Enshayzerol: Human Anatomy and Physiology

Unit 3: Nutrition and Metabolism

Chapter 7: The Digestive System

Anatomy and Physiology

Comprehensive Exam Reviewer

Every concept, term, diagram, and process from the lecture slides explained in detail for college exam preparation.

2. Layers of the G.I Tract

3. Peritoneum

4. Mouth (Oral Cavity)

5. Pharynx

6. Esophagus

7. Stomach

8. Pancreas

9. Liver and Gallbladder

10. Digestive Hormones

11. Small Intestines

12. Large Intestines

13. Digestion, Absorption, and Utilization

14. Nutrition Guidelines: Carbs, Protein, and Fats

16. Quick Review Checklist The digestive system is responsible for breaking down the food we eat into small, absorbable molecules that the body can use for energy, growth, and repair. It is made up of a long, continuous tube (the G.I tract) plus several supporting organs (the accessory organs) that assist digestion without food actually passing through them.

Definition: Digestive System

The organ system responsible for the physical and chemical breakdown of food, the absorption of nutrients into the blood/lymph, and the elimination of indigestible waste.

G.I Tract Organs versus Accessory Organs

The slides divide digestive organs into two groups:

- G.I Tract (Gastrointestinal Tract) Organs – the organs food physically passes through, in order: mouth to pharynx to esophagus to stomach to small intestine to large intestine to rectum to anus.

- Accessory Organs – organs that support digestion by producing secretions (enzymes, bile) but that food never directly passes through: salivary glands, liver, gallbladder, pancreas.

Definition: G.I Tract (Alimentary Canal)

Image summary: A diagram of the human digestive system showing the anatomical path of food and waste. It labels key organs including the mouth, salivary glands, esophagus, stomach, liver, gallbladder, pancreas, small intestine, large intestine, appendix, rectum, and anus. The purpose is to illustrate the sequential organs involved in the process of digestion and excretion.

Also called the alimentary canal — the continuous muscular tube running from the mouth to the anus through which food actually travels.

Six Major Functions of the Digestive System

Each function represents a stage in processing food, from the moment it enters the mouth to the moment waste leaves the body:

Ingestion—taking food into the mouth (the starting point of digestion).

Propulsion-moving food through the G.I tract, mainly through peristalsis (wave-like muscle contractions).

• Mechanical Digestion – physically breaking food into smaller pieces (e.g., chewing/mastication, churning in the stomach) to increase surface area for enzymes.

Chemical Digestion – breaking down large food molecules into smaller absorbable units using enzymes and acids.

• Absorption – moving digested nutrients across the intestinal wall into the blood or lymph.

Defecation – eliminating indigestible waste (feces) from the body through the anus.

Remember!

Mnemonic for the six functions, in order: "I Pooped My Corn Absolutely Delicious" to Ingestion, Propulsion, Mechanical digestion, Chemical digestion, Absorption, Defecation.

Mechanical digestion increases Surface Area (physical breakdown); chemical digestion breaks Chemical Bonds (using enzymes) — the two always work together.

2. Layers of the G.I Tract

From the esophagus to the anus, the wall of the G.I tract is built from the same four basic layers, arranged from the innermost (touching food) to the outermost. Knowing these layers helps explain how each organ carries out digestion, absorption, and movement.

Table: Columns: Layer, Location, Key Components / Function. Row 1. Layer: Mucosa. Location: Innermost layer (lines the lumen). Key Components / Function: Secretion, absorption, protection — the layer in direct contact with food. Row 2. Layer: Submucosa. Location: Just outside the mucosa. Key Components / Function: Connective tissue, blood vessels, lymphatics, and nerves — supplies and drains the mucosa. Row 3. Layer: Muscularis externa. Location: Middle muscular layer. Key Components / Function: Circular & longitudinal smooth muscle — produces peristalsis (the wave-like contractions that push food along). Row 4. Layer: Serosa / Adventitia. Location: Outermost layer. Key Components / Function: Protective outer covering (serosa in the abdominal cavity; adventitia where the tract attaches to surrounding structures).

Definition: Peristalsis

Image summary: An anatomical diagram showing a cross-section of the wall of the gastrointestinal tract. It labels the layers from the innermost lumen outward: the mucosa (epithelium, lamina propria, and muscularis mucosae), the submucosa containing glands and the submucosal (Meissner's) plexus, the muscularis containing circular and longitudinal muscle layers and the myenteric (Auerbach's) plexus, and the outer serosa. It also shows the associated blood vessels (artery and vein), nerves, and lymphatic tissue. The purpose of the diagram is to illustrate the histological organization and the neural networks that regulate digestive functions.

Rhythmic, wave-like contractions of the muscularis externa (alternating circular and longitudinal muscle contraction) that push food forward through the G.I tract.

☑ Remember!

Memorize the order from inside to outside: Mucosa leads to Submucosa leads to Muscularis externa leads to Serosa/Adventitia (mnemonic: "Most Students Master Sleep").

The Muscularis externa is the layer responsible for peristalsis — this is a favorite exam link (muscle layer = movement).

3. Peritoneum

The peritoneum is a large serous membrane that lines the abdominal cavity and covers most of the abdominal organs. It reduces friction between organs as they move (e.g., during digestion or breathing) and helps hold organs in place.

Definition: Peritoneum

A serous membrane consisting of two layers — the visceral peritoneum (covers the organs themselves) and the parietal peritoneum (lines the abdominal wall) — with a thin peritoneal cavity of lubricating fluid between them.

Key Structures of the Peritoneum

• Mesentery—a double fold of peritoneum that suspends the small intestine from the posterior abdominal wall and carries blood vessels, nerves, and lymphatics to the intestines.

Greater omentum – a large, fatty peritoneal fold that hangs down over the abdominal organs like an apron.

Lesseromentum – a smaller peritoneal fold connecting the stomach and liver.

Mesocolon – the peritoneal fold that attaches the colon (large intestine) to the posterior abdominal wall.

Functions of the Peritoneum

• Supports and holds abdominal organs in their proper position

- Reduces friction between organs through its lubricating peritoneal fluid

- Serves as a site of fat storage (especially in the greater omentum)

Remember!

Visceral peritoneum wraps around the Organs; parietal peritoneum lines the Abdominal Wall — 'visceral = organ, parietal = wall.'

The peritoneal cavity is a Potential space filled with a small amount of lubricating fluid, not a large open cavity — this reduces friction as organs slide against each other.

FIGURE 11.3. Abdominal Cavity—Sagittal Section (Showing the

Figure 11.3 summary: Two sagittal anatomical diagrams of the abdominal cavity. The first labels key organs such as the liver, stomach, intestines, and bladder, alongside the peritoneal structures. The second highlights the parietal peritoneum in red and the visceral peritoneum in blue, illustrating how the parietal layer lines the abdominal wall while the visceral layer covers the internal organs. The purpose is to depict the spatial relationship between abdominal organs and the dual layers of the peritoneal membrane.

4. Mouth (Oral Cavity)

Digestion begins in the mouth, where both mechanical and chemical digestion start.

- Teeth – responsible for mechanical digestion; they cut, tear, and grind food into smaller pieces (incisors cut, canines tear, premolars/molars grind).

- Tongue – positions food for chewing and swallowing, and contains taste buds for sensing flavor.

- Salivary Glands – produce saliva, which contains:

- o Amylase – an enzyme that begins the chemical digestion of starch (carbohydrates).

- o Mucin-a slippery substance that lubricates food, making it easier to swallow.

- O Lysozyme – an antibacterial enzyme that helps protect against oral pathogens.

- Bolus – the soft, rounded mass of chewed food mixed with saliva that is formed in the mouth and ready to be swallowed.

Image summary: Three anatomical diagrams of the human mouth and throat. The first shows a frontal view of the open oral cavity, detailing the teeth, gums, palate, and tongue. The second is a sagittal cross-section showing the relationship between the nasal cavity, oral cavity, pharynx, larynx, and esophagus. The third is a lateral view highlighting the parotid, submandibular, and sublingual salivary glands. Together, these diagrams illustrate the structure and primary components of the upper digestive and respiratory tracts.

Definition: Bolus

A soft, moist mass of chewed food combined with saliva, shaped by the tongue and cheeks in preparation for swallowing.

Remember!

Chemical digestion of carbohydrates Starts in the mouth (via salivary amylase) — this is a commonly tested fact.

Major salivary glands (not always all pictured but relevant background): parotid, submandibular, and sublingual glands.

5. Pharynx

The pharynx (throat) is a shared passageway used by both the digestive and respiratory systems — it carries food toward the esophagus and air toward the trachea (windpipe).

Divisions of the Pharynx

- Nasopharynx – uppermost region, behind the nasal cavity; used for air only.

- Oropharynx – middle region, behind the mouth; used for both food and air.

- Laryngopharynx – lowest region, connecting to both the esophagus (food) and the larynx (air).

Swallowing (Deglutition)

During swallowing, the epiglottis (a flap of cartilage) closes over the opening of the larynx to prevent food from entering the airway. The bolus is then pushed from the pharynx into the esophagus.

Definition: Epiglottis

Image summary: An anatomical diagram of the human pharynx in a sagittal cross-section of the head and neck. It labels the three divisions of the pharynx—the nasopharynx, oropharynx, and hypopharynx—and shows their relative positions between the nasal cavity, oral cavity, larynx, esophagus, and trachea. The diagram is intended to illustrate the structural anatomy and spatial organization of the pharyngeal region.

A flexible flap of elastic cartilage at the top of the larynx that folds down during swallowing to cover the airway, directing food into the esophagus instead of the trachea.

Remember!

The pharynx is the only part of the digestive tract also used by the Respiratory system — this dual role is why swallowing must be carefully coordinated.

If the epiglottis fails to close properly, food or liquid can enter the trachea, causing choking or as piration.

6. Esophagus

The esophagus is a muscular tube that connects the pharynx to the stomach. It has no digestive function of its own (no enzymes are secreted here) — its only job is to transport the bolus.

- Structure – a muscular tube running from the pharynx to the stomach, passing through the diaphragm.

- Peristalsis – wave-like muscle contractions move the bolus down toward the stomach, even against gravity (which is why you can swallow while lying down or upside down).

- Upper Esophageal Sphincter (U.E.S) – controls the passage of food from the pharynx into the esophagus.

- Lower Esophageal Sphincter (L.E.S) – controls the passage of food from the esophagus into the stomach, and normally prevents stomach contents from flowing backward.

Image summary: An anatomical diagram of the upper digestive tract showing the path from the throat (pharynx), through the upper esophageal sphincter, down the esophagus, through the lower esophageal sphincter and diaphragm, and into the stomach. The diagram identifies the key components and valves that regulate the flow of food and liquids from the mouth to the stomach.

gerd (Gastroesophageal Reflux Disease)

gerd occurs when the lower esophageal sphincter is weak or relaxes abnormally, allowing acidic stomach contents to reflux (flow backward) into the esophagus. Because the esophagus lacks the protective mucus lining of the stomach, this reflux causes irritation and the burning sensation known as heartburn.

Definition: Sphincter

Gastroesophageal reflux disease

Image summary: A diagram of the upper digestive system showing the esophagus connected to the stomach. A closed sphincter acts as a valve between the two, with arrows indicating pressure keeping it shut to prevent stomach contents from moving upward. The purpose of the diagram is to illustrate how the closed sphincter prevents acid reflux from the stomach into the esophagus.

Image summary: An anatomical diagram of the stomach and esophagus showing the lower esophageal sphincter in an open state. Blue droplets and a yellow arrow indicate stomach contents moving upward into the esophagus. The diagram illustrates that an open sphincter allowing reflux leads to gastroesophageal reflux disease.

A ring-shaped muscle that contracts to close an opening or passageway and relaxes to allow material through — sphincters act like one-way valves controlling movement along the G.I tract.

☑ Remember!

U.E.S = pharynx leads to esophagus. L.E.S = esophagus leads to stomach. Remember by Order: Upper comes first (top), Lower comes second (bottom).

gerd equals a Weak Lower Esophageal Sphincter leads to acid reflux leads to heartburn. This is a classic clinical-correlation exam question.

GERD

7. Stomach

The stomach is a muscular, J-shaped sacthat temporarily stores food, mechanically churns it, and begins the chemical digestion of proteins. It converts the bolus into a thick, acidic liquid called chyme.

Definition: Chyme

The thick, semi-liquid mixture of partially digested food and gastric juices produced by the churning action of the stomach.

Regions of the Stomach

Cardia – the region surrounding the opening where the esophagus connects to the stomach.

• Fundus – the rounded upper portion of the stomach, above and to the side of the cardia.

Body – the large central region of the stomach; the main site of mixing and digestion.

• Antrum – the lower portion of the stomach, which helps grind food and push chyme toward the pylorus.

Pylorus – the funnel-shaped end of the stomach that connects to the duodenum, guarded by the pyloric sphincter.

Functions of the Stomach • Churning – mechanically mixes food with gastric juices

• Protein digestion – begins the chemical breakdown of proteins

- Chyme formation – converts the bolus into chyme for release into the small intestine

Table: Columns: Cell Type, Secretion, Function. Row 1. Cell Type: Mucous cells. Secretion: Mucus. Function: Coats and protects the stomach lining from its own acid. Row 2. Cell Type: Parietal cells. Secretion: HCl (Hydrochloric acid) and Intrinsic factor. Function: HCl activates pepsin and kills microbes; intrinsic factor is needed for vitamin B12 absorption. Row 3. Cell Type: Chief cells. Secretion: Pepsinogen. Function: Inactive precursor that is converted to pepsin (a protein-digesting enzyme) by HCl. Row 4. Cell Type: G cells. Secretion: Gastrin (hormone). Function: Stimulates gastric acid secretion and stomach motility.

Image summary: An anatomical diagram of the human stomach labeling its primary regions. It identifies the cardia where the esophagus enters, the fundus at the top, the body as the main central region, the antrum leading toward the exit, and the pylorus as the final section connecting to the duodenum. The purpose of the image is to illustrate the structural divisions of the stomach.

Sections of the Stomach

☑ Remember!

Image summary: An anatomical diagram of the stomach identifying its key structures and layers. It labels the external regions such as the fundus, body, and pyloric antrum, as well as the muscularis externa consisting of longitudinal, circular, and oblique layers. The diagram illustrates the pathway from the esophagus through the lumen and rugae of the mucosa, ending at the pyloric sphincter and duodenum; its purpose is to provide a comprehensive overview of stomach anatomy.

Regions of the stomach in order (entrance to exit): Cardia leads to Fundus leads to Body leads to Antrum leads to Pylorus.

Parietal cells make H.C.L + Intrinsic factor. Chief cells make Pepsinogen (inactive) which H.C.L converts to Pepsin (active). G cells make Gastrin.

Without intrinsic factor from parietal cells, the body cannot absorb vitamin B.12 — this is why stomach surgery/disease can lead to B.12 deficiency.

8. Pancreas

The pancreas is a dual-function gland — it has both an endocrine role (releasing hormones into the blood) and an exocrine role (releasing digestive secretions into the small intestine via a duct). In the context of digestion, its exocrine function is the main focus.

- Endocrine function – produces insulin and glucagon, which regulate blood glucose levels (released directly into the blood).

- Exocrine function – produces digestive enzymes (amylase, lipase, proteases) and bicarbonate, released through a duct into the small intestine.

• Neutralizing acidic chyme – the bicarbonate in pancreatic secretions neutralizes the acidic chyme coming from the stomach, protecting the intestinal lining and creating the proper pH for intestinal enzymes to work.

Table: Columns: Enzyme/Secretion, Digests / Function. Row 1. Enzyme/Secretion: Amylase. Digests / Function: Breaks down carbohydrates (starches) into smaller sugars. Row 2. Enzyme/Secretion: Lipase. Digests / Function: Breaks down lipids (fats) into fatty acids and glycerol. Row 3. Enzyme/Secretion: Proteases. Digests / Function: Break down proteins into smaller peptides/amino acids. Row 4. Enzyme/Secretion: Bicarbonate ( HCO 3 raised to - ). Digests / Function: Neutralizes acidic chyme entering from the stomach.

☑ Remember!

Image summary: An anatomical diagram showing the gallbladder, bile duct, pancreas, and small intestine. The bile duct connects the gallbladder and liver area, traveling through the pancreas to empty into the small intestine. The diagram illustrates how bile and pancreatic secretions are delivered to the small intestine to aid in digestion.

Endocrine pancreas = hormones into the blood (insulin, glucagon). Exocrine pancreas = enzymes/bicarbonate into a duct leads to small intestine.

The pancreas provides enzymes for all Three major nutrients: carbohydrates (amylase), fats (lipase), and proteins (proteases) — it is the most versatile digestive gland.

9. Liver and Gallbladder

Liver

The liver is the body's largest internal organ and performs many metabolic functions beyond digestion:

- Metabolism – processes absorbed nutrients (carbohydrates, proteins, fats) for use or storage.

- Detoxification – breaks down drugs, alcohol, and metabolic waste products (e.g., ammonia) to make them safe for excretion.

• Plasma protein synthesis – produces important blood proteins, such as albumin and clotting factors.

• Storage – stores glycogen (a glucose-storage molecule), vitamins, and iron.

- Bile production – produces bile, which emulsifies (breaks up) large fat globules into smaller droplets, increasing surface area for lipase to act on.

Definition: Bile

A greenish-yellow fluid produced by the liver that emulsifies fats, breaking large fat globules into tiny droplets to increase the surface area available for lipase to digest them. Bile does not chemically digest fat itself — it only physically prepares it for enzymatic digestion.

Gallbladder

• Stores and concentrates bile produced by the liver.

• Releases bile into the small intestine (duodenum) when fatty food is present, typically triggered by the hormone C.C.K (see Section 10).

Remember!

Image summary: An anatomical diagram showing the biliary system and digestive organs, including the liver, gallbladder, pancreas, and duodenum. It illustrates how bile flows from the liver and gallbladder through the hepatic and cystic ducts into the common bile duct, which joins the pancreatic duct at the ampulla of Vater to enter the duodenum. The purpose of the diagram is to show the connection and drainage paths of the hepatobiliary and pancreatic systems into the small intestine.

The Liver makes bile; the Gallbladder stores and releases it — a very commonly confused exam pairing. Bile Emulsifies fat (mechanical-like action); it does not chemically break down fat — that is lipase's job.

10. Digestive Hormones

Digestion is tightly regulated by a set of hormones produced by the stomach and small intestine (duodenum). These hormones coordinate the timing of enzyme release, bile release, acid production, and even hunger.

Table: Columns: Hormone, Source, Main Function. Row 1. Hormone: Gastrin. Source: Stomach (G cells). Main Function: Stimulates gastric acid secretion and gastric motility. Row 2. Hormone: Secretin. Source: Duodenum. Main Function: Stimulates bicarbonate release from the pancreas; inhibits gastric acid secretion/activity. Row 3. Hormone: CCK (Cholecystokinin). Source: Duodenum. Main Function: Stimulates bile release from the gallbladder and enzyme secretion from the pancreas. Row 4. Hormone: GIP (Gastric Inhibitory Polypeptide). Source: Duodenum. Main Function: Inhibits gastric acid secretion; stimulates insulin release. Row 5. Hormone: Motilin. Source: Small intestine. Main Function: Regulates GI motility (movement) between meals. Row 6. Hormone: Ghrelin. Source: Stomach (during fasting). Main Function: Stimulates hunger/appetite when the stomach is empty.

Image summary: Two diagrams illustrating the roles of digestive hormones. The left panel shows a feedback loop where acid and fatty foods in the duodenum trigger the release of secretin and CCK to inhibit stomach motility and increase bile and bicarbonate secretion, while food in the stomach triggers gastrin to increase acid production and motility. The right panel lists five specific hormones—Gastrin, Secretin, Cholecystokinin, Motilin, and GIP—and their primary functions, such as regulating gastric acid, pH, bile release, and insulin secretion. Together, these diagrams show how digestive hormones coordinate the activity of the stomach, pancreas, liver, and duodenum to optimize digestion.

? Remember!

Mnemonic: "Girls Should Cook Great Meals, Guys!" leads to Gastrin, Secretin, C.C.K, G.I.P, Motilin, Ghrelin.

Secretin and G.I.P both Inhibit the stomach; Gastrin Stimulates the stomach — opposite actions on the same organ, a classic 'compare and contrast' exam item.

C.C.K = the 'fat-sensing' hormone — released when fatty food enters the duodenum, triggering both bile release (gallbladder) and enzyme release (pancreas).

Ghrelin is the only hormone here that stimulates Hunger rather than digestion — it rises when the stomach is empty and falls after eating.

11. Small Intestines

The small intestine is the primary site where digestion is completed and where the vast majority of nutrient absorption occurs. It is long and highly folded to maximize the surface area available for absorption.

Regions of the Small Intestine

- Duodenum – the first and shortest segment; receives chyme from the stomach along with bile (from the gallbladder) and pancreatic secretions (enzymes + bicarbonate).

● Jejunum – the middle segment; the primary site of nutrient absorption.

- Ileum – the final segment; absorbs remaining nutrients (including vitamin B.12 and bile salts) before connecting to the large intestine.

Structural Adaptations for Absorption

Image summary: An anatomical diagram of the human digestive system highlighting the parts of the small intestine. It shows the sequence of digestion flowing from the stomach into the duodenum, then through the jejunum and ileum, which are surrounded by the large intestine and lead toward the rectum, with the appendix attached to the lower right. The purpose is to illustrate the relative positions and connectivity of the different segments of the small intestine within the abdominal cavity.

The small intestine's wall has three levels of folding that dramatically increase its internal surface area:

- Circular folds (plicae circulares) – large, permanent ridges in the intestinal wall.

- Villi-finger-like projections covering the circular folds, each containing blood capillaries and a lacteral (lymph capillary) for nutrient absorption.

• Microvilli (brush border) – microscopic projections on the surface of each villus's epithelial cells, forming the 'brush border.'

Definition: Brush Border

Image summary: A diagram showing the hierarchical structure of the small intestine across four levels of magnification: the organ level, circular folds, villi, and finally the cellular level showing microvilli. This progression illustrates how the intestinal surface area is exponentially increased at each level to optimize nutrient absorption.

The dense covering of microvilli on the surface of small intestinal epithelial cells; it not only increases absorptive surface area but also contains brush border enzymes that complete the final steps of chemical digestion.

☑ Remember!

Three levels of folding, small to large: Microvilli (brush border) <Villi <Circular folds — each level multiplies the absorptive surface area.

The small intestine is the Main site of absorption for nutrients — this is one of the most important 'site of function' facts to memorize.

Order of the small intestine: Duodenum leads to Jejunum leads to Ileum (mnemonic: "Do You Import?").

12. Large Intestines

The large intestine (colon) receives indigestible material from the small intestine and prepares it for elimination. Unlike the small intestine, its main role is not digestion or nutrient absorption but water/electrolyte reabsorption and waste processing.

Regions of the Large Intestine

• Cecum – the pouch-like beginning of the large intestine, connected to the ileum.

- Colon – the main segment, further divided into ascending, transverse, descending, and sigmoid portions.

- Rectum – stores feces before elimination.

- Anal canal – the final passage through which feces are expelled from the body.

Functions of the Large Intestine

Image summary: An anatomical diagram of the human large intestine, labeling the key sections including the cecum, appendix, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon, and rectum. The diagram illustrates the continuous path of the colon from the cecum to the rectum to show the structural layout of the distal gastrointestinal tract.

- Absorbs water, electrolytes, and some vitamins – concentrating waste into solid/semi-solid feces.

- Gut microbiota – trillions of beneficial bacteria that live in the colon and carry out fermentation of undigested material and synthesize certain vitamins (such as vitamin K and some B vitamins).

• Feces formation and elimination – compacts waste into feces and eliminates it through defecation.

Definition: Gut Microbiota

The community of beneficial bacteria living in the large intestine that ferment undigested carbohydrates and fiber, and synthesize certain vitamins (e.g., vitamin K), benefiting the host's health.

Remember!

Small intestine = main site of Nutrient absorption. Large intestine = main site of Water and Electrolyte absorption.

Gut microbiota in the large intestine produce vitamin K and some B vitamins — a commonly tested detail linking digestion to overall health.

13. Digestion, Absorption, and Utilization

This section summarizes what ultimately happens to each major type of nutrient after it is broken down — where it goes, and how the body uses it.

Table: Columns: Nutrient, Broken Down Into, Absorption Route, Used For. Row 1. Nutrient: Carbohydrates. Broken Down Into: Monosaccharides (simple sugars). Absorption Route: Absorbed into the blood. Used For: ATP (energy), glycogen storage, or converted to fat. Row 2. Nutrient: Proteins. Broken Down Into: Amino acids. Absorption Route: Absorbed into the blood. Used For: Synthesis of new proteins, or used for energy. Row 3. Nutrient: Lipids (fats). Broken Down Into: Fatty acids. Absorption Route: Absorbed into lacteals to lymph. Used For: Energy or stored as fat.

- Vitamins and minerals – absorbed primarily in the small intestine.

- Water – absorbed in Both the small and large intestines.

☑ Remember!

Carbs and Proteins go into the Blood (via capillaries in the villi). Lipids go into the Lymph first (via lacteals), because fats are not water-soluble and cannot dissolve directly into blood plasma.

This is one of the most important physiology distinctions in the whole chapter: 'fats take the lymphatic route.'

Water is unique — it's absorbed along the Entire G.I tract, but especially in the small and large intestine.

14. Nutrition Guidelines: Carbohydrates, Protein, and Fats

Beyond the anatomy and physiology of digestion, the slides include practical nutrition guidance connecting digestion to everyday dietary choices.

Carbohydrate Recommendations (American Heart Association)

- Limit refined sugars – foods with simple carbohydrates provide 'empty calories' (energy with little nutritional value).

- Choose complex carbohydrates – fruits, vegetables, legumes, beans, lentils, and dried peas are rich in nutrients and fiber.

• Include whole grains – such as brown rice and whole-grain pasta, bread, and cereals.

Definition: Simple versus Complex Carbohydrates

Simple carbohydrates (sugars) are quickly digested and absorbed, providing a fast but short-lived energy spike with few nutrients ('empty calories'). Complex carbohydrates (starches and fiber) are digested more slowly and are typically packaged with vitamins, minerals, and fiber.

Protein Recommendations

According to the Recommended Dietary Allowance (R.D.A), the average adult needs about 0.8 grams of protein per kilogram of body weight (or 0.36 grams per pound) each day.

Definition: R.D.A (Recommended Dietary Allowance)

The average daily intake level of a nutrient sufficient to meet the nutritional needs of nearly all healthy individuals in a specific age/sex group.

- Example calculation: a person who weighs 75 kilograms (about 165 lbs) needs approximately 60 grams of protein per day (75 kilograms × 0.8 g/kg = 60 g).

Fat Recommendations

Fats are compared as unsaturated (generally considered healthier, found in foods like fish, avocado, nuts, and olive oil) versus saturated fats (found in foods like processed meats, butter, and full-fat dairy, and generally recommended in moderation).

Foods highlighted as sources of healthy fats include:

Fatty fish

• Chia seeds

Dark chocolate

Eggs

Avocado

Flaxseed

Nuts

• Olive oil

Tofu

Yogurt

☑ Remember!

Protein R.D.A formula: 0.8 grams protein times body weight in kg equals daily protein need in grams. Practice this calculation — it's a classic applied exam question.

'Complex' carbs and 'unsaturated' fats are generally the healthier choices emphasized in the slides — useful for any nutrition-related exam question framed around healthy eating.

The digestive system develops early in embryonic life and changes predictably as the body ages.

Development

- The G.I tract develops from the embryonic gut tube, a simple tube that forms early in embryonic development and later differentiates into the mouth-to-anus digestive tract.

- The liver and pancreas develop as outgrowths (buds) of this embryonic gut tube, which is why they remain connected to the small intestine via ducts in the adult.

Aging

- Saliva production – can make chewing and swallowing more difficult and increase risk of dental issues.

• Weaker sphincters – increases the risk of acid reflux/gerd in older adults.

- Slower motility – peristalsis becomes less efficient, slowing the movement of food through the G.I tract.

• Reduced secretions – less production of digestive enzymes and acids can impair digestion and nutrient absorption.

- Decline in liver function and absorption efficiency—can affect metabolism, detoxification, and nutrient uptake.

- Constipation – becomes more common with age, related to slower motility, reduced fluid intake, and weaker muscle tone.

? Remember!

The liver and pancreas are developmental Outgrowths of the gut tube — this explains their ductal connection to the small intestine even in adults.

Aging generally causes a Decrease in digestive efficiency: less saliva, weaker sphincters ( to gerd risk), slower motility, reduced secretions, and more constipation.

16. Quick Review Checklist

Before your exam, make sure you can confidently do each of the following:

- List the G.I tract organs in order from mouth to anus, and name the four accessory organs.

- List the six functions of the digestive system in order.

- Name the four layers of the G.I tract wall from innermost to outermost, and identify which layer produces peristalsis.

- Distinguish visceral versus parietal peritoneum, and name the major peritoneal structures (mesentery, omentum, mesocolon).

- Explain the roles of teeth, tongue, and salivary glands in the mouth, including the three components of saliva.

- Describe the divisions of the pharynx and the role of the epiglottis in swallowing.

- Explain the roles of the U.E.S and L.E.S, and how a weak L.E.S causes gerd.

- List the five regions of the stomach and identify the secretion/function of each stomach cell type.

- Distinguish the endocrine and exocrine functions of the pancreas.

- Explain the roles of the liver (bile production) versus the gallbladder (bile storage/release).

- Name all six digestive hormones and their sources/functions.

- List the three regions of the small intestine and its three levels of surface-area adaptation.

- Explain the main functions of the large intestine, including the role of gut microbiota.

• Trace where carbohydrates, proteins, and lipids go after absorption (blood versus lymph).

- Recall the A.H.A carbohydrate guidelines, the protein R.D.A formula, and examples of healthy fats.

- Summarize how the digestive system develops and how it changes with aging.