nucleotides.

Bariatric Surgery

Bariatric surgery bareatrik; baros = weight; iatreia = medical treatment) refers to any surgical procedure that limits the amount of food that can be ingested and absorbed. This leads to a significant weight loss in obese individuals. Among the available options are the following:

Gastric banding. In this procedure, a ring with an inner inflatable band filled with a solution is placed around the top of the stomach to create a small pouch. This makes a person feel full after eating only a small amount of food. The size of the band can be adjusted by adding or removing fluid through a small device called a port placed under the skin of the abdomen.

Sleeve gastrectomy. In this approach, about 80% of the stomach is removed to leave only a banana-shaped section called a gastric sleeve. This reduction in the size of the stomach makes a person feel full sooner.

Image summary: This is an anatomical diagram. The figure illustrates the structural changes resulting from a gastric bypass procedure, labeling the esophagus, the newly created gastric pouch, the bypassed portion of the stomach, the duodenum, and the jejunum. The diagram shows how the digestive path is rerouted to connect the small gastric pouch directly to the jejunum, effectively bypassing a large section of the stomach and the first part of the small intestine. This configuration indicates that food intake is restricted to a much smaller stomach capacity and that a portion of the intestinal tract is avoided during the initial stages of digestion.

Image summary: This is an anatomical diagram. The figure illustrates the process of gastric banding, showing the placement of an adjustable band around the upper portion of the stomach to create a small gastric pouch connected to the esophagus, with a tubing system leading to an external port. The diagram demonstrates how the procedure restricts the size of the stomach opening, which limits the amount of food that can enter the main body of the stomach, thereby inducing a feeling of fullness more quickly.

Image summary: This is an anatomical diagram. The figure illustrates a sleeve gastrectomy procedure, labeling the esophagus, the remaining gastric sleeve, the pylorus, the duodenum, and the portion of the stomach that has been removed. The diagram demonstrates that a significant vertical section of the stomach is excised, leaving behind a narrow, tube-like structure that connects the esophagus to the small intestine.

Gastric bypass. This surgery works by decreasing the amount of food that can be eaten and by decreasing the absorption of nutrients. First, the stomach is made smaller by creating a pouch with staples in the superior portion of the stomach. The resulting pouch is about the size of a walnut. Secondly, the pouch is connected to the jejunum of the small intestine. Therefore, food bypasses the remainder of the stomach and the duodenum.

32. List the regions of the small intestine and describe their functions.

33. In what ways are the mucosa and submucosa of the small intestine adapted for digestion and absorption?

34. Describe the types of movement that occur in the small intestine.

35. Explain the functions of pancreatic amylase, aminopeptidase, gastric lipase, and deoxyribonuclease.

36. What is the difference between digestion and absorption? How are the end products of carbohydrate, protein, and lipid digestion absorbed?

37. By what routes do absorbed nutrients reach the liver?

38. Describe the absorption of electrolytes, vitamins, and water by the small intestine.

Figure 24.24 Anatomy of the Large Intestine.

The regions of the large intestine are the cecum, colon, and rectum.

24.13 Large Intestine

Objective

- Describe the anatomy, histology, and functions of the large intestine.

The large intestine is the terminal portion of the digestive canal. The overall functions of the large intestine are the completion of absorption, the production of certain vitamins, the formation of feces, and the expulsion of feces from the body. The medical specialty that deals with the diagnosis and treatment of disorders of the rectum and anus is called proctology protokoloje; proct-= rectum).

Anatomy of the Large Intestine

The large intestine (Figure 24.24), which is about 1.5 m (5 ft) long and 6.5 centimeters (2.5 in.) in diameter in living humans and

Figure 24.24 summary: This figure consists of two anatomical diagrams. The first diagram provides an anterior view of the large intestine, labeling the major regions including the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal, along with associated structures like the vermiform appendix and haustra. The second diagram shows a coronal section of the anal canal, detailing the rectum, anal columns, and the internal and external anal sphincters. The figure illustrates the continuous path of the large intestine from the ileal orifice to the anus and highlights the structural differences between the involuntary internal sphincter and the voluntary external sphincter of the anal canal.

Functions of the Large Intestine

1. Haustral churning, peristalsis, and mass peristalsis drive contents of colon into rectum.

2. Bacteria in large intestine convert proteins to amino acids, break down amino acids, and

produce some B vitamins and vitamin K.

3. Absorption of some water, ions, and vitamins.

4. Formation of feces.

5. Defecation (emptying rectum).

cadavers, extends from the ileum to the anus. It is attached to the posterior abdominal wall by its mesocolon, which is a double layer of peritoneum (see Figure 24.5a). Structurally, the three major regions of the large intestine are the cecum, colon, and rectum (Figure 24.24a).

The opening from the ileum into the large intestine is called the ileal orifice (orifice of the ileal papilla). The orifice is guarded by a superior ileocolic lip; and inferior ileocecal lip, which are folds of the mucosa. The ileal orifice allows materials from the small intestine to pass into the large intestine.

Hanging inferior to the ileal orifice is the cecum, a small pouch about 6 centimeters (2.4 in.) long. Attached to the cecum is a twisted, coiled tube, measuring about 8 centimeters (3 in.) in length, called the appendix or vermiform appendix vermiform; vermiform = worm-shaped; appendix = appendage). The mesentery of the appendix, called the mesoappendix mesoapendiks, attaches the appendix to the inferior part of the mesentery of the ileum.

Clinical Connection

Appendicitis

Inflammation of the appendix, termed appendicitis, is preceded by obstruction of the lumen of the appendix by chyme, inflammation, a foreign body, a carcinoma of the cecum, stenosis, or kinking of the organ. It is characterized by high fever, elevated white blood cell count, and a neutrophil count higher than 75%. The infection that follows may result in edema and ischemia and may progress to gangrene and perforation within 24 hours. Typically, appendicitis begins with referred pain in the umbilical region of the abdomen, followed by anorexia (loss of appetite for food), nausea, and vomiting. After several hours the pain localizes in the right lower quadrant R.L.Q and is continuous, dull or severe, and intensified by coughing, sneezing, or body movements.

Early appendectomy (removal of the appendix) is recommended because it is safer to operate than to risk rupture, peritonitis, and gangrene. Although they required major abdominal surgery in the past, today appendectomies are usually performed laparoscopically.

The open end of the cecum merges with a long tube called the colon (= food passage), which is divided into ascending, transverse, descending, and sigmoid portions. Both the ascending and descending colon are retroperitoneal; the transverse and sigmoid colon are not. True to its name, the ascending colon ascends on the right side of the abdomen, reaches the inferior surface of the liver, and turns abruptly to the left to form the right colic (hepatic) flexure.

The colon continues across the abdomen to the left side as the transverse colon. It curves beneath the inferior end of the spleen on the left side as the left colic (splenic) flexure and passes inferiorly to the level of the iliac crest as the descending colon. The sigmoid colon (sigm-= S-shaped) begins near the left iliac crest, projects medially to the midline, and terminates as the rectum at about the level of the third sacral vertebra.

The rectum is about 15 centimeters (6 in.) in length and lies anterior to the sacrum and coccyx. The large intestine terminates as it joins the anal canal. The anal canal is the terminal 2 to 3 centimeters (1 in.) of the digestive canal that opens to the outside of the body (Figure 24.24b). The mucous membrane of the anal canal is arranged in longitudinal folds called anal columns that contain a network of arteries and veins. The opening of the anal canal to the exterior, called the anus, is guarded by an internal anal sphincter of smooth muscle (involuntary) and an external anal sphincter of skeletal muscle (voluntary). Normally these sphincters keep the anus closed except during the elimination of feces.

Histology of the Large Intestine

The wall of the large intestine contains the typical four layers found in the rest of the digestive canal: mucosa, submucosa, muscular layer, and serosa. The mucosa consists of simple columnar epithelium, lamina propria (areolar connective tissue), and muscularis mucosae (smooth muscle) (Figure 24.25a). The epithelium contains mostly absorptive and goblet cells (Figure 24.25b, d). The absorptive cells function primarily in water absorption; the goblet cells secrete mucus that lubricates the passage of the colonic contents. Both absorptive and goblet cells are located in long, straight, tubular intestinal glands (crypts of Lieberkühn) that extend the full thickness of the mucosa.

Solitary lymphoid nodules are also found in the lamina propria of the mucosa and may extend through the muscularis mucosae into the submucosa. Compared to the small intestine, the mucosa of the large intestine does not have as many structural adaptations that increase surface area. There are no circular folds or intestinal villi; however, microvilli are present on the absorptive cells.

Consequently, much more absorption occurs in the small intestine than in the large intestine.

The submucosa of the large intestine consists of areolar connective tissue. The muscular layer consists of an external layer of longitudinal smooth muscle and an internal layer of circular smooth muscle. Unlike other parts of the digestive canal, portions of the longitudinal muscles are thickened, forming three conspicuous bands called the teniae coli (Tene-e Ko-li; teniae = flat bands) that run most of the length of the large intestine (see Figure 24.24a). The teniae coli are separated by portions of the wall with less or no longitudinal muscle.

Tonic contractions of the bands gather the colon into a series of pouches called haustra hawstra = shaped like pouches; singular is haustrum), which give the colon a puckered appearance. A single layer of circular smooth muscle lies between teniae coli. The serosa of the large intestine is part

Clinical Connection

Polyps in the Colon

Polyps in the colon are generally slow-developing benign growths that arise from the mucosa of the large intestine. Often, they do not cause symptoms. If symptoms do occur, they include diarrhea, blood in the feces, and mucus discharged from the anus. The polyps are removed by colonoscopy or surgery because some of them may become cancerous.

Intestinal glands formed by simple columnar epithelial cells and goblet cells extend the full thickness of the mucosa. of the visceral peritoneum. Small pouches of visceral peritoneum filled with fat are attached to teniae coli and are called omental (fatty) appendices.

Image summary: This figure is a light micrograph. It displays a cross section of the wall of the large intestine, highlighting the distinct histological layers including the mucosa, submucosa, muscular layer, and serosa. Specific structures within these layers are identified, such as the lumen of the large intestine, lamina propria, intestinal glands, lymphoid nodules, and the muscularis mucosae. The image illustrates the organized stratification of the intestinal wall, showing that the mucosa is the innermost layer containing glandular structures and immune nodules, followed by the connective tissue of the submucosa, a thick layer of smooth muscle, and an outer serosal covering.

Mechanical Digestion in the Large Intestine

The passage of chyme from the ileum into the cecum is through the ileal orifice. Normally, the orifice remains partially closed so that the passage of chyme into the cecum usually occurs slowly. Immediately after a meal, a gastroileal reflex gastroileal intensifies peristalsis in the ileum and forces any chyme into the cecum. Whenever the cecum is distended, the degree of contraction widens the ileal orifice.

Movements of the colon begin when substances pass the ileal orifice. Because chyme moves through the small intestine at a fairly constant rate, the time required for a meal to pass into the colon is determined by gastric emptying time. As food passes through the ileal orifice, it fills the cecum and accumulates in the ascending colon.

One movement characteristic of the large intestine is haustral churning. In this process, the haustra remain relaxed and become distended while they fill up. When the distension reaches a certain point, the walls contract and squeeze the contents into the next haustrum. Peristalsis also occurs, although at a slower rate (3 to 12 contractions per minute) than in more proximal portions of the tract.

A final type of movement is mass peristalsis, a strong peristaltic wave that begins at about the middle of the transverse colon and quickly drives the contents of the colon into the rectum. Because food in the stomach initiates this gastrocollic reflex in the colon, mass peristalsis usually takes place three or four times a day, during or immediately after a meal.

Figure d summary: This is a light micrograph. The image displays a detailed cross-section of the large intestine mucosa, highlighting structural components such as the intestinal gland and its opening into the lumen. Key cellular elements identified include absorptive cells and goblet cells, alongside the supporting lamina propria tissue. The arrangement shows a deep tubular gland lined with a mixture of cell types that lead toward the intestinal cavity. Based on the cellular distribution, it can be inferred that the large intestine mucosa is specialized for secretion and absorption, with a high density of goblet cells indicating a significant role in mucus production to facilitate the movement of waste.

Chemical Digestion in the Large Intestine

The final stage of digestion occurs in the colon through the activity of bacteria that inhabit the lumen. Mucus is secreted by the glands of the large intestine, but no enzymes are secreted. Chyme is prepared for elimination by the action of bacteria, which ferment any remaining carbohydrates and release hydrogen, carbon dioxide, and methane gases.

These gases contribute to flatus (gas) in the colon, termed flatulence when it is excessive. Bacteria also convert any remaining proteins to amino acids and break down the amino acids into simpler substances: indole, skatole, hydrogen sulfide, and fatty acids. Some of the indole and skatole is eliminated in the feces and contributes to their odor; the rest is absorbed and transported to the liver, where these compounds are converted to less toxic compounds and excreted in the urine. Bacteria also decompose bilirubin to simpler pigments, including stercobilin, which gives feces their brown color.

Bacterial products that are absorbed in the colon include several vitamins needed for normal metabolism, among them some B vitamins and vitamin K.

Absorption and Feces Formation in the Large Intestine

By the time chyme has remained in the large intestine 3 to 10 hours, it has become solid or semisolid because of water absorption and is now called feces. Chemically, feces consist of water, inorganic salts, sloughed-off epithelial cells from the mucosa of the digestive canal, bacteria, products of bacterial decomposition, unabsorbed digested materials, and indigestible parts of food.

Although 90% of all water absorption occurs in the small intestine, the large intestine absorbs enough to make it an important organ in maintaining the body's water balance. Of the 0.5 to 1.0 liter of water that enters the large intestine, all but about 100 to 200 mL is normally absorbed via osmosis. The large intestine also absorbs ions, including sodium and chloride, and some vitamins.

Clinical Connection

Occult Blood

The term occult blood refers to blood that is hidden; it is not detectable by the human eye. The main diagnostic value of occult blood testing is to screen for colorectal cancer. Two substances often examined for occult blood are feces and urine. Several types of products are available for at-home testing for hidden blood in feces. The tests are based on color changes when reagents are added to feces. The presence of occult blood in urine may be detected at home by using dip-and-read reagent strips.

The Defecation Reflex

Mass peristaltic movements push fecal material from the sigmoid colon into the rectum. The resulting distension of the rectal wall stimulates stretch receptors, which initiates a defecation reflex that results in defecation, the elimination of feces from the rectum through the anus. The defecation reflex occurs as follows: In response to distension of the rectal wall, the receptors send sensory nerve impulses to the sacral spinal cord.

Motor impulses from the cord travel along parasympathetic nerves back to the descending colon, sigmoid colon, rectum, and anus. The resulting contraction of the longitudinal rectal muscles shortens the rectum, thereby increasing the pressure within it. This pressure, along with voluntary contractions of the diaphragm and abdominal muscles, plus parasympathetic stimulation, opens the internal anal sphincter.

The external anal sphincter is voluntarily controlled. If it is voluntarily relaxed, defecation occurs and the feces are expelled through the anus; if it is voluntarily constricted, defecation can be postponed. Voluntary contractions of the diaphragm and abdominal muscles aid defecation by increasing the pressure within the abdomen, which pushes the walls of the sigmoid colon and rectum inward. If defecation does not occur, the feces back up into the sigmoid colon until the next wave of mass peristalsis stimulates the stretch receptors, again creating the urge to defecate. In infants, the defecation reflex causes automatic emptying of the rectum because voluntary control of the external anal sphincter has not yet developed.

The amount of bowel movements that a person has over a given period of time depends on various factors such as diet, health, and stress. The normal range of bowel activity varies

Dietary Fiber

Dietary fiber consists of indigestible plant carbohydrates—such as cellulose, lignin, and pectin—found in fruits, vegetables, grains, and beans. Insoluble fiber, which does not dissolve in water, includes the woody or structural parts of plants such as the skins of fruits and vegetables and the bran coating around wheat and corn kernels. Insoluble fiber passes through the digestive canal largely unchanged but speeds up the passage of material through the tract.

Soluble fiber, which does dissolve in water, forms a gel that slows the passage of material through the digestive canal. It is found in abundance in beans, oats, barley, broccoli, prunes, apples, and citrus fruits.

People who choose a fiber-rich diet may reduce their risk of developing obesity, diabetes, atherosclerosis, gallstones, hemorrhoids, diverticulitis, appendicitis, and colorectal cancer. Soluble fiber also may help lower blood cholesterol. The liver normally converts cholesterol to bile salts, which are released into the small intestine to help fat digestion.

Having accomplished their task, the bile salts are reabsorbed by the small intestine and recycled back to the liver. Since soluble fiber binds to bile salts to prevent their reabsorption, the liver makes more bile salts to replace those lost in feces. Thus, the liver uses more cholesterol to make more bile salts and blood cholesterol level is lowered. from two or three bowel movements per day to three or four bowel movements per week.

Diarrhea (di-a-Re-a; dia-= through; -rrhea = flow) is an increase in the frequency, volume, and fluid content of the feces caused by increased motility of and decreased absorption by the intestines. When chyme passes too quickly through the small intestine and feces pass too quickly through the large intestine, there is not enough time for absorption. Frequent diarrhea can result in dehydration and electrolyte imbalances. Excessive motility may be caused by lactose intolerance, stress, and microbes that irritate the digestive canal mucosa.

Constipation (kon-sti-PÄ-shun; con-= together; -stip-= to press) refers to infrequent or difficult defecation caused by decreased motility of the intestines. Because the feces remain in the colon for prolonged periods, excessive water absorption occurs, and the feces become dry and hard. Constipation may be caused by poor habits (delaying defecation), spasms of the colon, insufficient fiber in the diet, inadequate fluid intake, lack of exercise, emotional stress, and certain drugs.

A common treatment is a mild laxative, such as milk of magnesia, which induces defecation. However, many physicians maintain that laxatives are habit-forming, and that adding fiber to the diet, increasing the amount of exercise, and increasing fluid intake are safer ways of controlling this common problem.

Table 24.6 summarizes the digestive activities in the large intestine, and Table 24.7 summarizes the functions of all digestive system organs.

Table 24.6 summary: This table outlines the various digestive activities of the large intestine, mapping specific anatomical structures to their associated activities and functions. It details how the lumen supports bacterial breakdown of nutrients and vitamin synthesis, while the mucosa handles lubrication and the absorption of water and solutes. Additionally, it describes the diverse roles of the muscular layer in moving waste through the colon via different types of contractions and the final process of elimination.

Table 24.7 summary: This table outlines the primary organs of the digestive system and their respective roles in processing food. It details how the mouth and its components handle initial mechanical breakdown and chemical initiation, the pharynx and esophagus facilitate transport, and the stomach manages mixing and protein digestion. It further describes the critical roles of the pancreas, liver, and gallbladder in providing secretions for nutrient breakdown in the small intestine, where the majority of absorption occurs, and concludes with the large intestine's role in water absorption and waste elimination.

39. What are the major regions of the large intestine?

40. How does the muscular layer of the large intestine differ from that of the rest of the digestive canal? What are haustra?

41. Describe the mechanical movements that occur in the large intestine.

42. What is defecation, and how does it occur?

43. What activities occur in the large intestine to change its contents into feces?

24.14 Phases of Digestion

Objectives

• Explain the three phases of digestion.

• Describe the major hormones regulating digestive activities.

Digestive activities occur in three overlapping phases: the cephalic phase, the gastric phase, and the intestinal phase.

Cephalic Phase

During the cephalic phase of digestion, the smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem. The brain stem then activates the facial (7), glossopharyngeal (9), and vagus (10) nerves. The facial and glossopharyngeal nerves stimulate the salivary glands to secrete saliva, while the vagus nerves stimulate the gastric glands to secrete gastric juice. The purpose of the cephalic phase of digestion is to prepare the mouth and stomach for food that is about to be eaten.

Gastric Phase

Once food reaches the stomach, the gastric phase of digestion begins. Neural and hormonal mechanisms regulate the gastric phase of digestion to promote gastric secretion and gastric motility.

• Neural regulation. Food of any kind distends the stomach and stimulates stretch receptors in its walls. Chemoreceptors in the stomach monitor the pH of the stomach chyme. When the stomach walls are distended or pH increases because proteins have entered the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors are activated, and a neural negative feedback loop is set in motion (Figure 24.26). From the stretch receptors and chemoreceptors, nerve impulses propagate to the submucosal neural plexus, where they activate parasympathetic and enteric neurons.

Figure 24.26 summary: This figure is a flowchart depicting a biological feedback loop. It illustrates the physiological process triggered by food entering the stomach, starting with the disruption of homeostasis through increased pH and stomach wall distension. The process follows a pathway from receptors that detect these changes, to a control center in the submucosal neural plexus, and finally to effectors including parietal cells and smooth muscles. The resulting response involves the secretion of hydrochloric acid and vigorous muscle contractions to increase acidity and mix stomach contents. The loop concludes with a negative feedback mechanism that returns the system to its pre-eating state once homeostasis is restored.

The resulting nerve impulses cause waves of peristalsis and continue to stimulate the flow of gastric juice from gastric glands. The peristaltic waves mix the food with gastric juice; when the waves become strong enough, a small quantity of chyme undergoes gastric emptying into the duodenum. The pH of the stomach chyme decreases (becomes more acidic) and the distension of the stomach walls lessens because chyme has passed into the small intestine, suppressing secretion of gastric juice.

• Hormonal regulation. Gastric secretion during the gastric phase is also regulated by the hormone gastrin. Gastrin is released from the G cells of the gastric glands in response to several stimuli: distension of the stomach by chyme, partially digested proteins in chyme, the high pH of chyme due to the presence of food in the stomach, caffeine in gastric chyme, and acetylcholine released from parasympathetic neurons. Once it is released, gastrin enters the bloodstream, makes a round-trip through the body, and finally reaches its target organs in the digestive system.

Gastrin stimulates gastric glands to secrete large amounts of gastric juice. It also strengthens the contraction of the lower esophageal sphincter to prevent reflux of acid chyme into the esophagus, increases motility of the stomach, and relaxes the pyloric sphincter, which promotes gastric emptying. Gastrin secretion is inhibited when the pH of gastric juice drops below 2.0 and is stimulated when the pH rises. This negative feedback mechanism helps provide an optimal low pH for the functioning of pepsin, the killing of microbes, and the denaturing of proteins in the stomach.

Figure 24.26 Neural negative feedback regulation of the pH of gastric juice and gastric motility during the gastric phase of digestion.

Food entering the stomach stimulates secretion of gastric juice and causes vigorous waves of peristalsis.

Intestinal Phase

The intestinal phase of digestion begins once food enters the small intestine. In contrast to reflexes initiated during the cephalic and gastric phases, which stimulate stomach secretory activity and motility, those occurring during the intestinal phase have inhibitory effects that slow the exit of chyme from the stomach. This prevents the duodenum from being overloaded with more chyme than it can handle.

In addition, responses occurring during the intestinal phase promote the continued digestion of foods that have reached the small intestine. These activities of the intestinal phase of digestion are regulated by neural and hormonal mechanisms.

• Neural regulation. Distension of the duodenum by the presence of chyme causes the enterogastric reflex enterogastrik. Stretch receptors in the duodenal wall send nerve impulses to the medulla oblongata, where they inhibit parasympathetic stimulation and stimulate the sympathetic nerves to the stomach. As a result, gastric motility is inhibited and there is an increase in the contraction of the pyloric sphincter, which decreases gastric emptying.

• Hormonal regulation. The intestinal phase of digestion is mediated by two major hormones secreted by the small intestine: cholecystokinin and secretin. Cholecystokinin (C.C.K) is secreted by the C.C.K cells of intestinal glands in the small intestine in response to chyme containing amino acids from partially digested proteins and fatty acids from partially digested triglycerides. C.C.K stimulates secretion of pancreatic juice that is rich in digestive enzymes. It also causes contraction of the wall of the gallbladder, which squeezes stored bile out of the gallbladder into the cystic duct and through the bile duct. In addition, C.C.K causes relaxation of the sphincter of the hepatopancreatic ampulla, which allows pancreatic juice and bile to flow into the duodenum. C.C.K also slows gastric emptying by promoting contraction of the pyloric sphincter, produces satiety (a feeling of fullness) by acting on the hypothalamus in the brain, promotes normal growth and maintenance of the pancreas, and enhances the effects of secretin. Acidic chyme entering the duodenum stimulates the release of secretin from the S cells of the intestinal glands in the small intestine.

In turn, secretin stimulates the flow of pancreatic juice that is rich in bicarbonate (H.C.O₃⁻) ions to buffer the acidic chyme that enters the duodenum from the stomach. In addition to this major effect, secretin inhibits secretion of gastric juice, promotes normal growth and maintenance of the pancreas, and enhances the effects of C.C.K. Overall, secretin causes buffering of acid in chyme that reaches the duodenum and slows production of acid in the stomach.

Table 24.8 summarizes the major hormones that control digestion.

Table 24.8 summary: This table outlines the primary hormones regulating digestion, detailing their specific triggers, sites of origin, and physiological impacts. Gastrin is stimulated by stomach distension and proteins to increase gastric juice and motility. Secretin is triggered by acidic chyme in the duodenum to promote bicarbonate-rich secretions from the pancreas and bile. Cholecystokinin is released in response to proteins and fats in the small intestine to stimulate enzyme-rich pancreatic juice, trigger bile release from the gallbladder, and signal satiety.

Other Hormones of the Digestive System

Besides gastrin, C.C.K, and secretin, there are many other hormones of the digestive system. For example, ghrelin, which is secreted by the stomach, plays a role in increasing appetite. Glucose-dependent insulinotropic peptide (G.I.P) and glucagon-like peptide (G.L.P), which are secreted by the small intestine in response to the presence of food, stimulate the release of insulin from the pancreas, thereby increasing the blood glucose concentration. G.I.P and G.L.P are collectively referred to as incretins; they provide a type of feedforward control that anticipates the increase in blood glucose occurring after a typical meal. At least 10 other so-called gut hormones are secreted by and have effects on the digestive canal.

They include motilin, substance P, and bombesin, which stimulate motility of the intestines; vasoactive intestinal polypeptide V.I.P, which stimulates secretion of ions and water by the intestines and inhibits gastric acid secretion; gastrin-releasing peptide, which stimulates release of gastrin; and somatostatin, which inhibits gastrin release. Some of these hormones are thought to act as local hormones (paracrines), whereas others are secreted into the blood or even into the lumen of the digestive canal. The physiological roles of these and other gut hormones are still under investigation.

Checkpoint

Image summary: This figure is a medical illustration. It depicts a human fetus positioned within a womb, connected to the uterine wall via an umbilical cord. The illustration shows the fetus in a curled fetal position, highlighting the developmental stage of the embryo within the protective environment of the uterus.

Objective

24.15 Development of the Digestive System

• Describe the development of the digestive system.

During the fourth week of development, the cells of the endoderm form a cavity called the primitive gut, the forerunner of the digestive canal (see Figure 29.12b). Soon afterward the mesoderm forms and splits into two layers (somatic and splanchnic), as shown in Figure 29.9d. The splanchnic mesoderm associates with the endoderm of the primitive gut; as a result, the primitive gut has a double-layered wall. The endodermal layer gives rise to the epithelial lining and glands of most of the digestive canal; the mesodermal layer produces the smooth muscle and connective tissue of the tract.

The primitive gut elongates and differentiates into an anterior foregut, an intermediate midgut, and a posterior hindgut (see Figure 29.12c). Until the fifth week of development, the midgut opens into the yolk sac; after that time, the yolk sac constricts and detaches from the midgut, and the midgut seals.

In the region of the foregut, a depression consisting of ectoderm, the stomodeum (sto-mo-De-um), appears (see Figure 29.12d), which develops into the oral cavity. The oropharyngeal membrane orofarinjeal is a depression of fused ectoderm and endoderm on the surface of the embryo that separates the foregut from the stomodeum. The membrane ruptures during the fourth week of development, so that the foregut is continuous with the outside of the embryo through the oral cavity.

Another depression consisting of ectoderm, the proctodeum (prok-to-De-um), forms in the hindgut and goes on to develop into the anus (see Figure 29.12d). The cloacal membrane (klo-A-kul) is a fused membrane of ectoderm and endoderm that separates the hindgut from the proctodeum. After it ruptures during the seventh week, the hindgut is continuous with the outside of the embryo through the anus. Thus, the digestive canal forms a continuous tube from mouth to anus.

The foregut develops into the pharynx, esophagus, stomach, and part of the duodenum. The midgut is transformed into the remainder of the duodenum, the jejunum, the ileum, and portions of the large intestine (cecum, appendix, ascending colon, and most of the transverse colon). The hindgut develops into the remainder of the large intestine, except for a portion of the anal canal that is derived from the proctodeum.

As development progresses, the endoderm at various places along the foregut develops into hollow buds that grow into the mesoderm. These buds will develop into the salivary glands, liver, gallbladder, and pancreas. Each of these organs retains a connection with the digestive canal through ducts.

Checkpoint

24.16 Aging and the Digestive System

Objective

• Describe the effects of aging on the digestive system.

Overall changes of the digestive system associated with aging include decreased secretory mechanisms, decreased motility of the digestive canal organs, loss of strength and tone of the muscular layer and its supporting structures, changes in neurosensory feedback regarding enzyme and hormone release, and diminished response to pain and internal sensations. In the upper portion of the digestive canal, common changes include reduced sensitivity to mouth irritations and sores, loss of taste, periodontal disease, difficulty in swallowing, hiatal hernia, gastritis, and peptic ulcer disease. Changes that may appear in the small intestine include duodenal ulcers, malabsorption, and maldigestion. Other pathologies that increase in incidence with age are appendicitis, gallbladder problems, jaundice, cirrhosis, and acute pancreatitis. Large intestinal changes such as constipation, hemorrhoids, and diverticular disease may also occur. Cancer of the colon or rectum is quite common, as are bowel obstructions and impactions.

Checkpoint 49. What are the general effects of aging on the digestive system?

Now that our exploration of the digestive system is complete, you can appreciate the many ways that this system contributes to homeostasis of other body systems by examining Focus on Homeostasis: Contributions of the Digestive System. Next, in Chapter 25, you will discover how the nutrients absorbed by the digestive canal enter into metabolic reactions in the body tissues.

Disorders: Homeostatic Imbalances

Dental Caries

Dental caries karez, or tooth decay, involves a gradual demineralization (softening) of the enamel and dentin. If untreated, microorganisms may invade the pulp, causing inflammation and infection, with subsequent death of the dental pulp and abscess of the alveolar bone surrounding the root's apex, requiring root canal therapy (see Section 24.5).

Dental caries begin when bacteria, acting on sugars, produce acids that demineralize the enamel. Dextran, a sticky polysaccharide produced from sucrose, causes the bacteria to stick to the teeth. Masses of bacterial cells, dextran, and other debris adhering to teeth constitute dental plaque plak. Saliva cannot reach the tooth surface to buffer the acid because the plaque covers the teeth.

Brushing the teeth after eating removes the plaque from flat surfaces before the bacteria can produce acids. Dentists also recommend that the plaque between the teeth be removed every 24 hours with dental floss.

Periodontal Disease

Periodontal disease is a collective term for a variety of conditions characterized by inflammation and degeneration of the gingivae, alveolar bone, periodontium, and cement. In one such condition, called pyorrhea, initial symptoms include enlargement and inflammation of the soft tissue and bleeding of the gums. Without treatment, the soft tissue may deteriorate and the alveolar bone may be resorbed, causing loosening of the teeth and recession of the gums. Periodontal diseases are often caused by poor oral hygiene; by local irritants, such as bacteria, impacted food, and cigarette smoke; or by a poor "bite."

Peptic Ulcer Disease

In the United States, 5 to 10% of the population develops peptic ulcer disease (P.U.D). An ulcer is a craterlike lesion in a membrane; ulcers that develop in areas of the digestive canal exposed to acidic gastric juice are called peptic ulcers. The most common complication of peptic ulcers is bleeding, which can lead to anemia if enough blood is lost. In acute cases, peptic ulcers can lead to shock and death. Three distinct causes of P.U.D are recognized: (1) the bacterium Helicobacter pylori helicobakter pi-Lo-re); (2) nonsteroidal anti-inflammatory drugs N.S.A.I.D's such as aspirin; and (3) hypersecretion of H.C.L, as occurs in Zollinger-Ellison syndrome zolinjer elison, a gastrin-producing tumor, usually of the pancreas.

Helicobacter pylori (previously named Campylobacter pylori) is the most frequent cause of P.U.D. The bacterium produces an enzyme called urease, which splits urea into ammonia and carbon dioxide. While shielding the bacterium from the acidity of the stomach, the ammonia also damages the protective mucous layer of the stomach and the underlying gastric cells. The microbe also produces catalase, an enzyme that may protect H. pylori from phagocytosis by neutrophils, plus several adhesion proteins that allow the bacterium to attach itself to gastric cells.

Several therapeutic approaches are helpful in the treatment of P.U.D. Cigarette smoke, alcohol, caffeine, and N.S.A.I.D's should be avoided because they can impair mucosal defensive mechanisms, which increases mucosal susceptibility to the damaging effects of H.C.L. In cases associated with H. pylori, treatment with an antibiotic drug often resolves the problem. Oral antacids such as Tums® or Maalox® can help temporarily by buffering gastric acid. When hypersecretion of H.C.L is the cause of P.U.D, H₂ blockers (such as Tagamet®) or proton pump inhibitors such as omeprazole (Prilosec®), which block secretion of H⁺ from parietal cells, may be used.

Image summary: This is an anatomical diagram. The figure illustrates the human digestive system, specifically focusing on the stomach and the duodenum, and identifies the common sites where peptic ulcers occur. The diagram indicates that peptic ulcers can develop in both the stomach and the duodenum, showing a lesion in each of these two regions.

Image summary: This figure is an anatomical diagram. It illustrates a cross-section of the stomach wall, identifying the various tissue layers including the mucosa, submucosa, muscular layer, and serosa, while highlighting the presence of a peptic ulcer. The diagram demonstrates that a peptic ulcer is a deep lesion that penetrates through the mucosal lining and extends into the deeper layers of the stomach wall.

Image summary: This figure is an endoscopic medical photograph. It displays the interior lining of a digestive organ, specifically highlighting the mucosal surface and a localized lesion identified as a peptic ulcer. The image demonstrates a clear contrast between the surrounding healthy tissue and the eroded area of the ulcer, indicating a focal point of tissue damage within the gastrointestinal lining.

Image summary: This is a scanning electron micrograph. The image displays several spiral-shaped Helicobacter pylori bacteria adhering to the surface of the gastric mucosa. The visual evidence shows that these bacteria are capable of colonizing and attaching themselves to the lining of the stomach.

Diverticular Disease

Diverticular disease divertikular refers to saclike outpouchings of the wall of the colon (especially the sigmoid colon), termed diverticula. They occur in places where the muscular layer has weakened and may become inflamed. Development of diverticula is known as diverticulosis (di–ver-tik'–u-Lo-sis). Many people who develop diverticulosis have no symptoms and experience no complications. Of those people known to have diverticulosis, 10 to 25% eventually develop an inflammation known as diverticulitis (di'–ver-tik-u-Li-tis). This condition may be characterized by pain, either constipation or increased frequency of defecation, nausea, vomiting, and low-grade fever. Because diets low in fiber contribute to development of diverticulitis, patients who change to high-fiber diets show marked relief of symptoms.

In severe cases, affected portions of the colon may require surgical removal. If diverticula rupture, the release of bacteria into the abdominal cavity can cause peritonitis.

Colorectal Cancer

Colorectal cancer is among the deadliest of malignancies, ranking second to lung cancer in males and third after lung cancer and breast cancer in females. Genetics plays a very important role; an inherited predisposition contributes to more than half of all cases of colorectal cancer. Intake of alcohol and diets high in animal fat and protein are associated with increased risk of colorectal cancer; dietary fiber, retinoids, calcium, and selenium may be protective. Signs and symptoms of colorectal cancer include diarrhea, constipation, cramping, abdominal pain, and rectal bleeding, either visible or occult (hidden in feces). Precancerous growths on the mucosal surface, called polyps, also increase the risk of developing colorectal cancer. Screening for colorectal cancer includes testing for blood in the feces, digital rectal examination, sigmoidoscopy, colonoscopy, and barium enema. Tumors may be removed endoscopically or surgically.

Hepatitis

Hepatitis is an inflammation of the liver that can be caused by viruses, drugs, and chemicals, including alcohol. Clinically, several types of viral hepatitis are recognized.

Hepatitis A (infectious hepatitis) is caused by the hepatitis A virus H.A.V and is spread via fecal contamination of objects such as food, clothing, toys, and eating utensils (fecal-oral route). It is generally a mild disease of children and young adults characterized by loss of appetite, malaise, nausea, diarrhea, fever, and chills. Eventually, jaundice appears.

This type of hepatitis does not cause lasting liver damage. Most people recover in 4 to 6 weeks. A vaccine is available.

Hepatitis B is caused by the hepatitis B virus (H.B.V) and is spread primarily by sexual contact and contaminated syringes and transfusion equipment. It can also be spread via saliva and tears. Hepatitis B virus can be present for years or even a lifetime, and it can produce cirrhosis and possibly cancer of the liver.

Individuals who harbor the active hepatitis B virus also become carriers. A vaccine is available.

Hepatitis C, caused by the hepatitis C virus H.C.V, is clinically similar to hepatitis B. Hepatitis C can cause cirrhosis and possibly liver cancer. In developed nations, donated blood is screened for the presence of hepatitis B and C.

Hepatitis D is caused by the hepatitis D virus H.D.V. It is transmitted like hepatitis B, and in fact a person must have been co-infected with hepatitis B before contracting hepatitis D. Hepatitis D results in severe liver damage and has a higher fatality rate than infection with hepatitis B virus alone. H.B.V vaccine is protective.

Hepatitis E is caused by the hepatitis E virus and is spread like hepatitis A. Although it does not cause chronic liver disease, hepatitis E virus has a very high mortality rate among pregnant women.

Focus on Homeostasis

Contributions of the Digestive System for All Body Systems

• The digestive system breaks down dietary nutrients into forms that can be absorbed and used by body cells for producing A.T.P and building body tissues

• Absorbs water, minerals, and vitamins needed for growth and function of body tissues

• Eliminates wastes from body tissues in feces

Image summary: This is an anatomical illustration. The figure depicts the human digestive system situated within a full body outline, showing the path from the mouth, through the esophagus, into the stomach and liver, and ending with the small and large intestines. The illustration demonstrates the continuous nature of the gastrointestinal tract and the relative spatial arrangement of the primary digestive organs within the abdominal cavity.

Image summary: This figure is an anatomical illustration. It depicts a full-body human figure composed of numerous horizontal lines, creating a layered or sliced effect across the entire anatomy. The illustration suggests a cross-sectional or tomographic representation of the human body, indicating a method of visualizing internal structures through sequential layering.

Image summary: This figure consists of a series of anatomical diagrams. The content displays three full-body human figures, each illustrating a different biological system: the skeletal system, the muscular system, and the nervous system. It can be inferred that the figure is intended to provide a comparative overview of how these distinct internal systems are distributed and structured throughout the entire human body.

Image summary: This figure is an anatomical illustration. It depicts a full-body human figure with specific internal organs highlighted in the abdominal region. The illustration indicates that certain internal structures are the primary focus of the diagram compared to the rest of the body.

Image summary: This is an anatomical diagram. The figure depicts the human circulatory system, showing the network of blood vessels distributed throughout the entire body from the head to the lower extremities. The illustration indicates that the circulatory system is an extensive and interconnected network that reaches all major organs and limbs, ensuring the distribution of blood throughout the body.

Image summary: This figure is an anatomical diagram. It depicts a full-body human female figure with internal organs and biological systems visible through a semi-transparent outer layer. The illustration highlights the placement of major organs within the thoracic and abdominal cavities. The figure demonstrates the spatial organization of human anatomy, showing how internal structures are distributed throughout the body.

Image summary: This figure is an anatomical diagram. It depicts a human figure with internal organs visible, specifically highlighting the respiratory system within the chest cavity. The illustration suggests a focus on the location and structure of the lungs relative to the overall human anatomy.

Image summary: This figure is an anatomical diagram. It depicts a human silhouette with internal organs highlighted, specifically focusing on the urinary system including the kidneys and the bladder. The diagram illustrates the spatial positioning and anatomical relationship between these organs within the human body, showing that the kidneys are located in the upper abdominal region and the bladder is situated lower in the pelvic area.

Image summary: This figure is an anatomical illustration. It depicts a human figure composed of horizontal stripes, with a specific highlighted area located in the pelvic region. The illustration suggests a focus on a particular anatomical site, indicating that the highlighted region is the primary area of interest or pathology within the body.

Integumentary System

• Small intestine absorbs vitamin D, which skin and kidneys modify to produce the hormone calcitriol

• Excess dietary calories are stored as triglycerides in adipose cells in dermis and subcutaneous tissue

Skeletal System

• Small intestine absorbs dietary calcium and phosphorus salts needed to build bone extracellular matrix

Muscular System

• Liver can convert lactic acid (produced by muscles during exercise) to glucose

Nervous System

• Gluconeogenesis (synthesis of new glucose molecules) in liver plus digestion and absorption of dietary carbohydrates provide glucose, needed for A.T.P production by neurons

Endocrine System

• Liver inactivates some hormones, ending their activity

• Pancreatic islets release insulin and glucagon

• Cells in mucosa of stomach and small intestine release hormones that regulate digestive activities

• Liver produces angiotensinogen

Cardiovascular System

• Digestive canal absorbs water that helps maintain blood volume and iron that is needed for synthesis of hemoglobin in red blood cells

• Bilirubin from hemoglobin breakdown is partially excreted in feces

• Liver synthesizes most blood plasma proteins

Lymphoid (Lymphatic) System and Immunity

• Acidity of gastric juice destroys bacteria and most toxins in stomach

• Lymphoid nodules in areolar connective tissue of mucosa of digestive canal destroy microbes

Respiratory System

• Pressure of abdominal organs against diaphragm helps expel air quickly during forced exhalation

Urinary System

• Absorption of water by digestive canal provides water needed to excrete waste products in urine

Genital (Reproductive) Systems

• Digestion and absorption provide adequate nutrients, including fats, for normal development of reproductive structures, for production of gametes (oocytes and sperm), and for fetal growth and development during pregnancy

Image summary: This figure is an anatomical illustration. It depicts a full body view of a human female figure from a frontal perspective. The illustration serves as a basic representation of female human anatomy and proportions.

Medical Terminology

Achalasia (ak'-a-LÃ-zê-a; α-= without; -chalasis = relaxation) A condition caused by malfunction of the myenteric neural plexus in which the lower esophageal sphincter fails to relax normally as food approaches. A whole meal may become lodged in the esophagus and enter the stomach very slowly. Distension of the esophagus results in chest pain that is often confused with pain originating from the heart.

Barrett's esophagus A pathological change in the epithelium of the esophagus from nonkeratinized stratified squamous epithelium to columnar epithelium so that the lining resembles that of the stomach or small intestine due to long-term exposure of the esophagus to stomach acid; increases the risk of developing cancer of the esophagus.

Borborygmus borborigmus A rumbling noise caused by the propulsion of gas through the intestines.

Bulimia (bü-LËM-ë-a; bu-= ox; limia = hunger) (or binge-purge syndrome) A disorder that typically affects young, single, middle-class white females, characterized by overeating at least twice a week followed by purging by self-induced vomiting, strict dieting or fasting, vigorous exercise, or use of laxatives or diuretics; it occurs in response to fears of being overweight or to stress, depression, and physiological disorders such as hypothalamic tumors.

Canker sore kangker Painful ulcer on the mucous membrane of the mouth that affects females more often than males, usually between ages 10 and 40; may be an autoimmune reaction or a food allergy.

Cirrhosis (si-Ro-sis) Distorted or scarred liver as a result of chronic inflammation due to hepatitis, chemicals that destroy hepatocytes, parasites that infect the liver, or alcoholism; the hepatocytes are replaced by fibrous or adipose connective tissue. Symptoms include jaundice, edema in the legs, uncontrolled bleeding, and increased sensitivity to drugs.

Colitis (ko-Li-tis) Inflammation of the mucosa of the colon and rectum in which absorption of water and salts is reduced, producing watery, bloody feces and, in severe cases, dehydration and salt depletion. Spasms of the irritated muscularis produce cramps. It is thought to be an autoimmune condition.

Colonoscopy kolonoskope; skop-= to view) The visual examination of the lining of the colon using an elongated, flexible, fiber-optic endoscope called a colonoscope. It is used to detect disorders such as polyps, cancer, and diverticulosis; to take tissue samples; and to remove small polyps. Most tumors of the large intestine occur in the rectum.

Colostomy kolostome; -stomy = provide an opening) The diversion of feces through an opening in the colon, creating a surgical "stoma" (artificial opening) that is made in the exterior of the abdominal wall. This opening serves as a substitute anus through which feces are eliminated into a bag worn on the abdomen.

Dysphagia (dis-FÃ-jê-a; dys-= abnormal; -phagia = to eat) Difficulty in swallowing that may be caused by inflammation, paralysis, obstruction, or trauma.

Flatus flatus Air (gas) in the stomach or intestine, usually expelled through the anus. If the gas is expelled through the mouth, it is called eructation or belching (burping). Flatus may result from gas released during the breakdown of foods in the stomach or from swallowing air or gas-containing substances such as carbonated drinks.

Food poisoning A sudden illness caused by ingesting food or drink contaminated by an infectious microbe (bacterium, virus, or protozoan) or a toxin (poison). The most common cause of food poisoning is the toxin produced by the bacterium Staphylococcus aureus. Most types of food poisoning cause diarrhea and/or vomiting, often associated with abdominal pain.

Gastroenteritis (gas'-tro-en-ter-l-tis; gastro-=stomach;-enteron-=intestine; -itis = inflammation) Inflammation of the lining of the stomach and intestine (especially the small intestine). It is usually caused by a viral or bacterial infection that may be acquired by contaminated food or water or by people in close contact. Symptoms include diarrhea, vomiting, fever, loss of appetite, cramps, and abdominal discomfort.

Gastroscopy gastroskope; skop-= to view) Endoscopic examination of the stomach in which the examiner can view the interior of the stomach directly to evaluate an ulcer, tumor, inflammation, or source of bleeding.

Halitosis (hal'-i-To-sis; halitus-= breath; -osis = condition) A foul odor from the mouth; also called bad breath.

Heartburn A burning sensation in a region near the heart due to irritation of the mucosa of the esophagus from hydrochloric acid in stomach contents. It is caused by failure of the lower esophageal sphincter to close properly, so that the stomach contents enter the inferior esophagus. It is not related to any cardiac problem.

Hemorrhoids hemoroyds; hemo-= blood; -rhoia = flow) Varicosed (enlarged and inflamed) superior rectal veins. Hemorrhoids develop when the veins are put under pressure and become engorged with blood. If the pressure continues, the wall of the vein stretches.

Such a distended vessel oozes blood; bleeding or itching is usually the first sign that a hemorrhoid has developed. Stretching of a vein also favors clot formation, further aggravating swelling and pain. Hemorrhoids may be caused by constipation, which may be brought on by low-fiber diets.

Also, repeated straining during defecation forces blood down into the rectal veins, increasing pressure in those veins and possibly causing hemorrhoids. Also called piles.

Hernia hernia Protrusion of all or part of an organ through a membrane or cavity wall, usually the abdominal cavity. Hiatus (diaphragmatic) hernia is the protrusion of a part of the stomach into the thoracic cavity through the esophageal hiatus of the diaphragm. Inguinal hernia is the protrusion of the hernia sac into the inguinal opening; it may contain a portion of the bowel in an advanced stage and may extend into the scrotal compartment in males, causing strangulation of the herniated part.

Inflammatory bowel disease inflamatore bowel Inflammation of the digestive canal that exists in two forms. (1) Crohn's disease is an inflammation of any part of the digestive canal in which the inflammation extends from the mucosa through the submucosa, muscular layer, and serosa. (2) Ulcerative colitis is an inflammation of the mucosa of the colon and rectum, usually accompanied by rectal bleeding. Curiously, cigarette smoking increases the risk of Crohn's disease but decreases the risk of ulcerative colitis.

Irritable bowel syndrome I.B.S Disease of the entire digestive canal in which a person reacts to stress by developing symptoms (such as cramping and abdominal pain) associated with alternating patterns of diarrhea and constipation. Excessive amounts of mucus may appear in feces; other symptoms include flatulence, nausea, and loss of appetite. The condition is also known as irritable colon or spastic colitis.

Malabsorption malabsorpshun; mal-= bad) A number of disorders in which nutrients from food are not absorbed properly. It may be due to disorders that result in the inadequate breakdown of food during digestion (due to inadequate digestive enzymes or juices), damage to the lining of the small intestine (from surgery, infections, and drugs like neomycin and alcohol), and impairment of motility. Symptoms may include diarrhea, weight loss, weakness, vitamin deficiencies, and bone demineralization.

Chapter Review

Review

Introduction

1. The breaking down of larger food molecules into smaller molecules is called digestion.

2. The organs involved in the breakdown of food are collectively known as the digestive system.

24.1 Overview of the Digestive System

1. The digestive system is composed of two main groups of organs: the digestive canal and accessory digestive organs.

2. The digestive canal is a continuous tube extending from the esophagus to the anus.

3. The accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

4. Digestion includes six basic processes: ingestion, secretion, mixing and propulsion, mechanical and chemical digestion, absorption, and defecation.

5. Mechanical digestion consists of mastication and movements of the digestive canal that aid chemical digestion.

6. Chemical digestion is a series of hydrolysis reactions that break down large carbohydrates, lipids, proteins, and nucleic acids in foods into smaller molecules that are usable by body cells.

24.2 Layers of the Digestive Canal

1. The basic arrangement of layers in most of the digestive canal, from deep to superficial, is the mucosa, submucosa, muscular layer, and serosa.

2. Associated with the lamina propria of the mucosa are extensive patches of lymphatic tissue called mucosa-associated lymphoid tissue (malt).

24.3 Neural Innervation of the Digestive Canal

1. The digestive canal is regulated by an intrinsic set of nerves known as the enteric nervous system (E.N.S) and by an extrinsic set of nerves that are part of the autonomic nervous system (A.N.S).

Malocclusion malokloozhun; mal-= bad; -occlusion = to fit together) Condition in which the surfaces of the maxillary (upper) and mandibular (lower) teeth fit together poorly.

Nausea nausea; nausea = seasickness) Discomfort characterized by a loss of appetite and the sensation of impending vomiting. Its causes include local irritation of the digestive canal, a systemic disease, brain disease or injury, overexertion, or the effects of medication or drug overdosage.

Traveler's diarrhea Infectious disease of the digestive canal that results in loose, urgent bowel movements, cramping, abdominal pain, malaise, nausea, and occasionally fever and dehydration. It is acquired through ingestion of food or water contaminated with fecal material typically containing bacteria (especially Escherichia coli); viruses or protozoan parasites are less common causes.

2. The E.N.S consists of neurons arranged into two plexuses: the myenteric neural plexus and the submucosal neural plexus.

3. The myenteric neural plexus, which is located between the longitudinal and circular smooth muscle layers of the muscular layer, regulates digestive canal motility.

4. The submucosal neural plexus, which is located in the submucosa, regulates digestive canal secretion.

5. Although the neurons of the E.N.S can function independently, they are subject to regulation by the neurons of the A.N.S.

6. Parasympathetic fibers of the vagus (10) nerves and pelvic splanchnic nerves increase digestive canal secretion and motility by increasing the activity of E.N.S neurons.

7. Sympathetic fibers from the thoracic and upper lumbar regions of the spinal cord decrease digestive canal secretion and motility by inhibiting E.N.S neurons.

24.4 Peritoneum

1. The peritoneum is the largest serous membrane of the body; it lines the wall of the abdominal cavity and covers some abdominal organs.

2. Folds of the peritoneum include the mesentery, mesocolon, falciform ligament, lesser omentum, and greater omentum.

24.5 Mouth

1. The mouth is formed by the cheeks, hard and soft palates, oral cavity, teeth, salivary glands, lips, and tongue.

2. The vestibule is the space bounded externally by the cheeks and lips and internally by the teeth and gums.

3. The oral cavity proper extends from the vestibule to the fauces.

4. The tongue, together with its associated muscles, forms the floor of the oral cavity. It is composed of skeletal muscle covered with mucous membrane. The upper surface and sides of the tongue are covered with lingual papillae, some of which contain taste buds. Glands in the tongue secrete lingual lipase, which digests triglycerides into fatty acids and diglycerides once in the acid environment of the stomach.

5. Most saliva is secreted by the major salivary glands, which lie outside the mouth and pour their contents into ducts that empty into the

oral cavity. There are three pairs of major salivary glands: parotid, submandibular, and sublingual glands.

6. Saliva lubricates food and starts the chemical digestion of carbohydrates. Salivation is controlled by the nervous system.

7. The teeth project into the mouth and are adapted for mechanical digestion.

8. A typical tooth consists of three principal regions: crown, root, and neck. Teeth are composed primarily of dentin and are covered by enamel, the hardest substance in the body. There are two dentitions: deciduous and permanent.

9. Through mastication, food is mixed with saliva and shaped into a soft, flexible mass called a bolus. Salivary amylase then begins the digestion of starches, and lingual lipase acts on triglycerides.

24.6 Pharynx

1. The pharynx is a funnel-shaped tube that extends from the choanae to the esophagus posteriorly and to the larynx anteriorly.

2. The pharynx has both respiratory and digestive functions.

24.7 Esophagus

1. The esophagus is a collapsible, muscular tube that connects the pharynx to the stomach.

2. It contains an upper and a lower esophageal sphincter.

24.8 Deglutition

1. Deglutition, or swallowing, moves a bolus from the mouth to the stomach.

2. Swallowing consists of voluntary, pharyngeal (involuntary), and esophageal (involuntary) stages.

24.9 Stomach

1. The stomach connects the esophagus to the duodenum.

2. The principal anatomical regions of the stomach are the cardia, fundus, body, and pyloric part.

3. Adaptations of the stomach for digestion include gastric folds; glands that produce mucus, hydrochloric acid, pepsin, gastric lipase, and intrinsic factor; and a three-layered muscular layer.

4. Mechanical digestion consists of propulsion and retropulsion.

5. Chemical digestion consists mostly of the conversion of proteins into peptides by pepsin.

6. The stomach wall is impermeable to most substances.

7. Among the substances the stomach can absorb are water, certain ions, drugs, and alcohol.

24.10 Pancreas

1. The pancreas consists of a head, a body, and a tail and is connected to the duodenum via the pancreatic duct and accessory duct.

2. Endocrine pancreatic islets secrete hormones, and exocrine pancreatic acini secrete pancreatic juice.

3. Pancreatic juice contains enzymes that digest starch (pancreatic amylase), proteins (trypsin, chymotrypsin, carboxypeptidase, and elastase), triglycerides (pancreatic lipase), and nucleic acids (ribonuclease and deoxyribonuclease).

24.11 Liver and Gallbladder

1. The liver has left and right lobes; the left lobe includes a quadrate lobe and a caudate lobe. The gallbladder is a sac located in a depression on the posterior surface of the liver that stores and concentrates bile.

2. The lobes of the liver are made up of lobules that contain hepatocytes (liver cells), sinusoids, stellate reticuloendothelial cells, and a central vein.

3. Hepatocytes produce bile that is carried by a duct system to the gallbladder for concentration and temporary storage.

4. Bile's contribution to digestion is the emulsification of dietary lipids.

5. The liver also functions in carbohydrate, lipid, and protein metabolism; processing of drugs and hormones; excretion of bilirubin; synthesis of bile salts; storage of vitamins and minerals; phagocytosis; and activation of vitamin D.

24.12 Small Intestine

1. The small intestine extends from the pyloric sphincter to the ileal orifice. It is divided into duodenum, jejunum, and ileum.

2. Its glands secrete fluid and mucus, and the circular folds, intestinal villi, and microvilli of its wall provide a large surface area for digestion and absorption.

3. Microvillous border enzymes digest alpha -dextrins, maltose, sucrose, lactose, peptides, and nucleotides at the surface of mucosal epithelial cells.

4. Pancreatic and intestinal microvillous border enzymes break down starches into maltose, maltotriose, and α-dextrins (pancreatic amylase), α-dextrins into glucose (α-dextrinase), maltose to glucose (maltase), sucrose to glucose and fructose (sucrase), lactose to glucose and galactose (lactase), and proteins into peptides (trypsin, chymotrypsin, and elastase). Also, enzymes break off amino acids at the carboxyl ends of peptides (carboxypeptidases) and break off amino acids at the amino ends of peptides (aminopeptidases). Finally, enzymes split dipeptides into amino acids (dipeptidases), triglycerides to fatty acids and monoglycerides (lipases), and nucleotides to pentoses and nitrogenous bases (nucleosidases and phosphatases).

5. Mechanical digestion in the small intestine involves segmentation and migrating motility complexes.

6. Absorption occurs via diffusion, facilitated diffusion, osmosis, and active transport; most absorption occurs in the small intestine.

7. Monosaccharides, amino acids, and short-chain fatty acids pass into the blood capillaries.

8. Long-chain fatty acids and monoglycerides are absorbed from micelles, resynthesized to triglycerides, and formed into chylomicrons.

9. Chylomicrons move into lymph plasma in the lymphatic capillary of an intestinal villus.

10. The small intestine also absorbs electrolytes, vitamins, and water.

24.13 Large Intestine

1. The large intestine extends from the ileal orifice to the anus.

2. Its regions include the cecum, colon, and rectum.

3. The mucosa contains many goblet cells, and the muscular layer consists of teniae coli and haustra.

4. Mechanical movements of the large intestine include haustral churning, peristalsis, and mass peristalsis.

5. The last stages of chemical digestion occur in the large intestine through bacterial action. Substances are further broken down, and some vitamins are synthesized.

6. The large intestine absorbs water, ions, and vitamins.

7. Feces consist of water, inorganic salts, epithelial cells, bacteria, and undigested foods.

8. The elimination of feces from the rectum is called defecation.

9. Defecation is a reflex action aided by voluntary contractions of the diaphragm and abdominal muscles and relaxation of the external anal sphincter.

24.14 Phases of Digestion

1. Digestive activities occur in three overlapping phases: cephalic, gastric, and intestinal.

2. During the cephalic phase of digestion, salivary glands secrete saliva and gastric glands secrete gastric juice in order to prepare the mouth and stomach for food that is about to be eaten.

3. The presence of food in the stomach causes the gastric phase of digestion, which promotes gastric juice secretion and gastric motility.

4. During the intestinal phase of digestion, food is digested in the small intestine. In addition, gastric motility and gastric secretion decrease in order to slow the exit of chyme from the stomach, which prevents the small intestine from being overloaded with more chyme than it can handle.

Critical Thinking Questions

1. Why would you not want to completely suppress H.C.L secretion in the stomach?

2. Trey has cystic fibrosis, a genetic disorder that is characterized by the production of excessive mucus, affecting several body systems (e.g., respiratory, digestive, genital). In the digestive system, the excess mucus blocks bile ducts in the liver and pancreatic ducts. How would this affect Trey's digestive processes?

Answers to Figure Questions

24.1 Digestive enzymes are produced by the salivary glands, tongue, stomach, pancreas, and small intestine.

24.2 In the context of the digestive system, absorption is the movement of the products of digestion from the lumen of the digestive canal into blood plasma or lymph plasma.

24.3 The lamina propria has the following functions: (1) It contains blood vessels and lymphatic vessels, which are the routes by which nutrients are absorbed from the digestive canal; (2) it supports the mucosal epithelium and binds it to the muscularis mucosae; and (3) it contains mucosa-associated lymphoid tissue, which helps protect against disease.

24.4 The neurons of the myenteric neural plexus regulate digestive canal motility, and the neurons of the submucosal neural plexus regulate digestive canal secretion.

24.5 Mesentery binds the small intestine to the posterior abdominal wall.

24.6 The uvula helps prevent foods and liquids from entering the nasal cavity during swallowing.

24.7 Chloride ions in saliva activate salivary amylase.

24.8 The main component of teeth is connective tissue, specifically dentin.

5. The activities that occur during the various phases of digestion are coordinated by neural pathways and by hormones. Table 24.8 summarizes the major hormones that control digestion.

24.15 Development of the Digestive System

1. The endoderm of the primitive gut forms the epithelium and glands of most of the digestive canal.

2. The mesoderm of the primitive gut forms the smooth muscle and connective tissue of the digestive canal.

24.16 Aging and the Digestive System

1. General changes include decreased secretory mechanisms, decreased motility, and loss of tone.

2. Specific changes may include loss of taste, pyorrhea, hernias, peptic ulcer disease, constipation, hemorrhoids, and diverticular diseases.

3. Antonio had dinner at his favorite Italian restaurant. His menu consisted of a salad, a large plate of spaghetti, garlic bread, and wine. For dessert, he consumed “death by chocolate” cake and a cup of coffee. He topped off his evening with a cigarette and brandy. He returned home and, while lying on his couch watching television, he experienced a pain in his chest. He called 911 because he was certain he was having a heart attack. Antonio was told his heart was fine, but he needed to watch his diet. What happened to Antonio?

24.9 The first, second, and third molars do not replace any deciduous teeth.

24.10 The esophageal mucosa and submucosa contain mucus-secreting glands.

24.11 Both. Initiation of swallowing is voluntary and the action is carried out by skeletal muscles. Completion of swallowing—moving a bolus along the esophagus and into the stomach—is involuntary and involves peristalsis by smooth muscle.

24.12 After a large meal, the gastric folds stretch and disappear as the stomach fills.

24.13 Parietal cells in gastric glands secrete H.C.L, which is a component of gastric juice. H.C.L kills microbes in food, denatures proteins, and converts pepsinogen into pepsin.

24.14 Hydrogen ions secreted into gastric juice are derived from carbonic acid H 2 C O 3.

24.15 Histamine is a paracrine agent released by mast cells in the lamina propria.

24.16 The pancreatic duct contains pancreatic juice (fluid and digestive enzymes); the bile duct contains bile; the hepatopancreatic ampulla contains pancreatic juice and bile.

24.17 The phagocytic cell in the liver is the stellate reticuloendothelial cell.

24.18 While a meal is being absorbed, nutrients, O 2 , and certain toxic substances are removed by hepatocytes from blood flowing through liver sinusoids.

24.19 The ileum is the longest part of the small intestine.

24.20 Nutrients being absorbed in the small intestine enter the blood plasma via blood capillaries or the lymph plasma via lymphatic capillaries.

24.21 The fluid that is secreted by duodenal glands—alkaline mucus—neutralizes gastric acid and protects the mucosal lining of the duodenum.

24.22 Because monoglycerides are hydrophobic (nonpolar) molecules, they can dissolve in and diffuse through the lipid bilayer of the plasma membrane.

24.23 The stomach and pancreas are the two digestive system organs that secrete the largest volumes of fluid.

24.24 The ascending and descending portions of the colon are retroperitoneal.

24.25 Goblet cells in the large intestine secrete mucus to lubricate colonic contents.

24.26 The pH of gastric juice rises due to the buffering action of some amino acids in food proteins.

January 1 2023 Chinese text

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t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t dass, garn归t vgi.n to anw vna.na.n.t.t.

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