Image summary: This is a photograph. It depicts a healthcare professional measuring the blood pressure of an older male patient using a sphygmomanometer. The scene indicates a medical consultation or routine health checkup where a patient's vital signs are being monitored.

The Cardiovascular System: Blood Vessels and Hemodynamics

Blood Vessels, Hemodynamics, and Homeostasis

Blood vessels contribute to homeostasis by providing the structures for the flow of blood to and from the heart and the exchange of nutrients and wastes in tissues. They also play an important role in adjusting the velocity and volume of blood flow.

The cardiovascular system contributes to homeostasis of other body systems by transporting and distributing blood throughout the body to deliver materials (such as oxygen, nutrients, and hormones) and carry away wastes. The structures involved in these important tasks are the blood vessels, which form a closed system of tubes that carries blood away from the heart, transports it to the tissues of the body, and then returns it to the heart. The left side of the heart pumps blood through an estimated 100,000 kilometers (60,000 mi) of blood vessels.

The right side of the heart pumps blood through the lungs, enabling blood to pick up oxygen and unload carbon dioxide. Chapters 19 and 20 described the composition and functions of blood and the structure and function of the heart. In this chapter, we focus on the structure and functions of the various types of blood vessels; on the forces involved in circulating blood throughout the body; and on the blood vessels that constitute the major circulatory routes.

21.1 Structure and Function of Blood Vessels

Objectives

- Contrast the structure and function of arteries, arterioles, capillaries, venules, and veins.

- Outline the vessels through which the blood moves in its passage from the heart to the capillaries and back.

• Distinguish between pressure reservoirs and blood reservoirs.

The five main types of blood vessels are arteries, arterioles, capillaries, venules, and veins (see Figure 21.17). Arteries (Arter-ez) carry blood away from the heart to other organs. Large, elastic arteries leave the heart and divide into medium-sized, muscular arteries that branch out into the various regions of the body. Medium-sized arteries then divide into small arteries, which in turn divide into still smaller arteries called arterioles (ar-Tlr-e-ols). As the arterioles enter a tissue, they branch into numerous tiny vessels called blood capillaries capilarez = hairlike) or simply capillaries.

Figure 21.17 summary: This is a schematic diagram illustrating the human circulatory system. The figure depicts the flow of blood through the heart, lungs, and various systemic organs, including the head, neck, upper limbs, liver, spleen, stomach, intestines, pelvis, and lower limbs, distinguishing between oxygenated and deoxygenated blood. The diagram demonstrates that oxygenated blood is pumped from the left ventricle through the aorta to systemic capillaries, while deoxygenated blood returns via the vena cavae to the right atrium. It further shows the pulmonary circuit where deoxygenated blood is sent to the lungs via the pulmonary trunk and returns oxygenated to the left atrium through pulmonary veins, highlighting the continuous loop of blood circulation and gas exchange between the heart, lungs, and body tissues.

The thin walls of capillaries allow the exchange of substances between the blood and body tissues. Groups of capillaries within a tissue reunite to form small veins called venules venuls = little veins). These in turn merge to form progressively larger blood vessels called veins.

Veins vanz are the blood vessels that convey blood from the tissues back to the heart.

Clinical Connection

Angiogenesis and Disease

Angiogenesis anjeogenesis; angio-= blood vessel; -genesis = production) refers to the growth of new blood vessels. It is an important process in embryonic and fetal development, and in postnatal life serves important functions such as wound healing, formation of a new uterine lining after menstruation, formation of the corpus luteum after ovulation, and development of blood vessels around obstructed arteries in the coronary circulation. Several proteins (peptides) are known to promote and inhibit angiogenesis.

Clinically angiogenesis is important because cells of a malignant tumor secrete proteins called tumor angiogenesis factors that stimulate blood vessel growth to provide nourishment for the tumor cells. Scientists are seeking chemicals that would inhibit angiogenesis and thus stop the growth of tumors. In diabetic retinopathy retinopathê, angiogenesis may be important in the development of blood vessels that actually cause blindness, so finding inhibitors of angiogenesis may also prevent the blindness associated with diabetes.

Basic Structure of a Blood Vessel

The wall of a blood vessel consists of three layers, or tunics, of different tissues: an epithelial inner lining, a middle layer consisting of smooth muscle and elastic connective tissue, and a connective tissue outer covering. The three structural layers of a generalized blood vessel from innermost to outermost are the tunica intima, tunica media, and tunica externa (Figure 21.1). Modifications of this basic design account for the five types of blood vessels and the structural and functional differences among the various vessel types. Always remember that structural variations correlate to the differences in function that occur throughout the cardiovascular system.

Figure 21.1 summary: This figure consists of anatomical diagrams. The diagrams illustrate the cross-sectional structures of an artery, a vein, and a capillary, detailing the various layers of the vessel walls. The artery and vein are shown with three distinct layers: the tunica intima containing the endothelium and basement membrane, the tunica media composed of smooth muscle and elastic membranes, and the tunica externa. The vein is specifically shown to contain valves. The capillary is depicted as a much simpler structure consisting only of endothelium and a basement membrane. From these diagrams, it can be inferred that arteries and veins have more complex, thicker walls to handle different pressure levels, whereas capillaries have the thinnest walls to facilitate the exchange of materials between blood and tissues.

Tunica Intima The tunica intima toonika; tunic = garment or coat; intima = innermost) or tunica interna forms the inner lining of a blood vessel and is in direct contact with the blood as it flows through the lumen loomen, or interior opening, of the vessel (Figure 21.1a, b). Although the tunica intima has multiple parts, these tissue components contribute minimally to the thickness of the vessel wall. Its innermost layer is called endothelium, which is continuous with the endocardial lining of the heart. The endothelium is a thin layer of flattened cells that lines the inner surface of the entire cardiovascular system (heart and blood vessels).

Until recently, endothelial cells were regarded as little more than a passive barrier between the blood and the remainder of the vessel wall. It is now known that endothelial cells are active participants in a variety of vessel-related activities, including physical influences on blood flow, secretion of locally acting chemical mediators that influence the contractile state of the vessel's overlying smooth muscle, and assistance with capillary permeability. In addition, their smooth luminal surface facilitates efficient blood flow by reducing surface friction.

The second component of the tunica intima is a basement membrane deep to the endothelium. It provides a physical support base for the epithelial layer. Its framework of collagen fibers affords the basement membrane significant tensile strength, yet its properties also provide resilience for stretching and recoil.

The basement membrane anchors the endothelium to the underlying connective tissue while also regulating molecular movement. It appears to play an important role in guiding cell movements during tissue repair of blood vessel walls. The outermost part of the tunica intima, which forms the boundary between the tunica intima and tunica media, is the internal elastic membrane.

The internal elastic membrane is a thin sheet of elastic fibers with a variable number of window-like openings that give it the look of Swiss cheese. These openings facilitate diffusion of materials through the tunica intima to the thicker tunica media.

Tunica Media The tunica media (media = middle) is a muscular and connective tissue layer that displays the greatest variation among the different vessel types (Figure 21.1a, b). In most vessels, it is a relatively thick layer comprising mainly smooth muscle cells and substantial amounts of elastic fibers. The primary role of the smooth muscle cells, which extend circularly around the lumen as a ring encircles your finger, is Figure 21.1 Comparative structure of blood vessels. The capillary (c) is enlarged relative to the artery (a) and vein (b).

Arteries carry blood from the heart to tissues; veins carry blood from tissues to the heart. to regulate the diameter of the lumen. An increase in sympathetic stimulation typically stimulates the smooth muscle to contract, squeezing the vessel wall and narrowing the lumen. Such a decrease in the diameter of the lumen of a blood vessel is called vasoconstriction vasokonstrikshun. In contrast, when sympathetic stimulation decreases, or in the presence of certain chemicals (such as nitric oxide, H⁺, and lactic acid) or in response to blood pressure, smooth muscle fibers relax.

The resulting increase in lumen diameter is called vasodilation vasodilashun. As you will learn in more detail shortly, the rate of blood flow through different parts of the body is regulated by the extent of smooth muscle contraction in the walls of particular vessels. Furthermore, the extent of smooth muscle contraction in particular vessel types is crucial in the regulation of blood pressure.

In addition to regulating blood flow and blood-pressure, smooth muscle contracts when a small artery or arteriole is damaged (vascular spasm) to help limit loss of blood through the injured vessel. Smooth muscle cells also help produce the elastic fibers within the tunica media that allow the vessels to stretch and recoil under the applied pressure of the blood.

The tunica media is the most variable of the tunics. As you study the different types of blood vessels in the remainder of this chapter, you will see that the structural differences in this layer account for the many variations in function among the different vessel types. Separating the tunica media from the tunica externa is a network of elastic fibers, the external elastic membrane, which is part of the tunica media.

Tunica Externa The outer covering of a blood vessel, the tunica externa (externa = outermost) or adventitia adventisha, consists of elastic and collagen fibers (Figure 21.1a, b). The tunica externa contains numerous nerves and, especially in larger vessels, tiny blood vessels that supply the tissue of the vessel wall. These small vessels that supply blood to the tissues of the vessel are called vasa vasorum (VÅ-sa va-SÖ-rum; vas = vessel), or vessels to the vessels. They are easily seen on large vessels such as the ay-or-tuh. In addition to the important role of supplying the vessel wall with nerves and self-vessels, the tunica externa helps anchor the vessels to surrounding tissues.

Figure 11.1 summary: This figure consists of a series of micrographs. The top images show transverse sections of an artery and a vein, detailing the structural layers including the tunica externa, tunica media, and tunica intima, along with the internal and external elastic membranes and the endothelium. The bottom images provide high-magnification views of capillaries, showing red blood cells moving through the vessel and leaking out into the surrounding connective tissue. The comparison between the artery and vein reveals that the artery has a significantly thicker muscular wall and a more defined circular shape compared to the vein. The capillary images demonstrate the extreme thinness of the capillary wall, which facilitates the exchange of materials and the movement of red blood cells.

Arteries

Because arteries were found empty at death, in ancient times they were thought to contain only air. The wall of an artery has the three layers of a typical blood vessel, but has a thick muscular-to-elastic tunica media (Figure 21.1a). Due to their plentiful elastic fibers, arteries normally have high compliance, which means that their walls stretch easily or expand without tearing in response to a small increase in pressure.

Elastic Arteries Elastic arteries are the largest arteries in the body, ranging from the garden hose-sized ay-or-tuh and pulmonary trunk to the finger-sized branches of the ay-or-tuh. They have the largest diameter among arteries, but their vessel walls (approximately one-tenth of the vessel's total diameter) are relatively thin compared with the overall size of the vessel. These vessels are characterized by well-defined internal and external elastic membrane, along with a thick tunica media that is dominated by elastic fibers, called elastic lamellae lamelle little plates).

Elastic arteries include the two major trunks that exit the heart (the ay-or-tuh and the pulmonary trunk), along with the ay-or-tuh's major initial branches, such as the brachiocephalic, subclavian, common carotid, and common iliac arteries (see Figure 21.20a). Elastic arteries perform an important function: They help propel blood onward while the ventricles are relaxing. As blood is ejected from the heart into elastic arteries, their walls stretch, easily accommodating the surge of blood. As they stretch, the elastic fibers momentarily store mechanical energy, functioning as a pressure reservoir rezervwar (Figure 21.2a). Then, the elastic fibers recoil and convert stored (potential) energy in the vessel into kinetic energy of the blood. Thus, blood continues to move through the arteries even while the ventricles are relaxed (Figure 21.2b). Because they conduct Q In atherosclerosis, the walls of elastic arteries become less compliant (stiffer). What effect does reduced compliance have on the pressure reservoir function of arteries? blood from the heart to medium-sized, more muscular arteries, elastic arteries also are called conducting arteries.

Muscular Arteries Medium-sized arteries are called muscular arteries because their tunica media contains more smooth muscle and fewer elastic fibers than elastic arteries. The large amount of smooth muscle, approximately three-quarters of the total mass, makes the walls of muscular arteries relatively thick. Thus, muscular arteries are capable of greater vasoconstriction and vasodilation to adjust the rate of blood flow.

Muscular arteries have a well-defined internal elastic membrane but a thin external elastic membrane. These two elastic membranes form the inner and outer boundaries of the muscular tunica media. In large arteries, the thick tunica media can have as many as 40 layers of circumferentially arranged smooth muscle fibers; in smaller arteries there are as few as three layers.

Muscular arteries span a range of sizes from the pencilsized femoral and axillary arteries to string-sized arteries that enter organs, measuring as little as 0.5 millimeters (1/64 inch) in diameter. Compared to elastic arteries, the vessel wall of muscular arteries comprises a larger percentage (25%) of the total vessel diameter. Because the muscular arteries continue to branch and ultimately distribute blood to each of the various organs, they are called distributing arteries. Examples include the brachial artery in the arm and radial artery in the forearm (see Figure 21.20a).

The tunica externa is often thicker than the tunica media in muscular arteries. This outer layer contains fibroblasts, collagen fibers, and elastic fibers, all oriented longitudinally. The loose structure of this layer permits changes in the diameter of the vessel to take place but also prevents shortening or retraction of the vessel when it is cut.

Because of the reduced amount of elastic tissue in the walls of muscular arteries, these vessels do not have the ability to recoil and help propel the blood like the elastic arteries. Instead, the thick, muscular tunica media is primarily responsible for the functions of the muscular arteries. The ability of the muscle to contract and maintain a state of partial contraction is referred to as vascular tone. Vascular tone stiffens the vessel wall and is important in maintaining vessel pressure and efficient blood flow.

Figure 21.2 summary: This figure is a series of anatomical diagrams. The diagrams illustrate the pressure reservoir function of the elastic arteries during the cardiac cycle, specifically focusing on the interaction between the left ventricle and the aorta. The first part shows the left ventricle contracting during systole to eject blood into the aorta and elastic arteries, causing them to expand. The second part depicts the left ventricle relaxing during diastole, which allows the elastic aorta and arteries to recoil. This recoil ensures that blood continues to flow toward the capillaries even when the heart is not actively pumping, maintaining a continuous flow of blood through the systemic circulation.

Anastomoses

Most tissues of the body receive blood from more than one artery. The union of the branches of two or more arteries supplying the same body region is called an anastomosis (a-nas'-to-Mo-sis = connecting; plural is anastomoses) (see Figure 21.22c). Anastomoses between arteries provide alternative routes for blood to reach a tissue or organ. If blood flow stops for a short time when normal movements compress a vessel, or if a vessel is blocked by disease, injury, or surgery, then circulation to a part of the body is not necessarily stopped. The alternative route of blood flow to a body part through an anastomosis is known as collateral circulation.

Anastomoses may also occur between veins and between arterioles and venules. Arteries that do not anastomose are known as end arteries. Obstruction of an end artery interrupts the blood supply to a whole segment of an organ, producing necrosis (death) of that segment.

Alternative blood routes may also be provided by nonanastomosing vessels that supply the same region of the body.

Arterioles

Literally meaning small arteries, arterioles are abundant microscopic vessels that regulate the flow of blood into the capillary networks of the body's tissues (see Figure 21.3). The approximately 400 million arterioles have diameters that range in size from 15 mu m to 300 mu m. The wall thickness of arterioles is one-half of the total vessel diameter.

Figure 21.3 summary: This figure consists of two anatomical diagrams illustrating blood flow patterns within a capillary bed. The diagrams show the pathway of blood from the heart through an arteriole and metarteriole, passing through a network of capillaries and a thoroughfare channel, before exiting via a postcapillary venule and muscular venule back toward the heart. In the first scenario, the precapillary sphincters are relaxed, allowing blood to distribute widely across the entire capillary bed. In the second scenario, these sphincters are contracted, which restricts blood flow from entering the smaller capillaries and redirects it primarily through the thoroughfare channel. It can be inferred that the contraction and relaxation of precapillary sphincters serve as a regulatory mechanism to control the volume of blood perfusing the capillary network. When sphincters are relaxed, perfusion is maximized for nutrient and gas exchange; when they are contracted, blood bypasses the exchange vessels to move more directly through the system.

Arterioles have a thin tunica intima with a thin, fenestrated (with small pores) internal elastic membrane that disappears at the terminal end. The tunica media consists of one to two layers of smooth muscle cells having a circular orientation in the vessel wall. The terminal end of the arteriole, the region called the metarteriole (met'-ar-TÉR-ê-ôl; meta = after), tapers toward the capillary junction. At the metarteriole-capillary junction, the distal-most muscle cell forms the precapillary sphincter sfinkter = to bind tight), which monitors the blood flow into the capillary; the other muscle cells in the arteriole regulate the resistance (opposition) to blood flow (see Figure 21.3).

The tunica externa of the arteriole consists of areolar connective tissue containing abundant un-my-elin-ay-ted sympathetic nerves. This sympathetic nerve supply, along with the actions of local chemical mediators, can alter the diameter of arterioles and thus vary the rate of blood flow and resistance through these vessels.

Arterioles play a key role in regulating blood flow from arteries into capillaries by regulating resistance, the opposition to blood flow due to friction between blood and the walls of blood vessels. Because of this they are known as resistance vessels. In a blood vessel, resistance is due mainly to friction between blood and the inner walls of blood vessels.

When the blood vessel diameter is smaller, the friction is greater, so there is more resistance. Contraction of the smooth muscle of an arteriole causes vasoconstriction, which increases resistance even more and decreases blood flow into capillaries supplied by that arteriole. By contrast, relaxation of the smooth muscle of an arteriole causes vasodilation, which decreases resistance and increases blood flow into capillaries.

A change in arteriole diameter can also affect blood pressure: Vasoconstriction of arterioles increases blood pressure, and vasodilation of arterioles decreases blood pressure.

Capillaries

, the smallest of blood vessels, have diameters of 5 to 10 mu m and form the U-turns that connect the arterial outflow to the venous return (Figure 21.3). Since red blood cells have a diameter of 8 mu m, they must often fold on themselves in order to pass single file through the lumens of these vessels. Capillaries form an extensive network, approximately Figure 21.3 Arterioles, capillaries, and venules. Precapillary sphincters regulate the flow of blood through capillary beds.

In capillaries, nutrients, gases, and wastes are exchanged between the blood and interstitial fluid.

20 billion in number, of short (hundreds of micrometers in length), branched interconnecting vessels that course among the individual cells of the body. This network forms an enormous surface area to make contact with the body's cells. The flow of blood from a metarteriole through capillaries and into a postcapillary venule (venule that receives blood from a capillary) is called the microcirculation (micro = small) of the body.

The primary function of capillaries is the exchange of substances between the blood and interstitial fluid. Because of this, these thin-walled vessels are referred to as exchange vessels.

Capillaries are found near almost every cell in the body, but their number varies with the metabolic activity of the tissue they serve. Body tissues with high metabolic requirements, such as muscles, the brain, the liver, the kidneys, and the nervous system, use more O 2 and nutrients and thus have extensive capillary networks. Tissues with lower metabolic requirements, such as tendons and ligaments, contain fewer capillaries. Capillaries are absent in a few tissues, such as all covering and lining epithelia, the cornea and lens of the eye, and cartilage.

The structure of capillaries is well suited to their function as exchange vessels because they lack both a tunica media and a tunica externa. Since capillary walls are composed of only a single layer of endothelial cells (see Figure 21.1e) and a basement membrane, a substance in the blood must pass through just one cell layer to reach the interstitial fluid and tissue cells. Exchange of materials occurs only through the walls of capillaries and the beginning of venules; the walls of arteries, arterioles, most venules, and veins present too thick a barrier. Capillaries form extensive branching networks that increase the surface area available for rapid exchange of materials. In most tissues, blood flows through only a small part of the capillary network when metabolic needs are low. However, when a tissue is active, such as contracting muscle, the entire capillary network fills with blood.

Throughout the body, capillaries function as part of a capillary bed (Figure 21.3), a network of 10 to 100 capillaries that arises from a single metarteriole. In most parts of the body, blood can flow through a capillary network from an arteriole into a venule as follows:

1. Capillaries. In this route, blood flows from an arteriole into capillaries and then into venules (postcapillary venules). As noted earlier, at the junctions between the metarteriole and the capillaries are rings of smooth muscle

fibers called precapillary sphincters that control the flow of blood through the capillaries. When the precapillary sphincters are relaxed (open), blood flows into the capillaries (Figure 21.3a); when precapillary sphincters contract (close or partially close), blood flow through the capillaries ceases or decreases (Figure 21.3b). Typically, blood flows intermittently through capillaries due to alternating contraction and relaxation of the smooth muscle of metarterioles and the precapillary sphincters. This intermittent contraction and relaxation, which may occur 5 to 10 times per minute, is called vasomotion (va-so-Mo-shun). In part, vasomotion is due to chemicals released by the endothelial cells; nitric oxide is one example. At any given time, blood flows through only about 25% of the capillaries.

2. Thoroughfare channel. The proximal end of a metarteriole is surrounded by scattered smooth muscle fibers whose contraction and relaxation help regulate blood flow. The distal end of the vessel has no smooth muscle; it resembles a capillary and is called a thoroughfare channel. Such a channel provides a direct route for blood from an arteriole to a venule, thus bypassing capillaries.

The body contains three different types of capillaries: continuous capillaries, fenestrated capillaries, and sinusoids (Figure 21.4). Most capillaries are continuous capillaries, in which the plasma membranes of endothelial cells form a continuous tube that is interrupted only by intercellular clefts, gaps between neighboring endothelial cells (Figure 21.4a). Continuous capillaries are found in the central nervous system, lungs, muscle tissue, and the skin.

Figure 21.4 summary: This figure consists of three anatomical diagrams illustrating different types of capillaries. The diagrams depict the structural organization of the vessel wall, highlighting components such as the lumen, endothelial cell nuclei, basement membranes, and intercellular clefts. The first diagram shows a continuous capillary where the endothelial cells form a complete lining. The second diagram illustrates a fenestrated capillary, which is characterized by the presence of small pores or fenestrations within the endothelial cells. The third diagram displays a sinusoid, which features a highly permeable structure with an incomplete basement membrane and larger gaps between cells. These variations in structure indicate a progression in permeability, with continuous capillaries being the least permeable and sinusoids being the most permeable, allowing for the passage of larger molecules and cells.

Other capillaries of the body are fenestrated capillaries fenestrated; fenestr-= window). The plasma membranes of the endothelial cells in these capillaries have many fenestrations fenestrashuns, small pores (holes) ranging from 70 to 100 nm in diameter (Figure 21.4b). Fenestrated capillaries are found in the kidneys, intestinal villi, choroid plexuses of the ventricles in the brain, ciliary processes of the eyes, and most endocrine glands.

Sinusoids (Si-nu-soyds; sinus = curve) are wider and more winding than other capillaries. Their endothelial cells may have unusually large fenestrations. In addition to having an incomplete or absent basement membrane (Figure 21.4c), sinusoids have very large intercellular clefts that allow proteins and in some cases even blood cells to pass from a tissue into the bloodstream.

For example, newly formed blood cells enter the bloodstream through the sinusoids of red bone marrow. In addition, sinusoids contain specialized lining cells that are adapted to the function of the tissue. Sinusoids in the liver, for example, contain phagocytic cells that remove bacteria and other debris from the blood.

The spleen, anterior pituitary, and parathyroid and suprarenal glands also have sinusoids.

Usually blood passes from the heart and then in sequence through arteries, arterioles, capillaries, venules, and veins and then back to the heart. In some parts of the body, however, blood passes from one capillary network into another through a vein called a portal vein. Such a circulation of blood is called a portal system. The name of the portal system gives the name

Figure 21.4 Types of Capillaries.

Capillaries are microscopic blood vessels that connect arterioles and venules. of the second capillary location. For example, there are portal systems associated with the liver (hepatic portal circulation; see Figure 21.29) and the pituitary gland (hypophysical portal system; see Figure 18.5).

Figure 21.29 summary: This figure consists of an anatomical illustration and a corresponding flow chart. The anatomical diagram displays the venous network of the abdominal cavity, specifically highlighting the vessels that converge into the hepatic portal vein from organs such as the stomach, spleen, pancreas, and intestines. The flow chart maps the systemic circulation path, showing the movement of blood from the heart through the abdominal aorta and proper hepatic artery to the liver, and the return path from various mesenteric and splenic tributaries through the hepatic portal vein, liver, hepatic veins, and finally into the inferior vena cava. The figure demonstrates that the hepatic portal system acts as a critical intermediary, collecting nutrient-rich blood from the digestive tract and directing it to the liver for processing before it enters the general systemic circulation.

Venules

Unlike their thick-walled arterial counterparts, venues and veins have thin walls that do not readily maintain their shape. Venules drain the capillary blood and begin the return flow of blood back toward the heart (see Figure 21.3).

As noted earlier, venules that initially receive blood from capillaries are called postcapillary venules. They are the smallest venules, measuring 10 mu m to 50 mu m in diameter, and have loosely organized intercellular junctions (the weakest endothelial contacts encountered along the entire vascular tree) and thus are very porous. They function as significant sites of exchange of nutrients and wastes and white blood cell emigration, and for this reason form part of the microcirculatory exchange unit along with the capillaries.

As the postcapillary venules move away from capillaries, they acquire one or two layers of circularly arranged smooth muscle fibers. These muscular venules (50 mu m to 200 mu m) have thicker walls across which exchanges with the interstitial fluid can no longer occur. The thin walls of the postcapillary and muscular venules are the most distensible elements of the vascular system; this allows them to expand and serve as excellent reservoirs for accumulating large volumes of blood. Blood volume increases of 360% have been measured in the postcapillary and muscular venules.

Veins

While veins do show structural changes as they increase in size from small to medium to large, the structural changes are not as distinct as they are in arteries. Veins, in general, have very thin walls relative to their total diameter (average thickness is less than one-tenth of the vessel diameter). They range in size from 0.5 millimeters in diameter for small veins to 3 centimeters in the large superior and interior venae cavae entering the heart.

Although veins are composed of essentially the same three layers as arteries, the relative thicknesses of the layers are different. The tunica intima of veins is thinner than that of arteries; the tunica media of veins is much thinner than in arteries, with relatively little smooth muscle and elastic fibers. The tunica externa of veins is the thickest layer and consists of collagen and elastic fibers.

Veins lack the internal or external elastic membrane found in arteries (see Figure 21.1b). They are distensible enough to adapt to variations in the volume and pressure of blood passing through them, but are not designed to withstand high pressure. The lumen of a vein is larger than that of a comparable artery, and veins often appear collapsed (flattened) when sectioned.

The pumping action of the heart is a major factor in moving venous blood back to the heart. The contraction of skeletal muscles in the lower limbs also helps boost venous return to the heart (see Figure 21.9). The average blood pressure in veins is considerably lower than in arteries. The difference in pressure can be noticed when blood flows from a cut vessel. Blood leaves a cut vein in an even, slow flow but spurts rapidly from a cut artery.

Figure 21.9 summary: This is a diagram illustrating the skeletal muscle pump mechanism in the lower leg. The figure depicts three sequential stages of muscle action on a vein containing proximal and distal valves. In the first stage, the muscle is relaxed and blood flows upward. In the second stage, muscle contraction compresses the vein, pushing blood upward through the proximal valve while the distal valve closes to prevent backflow. In the third stage, the muscle relaxes again, allowing the proximal valve to close and the distal valve to open, which refills the vein from below. The sequence demonstrates how rhythmic muscle contractions act as a secondary pump to facilitate venous return toward the heart by ensuring unidirectional blood flow.

Most of the structural differences between arteries and veins reflect this pressure difference. For example, the walls of veins are not as strong as those of arteries. (Figure 21.5). The low blood pressure in veins allows blood returning to the heart to slow and even back up; the valves aid in venous return by preventing the backflow of blood.

Figure 21.5 summary: This figure consists of anatomical photographs and diagrams. It displays different views of a vein valve, including a transverse section from a superior perspective and a longitudinal coronal section. The images highlight the cusps of the valve, which are structural folds within the vein. The figure demonstrates that the valve cusps are positioned to allow blood to flow in one direction toward the heart while preventing backward flow.

Many veins, especially those in the limbs, also contain valves, thin folds of tunica intima that form flaplike cusps. The valve cusps project into the lumen, pointing toward the heart. A vascular (venous) sinus is a vein with a thin endothelial wall that has no smooth muscle to alter its diameter.

In a vascular sinus, the surrounding dense connective tissue replaces the tunica media and tunica externa in providing support. For example, dural venous sinuses, which are supported by the dura mater, convey deoxygenated blood from the brain to the heart. Another example of a vascular sinus is the coronary sinus of the heart (see Figure 20.3c).

While veins follow paths similar to those of their arterial counterparts, they differ from arteries in a number of ways, aside from the structures of their walls. First, veins are more numerous than arteries for several reasons. Some veins are paired and accompany medium-to small-sized muscular arteries.

These double sets of veins escort the arteries and connect with one another via venous channels called anastomotic veins anastomotik. The anastomotic veins cross the accompanying artery to form ladderlike rungs between the paired veins (see Figure 21.26c). The greatest number of paired veins occurs within the limbs. The subcutaneous tissue deep to the skin is another source of veins. These

Figure 21.5 Venous Valves.

Varicose Veins and Spider Veins

Weak or damaged venous valves (incompetent valves) can cause veins to become dilated and twisted in appearance. These are called varicose veins varikos or varicosities (varic = a swollen vein). The condition may occur in the veins of almost any body part, but it is most common in the superficial veins of the lower limbs, anal canal, and esophagus. Varicose veins in the lower limbs can range from cosmetic problems to serious medical conditions.

The valvular defect may be congenital, or it may result from mechanical strain (pregnancy or prolonged standing) or aging. The leaking venous valves allow the backflow of blood from the deep veins to the less efficient superficial veins, where the blood pools. This creates pressure that distends the vein and allows fluid to leak into surrounding tissues. As a result, the affected vein and the tissue around it may become inflamed and painfully tender.

Veins close to the surface of the legs, especially the saphenous vein (see Section 21.19), are highly susceptible to varicosities; deeper veins are not as vulnerable because surrounding skeletal muscles prevent their walls from stretching excessively. Varicose veins in the anal canal are referred to as hemorrhoids hemoroids. Esophageal varices result from dilated veins in the walls of the lower part of the esophagus and sometimes the upper part of the stomach. Bleeding esophageal varices are life threatening and are usually a result of chronic liver disease.

Varicose veins may be congenital or related to a mechanical stress, such as pregnancy. There is an increased blood flow to the uterus to support the developing fetus, but decreased blood flow from the lower limbs back to the heart. The enlarged uterus may also compress the inferior vena cava. Varicose veins may also be caused by aging. As people get older, their veins lose elasticity and valves become incompetent.

Females are more likely to develop varicose veins, probably as a result of hormonal changes during premenstruation, pregnancy, menopause, or hormone replacement therapy. Heredity, obesity, and sitting or standing for a long period of time are also contributing factors. Several treatment options for varicose veins in the lower limbs are available.

Lifestyle changes can prevent varicose veins from getting worse, reduce pain, and delay the onset of developing new diseased veins. These changes include losing weight if needed, avoiding sitting or standing for long periods, elevating the limbs when sitting or sleeping, and becoming physically active. Compression stockings can be used to apply pressure to the limbs to keep blood from pooling and reduce swelling.

There are also several surgical options to either close off or remove varicose veins. Among the surgical options are the following:

• Sclerotherapy sklerotherapê, the injection of a solution into a varicose vein, damaging the tunica intima of the vein and causing scarring that occludes the vein.

- Endovenous laser ablation evla, application of intense light energy that damages the tunica intima; the subsequent scarring leads to the occlusion of the vein.

• Endovenous radiofrequency ablation evra, the application of radio-frequency energy to heat and destroy the tunica intima, producing scarring that leads to occlusion of the vein.

• Ambulatory phlebectomy filebektomê, the surgical removal of superficial varicose veins through slit-like incisions in the skin.

• Stripping, the surgical removal of part or the entire length of the great saphenous vein in which a flexible wire is threaded through the vein and then pulled out to strip (remove) it from the body.

Spider veins are dilated venules that are close to the skin, especially in the lower limbs and face. They appear as red, blue, or purple patterns that resemble a spider web (for which they are named) or the branches of a tree. Spider veins do not usually pose any health concern, but many people have them treated for cosmetic reasons. Contributing factors involved in the development of spider veins are heredity, age, sitting or standing for prolonged periods, pregnancy, hormonal changes, and gender (spider veins are twice as common in females). Treatment involves sclerotherapy and lasers. veins, called superficial veins, course through the subcutaneous tissue unaccompanied by parallel arteries. Along their course, the superficial veins form small connections (anastomoses) with the deep veins that travel between the skeletal muscles. These connections allow communication between the deep and superficial flow of blood. The amount of blood flow through superficial veins varies from location to location within the body.

Image summary: This figure consists of two anatomical diagrams and a clinical photograph. The diagrams compare the internal structure of a normal vein with that of a varicose vein, while the photograph shows the external appearance of varicose veins on a human leg. In the normal vein, the valve is closed to ensure blood flows in a single direction. In contrast, the varicose vein features an incompetent valve that allows blood to flow backward, leading to a dilated and bulging vessel. The accompanying photograph confirms that this internal failure results in a twisted and protruding appearance of the veins beneath the skin. It can be concluded that valve incompetence leads to blood pooling and the structural deformation characteristic of varicose veins.

In the upper limb, the superficial veins are much larger than the deep veins and serve as the major pathways from the capillaries of the upper limb back to the heart. In the lower limb, the opposite is true; the deep veins serve as the principal return pathways. In fact, one-way valves in small anastomosing vessels allow blood to pass from the superficial veins to the deep veins, but prevent the blood from passing in the reverse direction.

This design has important implications in the development of varicose veins. In some individuals the superficial veins can be seen as blue-colored tubes passing under the skin. While the venous blood is a deep dark red, the veins appear blue because their thin walls and the tissues of the skin absorb the red-light wavelengths, allowing the blue light to pass through the surface to our eyes where we see them as blue.

A summary of the distinguishing features of blood vessels is presented in Table 21.1.

Table 21.1 summary: This table compares the structural characteristics and physiological roles of various blood vessels, ranging from large arteries to small veins. It illustrates a progression in vessel size and layer composition, noting that larger arteries possess thick, specialized layers like the tunica media to facilitate blood conduction and distribution. As vessels decrease in size toward capillaries, the middle and outer layers diminish or disappear entirely to allow for nutrient and waste exchange. Conversely, in the venous system, the vessel walls become relatively thinner compared to arteries, with the outermost layer becoming the most prominent in larger veins, which serve to return blood to the heart.

Blood Distribution

The largest portion of your blood volume at rest—about 64%—is in systemic veins and venules (Figure 21.6). Systemic arteries and arterioles hold about 13% of the blood volume, systemic capillaries hold about 7%, pulmonary blood vessels

Figure 21.6 summary: This figure is a pie chart. It illustrates the distribution of total blood volume across various components of the circulatory system, including systemic veins and venules, systemic arteries and arterioles, pulmonary vessels, the heart, and systemic capillaries. The vast majority of blood volume is contained within the systemic veins and venules, which act as blood reservoirs. Systemic arteries and arterioles hold a smaller portion, while pulmonary vessels, the heart, and systemic capillaries contain the least amount of blood volume.

Figure 21.6 Blood Distribution in the Cardiovascular System at Rest.

Because systemic veins and venules contain more than half of the total blood volume, they are called blood reservoirs. hold about 9%, and the heart holds about 7%. Because systemic veins and venules contain a large percentage of the blood volume, they function as blood reservoirs from which blood can be diverted quickly if the need arises. For example, during increased muscular activity, the cardiovascular center in the brain stem sends a larger number of sympathetic impulses to veins. The result is venoconstriction, constriction of veins, which reduces the volume of blood in reservoirs and allows a greater blood volume to flow to skeletal muscles, where it is needed most. A similar mechanism operates in cases of hemorrhage, when blood volume and pressure decrease; in this case, venoconstriction helps counteract the drop in blood pressure. Among the principal blood reservoirs are the veins of the abdominal organs (especially the liver and spleen) and the veins of the skin.

21.2 Capillary Exchange

Objective

• Discuss the pressures that cause movement of fluids between capillaries and interstitial spaces.

The mission of the entire cardiovascular system is to keep blood flowing through capillaries to allow capillary exchange, the movement of substances between blood and interstitial fluid. The 7% of the blood in systemic capillaries at any given time is continually exchanging materials with interstitial fluid. Substances enter and leave capillaries by three basic mechanisms: diffusion, transcytosis, and bulk flow.

Diffusion

The most important method of capillary exchange is simple diffusion. Many substances, such as oxygen (O 2) , carbon dioxide (C-O 2) , glucose, amino acids, and hormones, enter and leave capillaries by simple diffusion. Because O 2 and nutrients normally are present in higher concentrations in blood, they diffuse down their concentration gradients into interstitial fluid and then into body cells. C-O 2 and other wastes released by body cells are present in higher concentrations in interstitial fluid, so they diffuse into blood.

Substances in blood or interstitial fluid can cross the walls of a capillary by diffusing through the intercellular clefts or fenestrations or by diffusing through the endothelial cells (see Figure 21.4). Water-soluble substances such as glucose and amino acids pass across capillary walls through intercellular clefts or fenestrations. Lipid-soluble materials, such as O 2 , C-O 2 , and steroid hormones, may pass across capillary walls directly through the lipid bilayer of endothelial cell plasma membranes. Most plasma proteins and red blood cells cannot pass through capillary walls of continuous and fenestrated capillaries because they are too large to fit through the intercellular clefts and fenestrations.

In sinusoids, however, the intercellular clefts are so large that they allow even proteins and blood cells to pass through their walls. For example, hepatocytes (liver cells) synthesize and release many plasma proteins, such as fibrinogen (the main clotting protein) and albumin, which then diffuse into the bloodstream through sinusoids. In red bone marrow, blood cells are formed (hemopoiesis) and then enter the bloodstream through sinusoids.

In contrast to sinusoids, the capillaries of the brain allow only a few substances to move across their walls. Most areas of the brain contain continuous capillaries; however, these capillaries are very “tight.” The endothelial cells of most brain capillaries are sealed together by tight junctions. The resulting blockade to movement of materials into and out of brain capillaries is known as the blood–brain barrier (see Section 14.1).

In brain areas that lack the blood-brain barrier, for example, the hypothalamus, pineal gland, and pituitary gland, materials undergo capillary exchange more freely.

Transcytosis

A small quantity of material crosses capillary walls by transcytosis (tranz'-si-To-sis; trans-= across; -cyt-= cell; -osis = process). In this process, substances in blood plasma become enclosed within tiny pinocytic vesicles that first enter endothelial cells by endocytosis, then move across the cell and exit on the other side by exocytosis. This method of transport is important mainly for large, lipid-insoluble molecules that cannot cross capillary walls in any other way. For example, the hormone insulin (a small protein) enters the bloodstream by transcytosis, and certain antibodies (also proteins) pass from the maternal circulation into the fetal circulation by transcytosis.

Bulk Flow: Filtration and Reabsorption

Bulk flow is a passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction. The substances move at rates far greater than can be accounted for by diffusion alone. Bulk flow occurs from an area of higher pressure to an area of lower pressure, and it continues as long as a pressure difference exists.

Diffusion is more important for solute exchange between blood and interstitial fluid, but bulk flow is more important for regulation of the relative volumes of blood and interstitial fluid. Pressure-driven movement of fluid and solutes from blood capillaries into interstitial fluid is called filtration. Pressure-driven movement from interstitial fluid into blood capillaries is called reabsorption.

Two pressures promote filtration: blood hydrostatic pressure (B.H.P), the pressure generated by the pumping action of the heart, and interstitial fluid osmotic pressure interstishal. The main pressure promoting reabsorption of fluid is blood colloid osmotic pressure. The balance of these pressures, called net filtration pressure N.F.P, determines whether the volumes of blood and interstitial fluid remain steady or change. Overall, the volume of fluid and solutes reabsorbed normally is almost as large as the volume filtered. This near equilibrium is known as Starling's law of the capillaries. Let's see how these hydrostatic and osmotic pressures balance.

Within vessels, the hydrostatic pressure is due to the pressure that water in blood plasma exerts against blood vessel walls. The blood hydrostatic pressure (B.H.P) is about 35 millimeters of mercury (mmHg) at the arterial end of a capillary, and about 16 mmHg at the capillary's venous end (Figure 21.7). B.H.P "pushes" fluid out of capillaries into interstitial fluid. The opposing pressure of the interstitial fluid, called interstitial fluid hydrostatic pressure (I.F.H.P), "pushes" fluid from interstitial spaces back into capillaries. However, I.F.H.P is close to zero. (I.F.H.P is difficult to measure, and its reported values vary from small positive values to small negative values.) For our discussion we assume that I.F.H.P equals 0 mmHg all along the capillaries.

Figure 21.7 summary: This figure is a schematic diagram illustrating the physiological process of fluid exchange between blood capillaries and the surrounding interstitial space. The diagram depicts a blood capillary receiving blood from an arteriole and draining into a venule, surrounded by tissue cells and a lymphatic capillary. It details the various pressures acting on the fluid, including blood hydrostatic pressure, blood colloid osmotic pressure, interstitial fluid hydrostatic pressure, and interstitial fluid osmotic pressure. The figure provides the mathematical relationship for calculating net filtration pressure by contrasting pressures that promote filtration against those that promote reabsorption at both the arterial and venous ends of the capillary. It can be inferred that fluid movement is dynamic and bidirectional along the length of the capillary. At the arterial end, the dominant hydrostatic pressure leads to net filtration, pushing fluid out into the interstitial space. Conversely, at the venous end, the decrease in hydrostatic pressure allows the colloid osmotic pressure to prevail, resulting in net reabsorption of fluid back into the bloodstream. The remaining interstitial fluid is eventually collected by the lymphatic system and returned to the blood plasma.

The difference in osmotic pressure across a capillary wall is due almost entirely to the presence of blood plasma proteins, which are too large to pass through either fenestrations or gaps between endothelial cells. Blood colloid osmotic pressure (B.C.O.P) is a force caused by the colloidal suspension of these large proteins in blood plasma that averages 26 mmHg in most capillaries. The effect of B.C.O.P is to "pull" fluid from interstitial spaces into capillaries.

Opposing B.C.O.P is interstitial fluid osmotic pressure (I.F.O.P), which "pulls" fluid out of capillaries into interstitial fluid. Normally, I.F.O.P is very small—0.1 to 5 mmHg—because only tiny amounts of protein are present in interstitial fluid. The small amount of protein that leaks from blood plasma into interstitial fluid does not accumulate there because it passes into lymph plasma in lymphatic capillaries and is eventually returned to the blood. For discussion, we can use a value of 1 mmHg for I.F.O.P.

Whether fluids leave or enter capillaries depends on the balance of pressures. If the pressures that push fluid out of capillaries exceed the pressures that pull fluid into capillaries, fluid will move from capillaries into interstitial spaces (filtration). If, however, the pressures that push fluid out of interstitial spaces into capillaries exceed the pressures that pull fluid out of capillaries, then fluid will move from interstitial spaces into capillaries (reabsorption).

The net filtration pressure N.F.P, which indicates the direction of fluid movement, is calculated as follows:

Math summary: This calculation determines the net filtration pressure by finding the difference between two groups of forces. It subtracts the sum of pressures promoting reabsorption from the sum of pressures promoting filtration to determine the direction of fluid movement.

At the arterial end of a capillary,

Math summary: This calculation determines the net filtration pressure by subtracting the total inward pressure from the total outward pressure. The process subtracts the sum of inward forces from the sum of outward forces to result in a net pressure of ten millimeters of mercury.

Thus, at the arterial end of a capillary, there is a net outward pressure of 10 mmHg, and fluid moves out of the capillary into interstitial spaces (filtration).

At the venous end of a capillary,

Math summary: This calculation computes the net filtration pressure by subtracting the venous end hydrostatic pressure from the capillary blood pressure. The process subtracts twenty-six millimeters of mercury from seventeen millimeters of mercury to produce a final result of negative nine millimeters of mercury.

At the venous end of a capillary, the negative value (-9 mmHg) represents a net inward pressure, and fluid moves into the capillary from tissue spaces (reabsorption).

On average, about 85% of the fluid filtered out of capillaries is reabsorbed. The excess filtered fluid and the few blood plasma proteins that do escape from blood into interstitial fluid enter lymphatic capillaries (see Figure 22.2). As lymph plasma drains into the junction of the jugular and subclavian veins in the upper thorax (see Figure 22.3), these materials return to the blood. Every day about 20 liters of fluid filter out of capillaries in tissues throughout the body. Of this fluid, 17 liters are reabsorbed and 3 liters enter lymphatic capillaries (excluding filtration during urine formation).

Figure 21.7 Dynamics of capillary exchange (Starling's law of the capillaries). Excess filtered fluid drains into lymphatic capillaries.

Blood hydrostatic pressure pushes fluid out of capillaries (filtration), and blood colloid osmotic pressure pulls fluid into capillaries (reabsorption).

Clinical Connection

Edema

If filtration greatly exceeds reabsorption, the result is edema (e-DÊ-ma = swelling), an abnormal increase in interstitial fluid volume. Edema is not usually detectable in tissues until interstitial fluid volume has risen to 30% above normal. Edema can result from either excess filtration or inadequate reabsorption.

Two situations may cause excess filtration:

• Increased capillary blood pressure causes more fluid to be filtered from capillaries.

• Increased permeability of capillaries raises interstitial fluid osmotic pressure by allowing some blood plasma proteins to escape. Such leakiness may be caused by the destructive effects of chemical, bacterial, thermal, or mechanical agents on capillary walls.

One situation commonly causes inadequate reabsorption:

• Decreased concentration of plasma proteins lowers the blood colloid osmotic pressure. Inadequate synthesis or dietary intake or loss of blood plasma proteins is associated with liver disease, burns, malnutrition (for example, kwashiorkor; see Disorders: Homeostatic Imbalances in Chapter 25), and kidney disease.

6. How can substances enter and leave blood plasma?

7. How do hydrostatic and osmotic pressures determine fluid movement across the walls of capillaries?

8. Define edema and describe how it develops.

21.3 Hemodynamics: Factors Affecting Blood Flow

Objectives

• Explain the factors that regulate the volume of blood flow.

• Explain how blood pressure changes throughout the cardiovascular system.

- Describe the factors that determine mean arterial pressure and systemic vascular resistance.

• Describe the relationship between cross-sectional area and velocity of blood flow.

Hemodynamics hemodinamiks; hemo-= blood; dynamics = power) refers to the forces involved in circulating blood throughout the body. Blood flow is the volume of blood that flows through any tissue in a given time period (in milliliters per minute). Total blood flow is cardiac output (C-O), the volume of blood that circulates through systemic (or pulmonary) blood vessels each minute. In Chapter 20 we saw that cardiac output depends on heart rate and stroke volume: Cardiac output (C-O) = heart rate H.R × stroke volume S.V. How the cardiac output becomes distributed into circulatory routes that serve various body tissues depends on two more factors: (1) the pressure difference that drives the blood flow through a tissue and (2) the resistance to blood flow in specific blood vessels.

Blood flows from regions of higher pressure to regions of lower pressure; the greater the pressure difference, the greater the blood flow. But the higher the resistance, the smaller the blood flow.

Blood Pressure

As you have just learned, blood flows from regions of higher pressure to regions of lower pressure; the greater the pressure difference, the greater the blood flow. Contraction of the ventricles generates blood pressure (B.P), the hydrostatic pressure exerted by blood on the walls of a blood vessel. B.P is determined by cardiac output (see Section 20.5), blood volume, and vascular resistance (described shortly).

B.P is highest in the ay-or-tuh and large systemic arteries; in a resting, young adult, B.P rises to about 110 mmHg during systole (ventricular contraction) and drops to about 70 mmHg during diastole (ventricular relaxation). Systolic blood pressure S.B.P sistolik is the highest pressure attained in arteries during systole, and diastolic blood pressure D.B.P diastolik is the lowest arterial pressure during diastole (Figure 21.8). As blood leaves the ay-or-tuh and flows through the systemic circulation, its pressure falls progressively as the distance from the left ventricle increases. Blood pressure decreases to about 35 mmHg as blood passes from systemic arteries through systemic arterioles and into capillaries, where the pressure fluctuations disappear.

Figure 21.8 summary: This is a line chart. The figure illustrates the change in blood pressure as blood flows through different segments of the circulatory system, starting from the aorta and moving through arteries, arterioles, capillaries, venules, veins, and finally the venae cavae. The blood pressure is highest and most pulsatile in the aorta and arteries, with distinct peaks and troughs representing systolic and diastolic pressure. As blood moves into the arterioles and capillaries, the pressure drops significantly and the pulsations disappear. The pressure continues to decline steadily through the venules and veins, reaching its lowest point in the venae cavae.

At the venous end of capillaries, blood pressure has dropped to about 16 mmHg. Blood pressure continues to drop as blood enters systemic venules and then veins because these vessels are farthest from the left ventricle. Finally, blood pressure reaches 0 mmHg as blood flows into the right ventricle.

Mean arterial pressure (M.A.P), the average blood pressure in arteries, is roughly one-third of the way between the diastolic and systolic pressures. It can be estimated as follows:

Math summary: This calculation determines the mean arterial pressure by taking the diastolic blood pressure and adding a fraction of the difference between the systolic and diastolic pressures. The process subtracts the diastolic value from the systolic value, scales that difference by one third, and adds the result to the initial diastolic input.

Thus, in a person whose B.P is 110/70 mmHg, M.A.P is about 83 mmHg [70 plus 1 third times (110 minus 70)].

We have already seen that cardiac output equals heart rate multiplied by stroke volume. Another way to calculate cardiac output is to divide mean arterial pressure (M.A.P) by resistance (R): C-O = M.A.P ÷ R. By rearranging the terms of this equation, you can see that M.A.P = C-O × R. If cardiac output rises due to an increase in stroke volume or heart rate, then the mean arterial pressure rises as long as resistance remains steady. Likewise, a decrease in cardiac output causes a decrease in mean arterial pressure if resistance does not change.

Blood pressure also depends on the total volume of blood in the cardiovascular system. The normal volume of blood in an adult is about 5 liters (5.3 qt). Any decrease in this volume, as from hemorrhage, decreases the amount of blood that is circulated through the arteries each minute. A modest decrease can be compensated for by homeostatic mechanisms that help maintain blood pressure (described in Section 21.4), but if the decrease in blood volume is greater than 10% of the total, blood pressure drops. Conversely, anything that increases blood volume, such as water retention in the body, tends to increase blood pressure.

Vascular Resistance

As noted earlier, vascular resistance is the opposition to blood flow due to friction between blood and the walls of blood vessels. Vascular resistance depends on (1) size of the blood vessel lumen, (2) blood viscosity, and (3) total blood vessel length.

1. Size of the lumen. The smaller the lumen of a blood vessel, the greater its resistance to blood flow. Resistance is inversely proportional to the fourth power of the diameter (d) of the blood vessel's lumen (R ∝ 1/d⁴). The smaller the diameter of the blood vessel, the greater the resistance it offers to blood flow. For example, if the diameter of a blood vessel decreases by one-half, its resistance to blood flow increases 16 times. Vasoconstriction narrows the lumen, and vasodilation widens it. Normally, moment-to-moment fluctuations in blood flow through a given tissue are due to vasoconstriction and vasodilation of the tissue's arterioles. As arterioles dilate, resistance decreases, and blood pressure falls. As arterioles constrict, resistance increases, and blood pressure rises.

2. Blood viscosity. The viscosity viskositë = thickness) of blood depends mostly on the ratio of red blood cells to blood plasma (fluid) volume, and to a smaller extent on the concentration of proteins in blood plasma. The higher the blood's viscosity, the higher the resistance. Any condition that increases the viscosity of blood, such as dehydration or polycythemia (an unusually high number of red blood cells), thus increases blood pressure. A depletion of blood plasma proteins or red blood cells, due to anemia or hemorrhage, decreases viscosity and thus decreases blood pressure.

3. Total blood vessel length. Resistance to blood flow through a vessel is directly proportional to the length of the blood vessel. The longer a blood vessel, the greater the resistance. Obese people often have hypertension (elevated blood pressure) because the additional blood vessels in their adipose tissue increase their total blood vessel length. An estimated 650 kilometers (about 400 miles) of additional blood vessels develop for each extra kilogram (2.2 pounds) of fat.

Systemic vascular resistance (S.V.R), also known as total peripheral resistance T.P.R, refers to all of the vascular resistances offered by systemic blood vessels. The diameters of arteries and veins are large, so their resistance is very small because most of the blood does not come into physical contact with the walls of the blood vessel. The smallest vessels—arterioles, capillaries, and venules—contribute the most resistance.

A major function of arterioles is to control S.V.R—and therefore blood pressure and blood flow to particular tissues—by changing their diameters. Arterioles need to vasodilate or vasoconstrict only slightly to have a large effect on S.V.R. The main center for regulation of S.V.R is the vasomotor center in the brain stem (described shortly).

Venous Return

Venous return, the volume of blood flowing back to the heart through the systemic veins, occurs due to the pressure generated by contractions of the heart's left ventricle. The pressure difference from venules (averaging about 16 mmHg) to the right ventricle (0 mmHg), although small, normally is sufficient to cause venous return to the heart. If pressure increases in the right atrium or ventricle, venous return will decrease.

One cause of increased pressure in the right atrium is an incompetent (leaky) tricuspid valve, which lets blood regurgitate (flow backward) as the ventricles contract. The result is decreased venous return and buildup of blood on the venous side of the systemic circulation.

When you stand up, for example, at the end of an anatomy and physiology lecture, the pressure pushing blood up the veins in your lower limbs is barely enough to overcome the force of gravity pushing it back down. Besides the heart, two other mechanisms “pump” blood from the lower body back to the heart: (1) the skeletal muscle pump and (2) the respiratory pump. Both pumps depend on the presence of valves in veins.

The skeletal muscle pump operates as follows (Figure 21.9):

- While you are standing at rest, both the venous valve closer to the heart (proximal valve) and the one farther from the heart (distal valve) in this part of the leg are open, and blood flows upward toward the heart.

- ② Contraction of leg muscles, such as when you stand on tiptoes or take a step, compresses the vein. The compression pushes blood through the proximal valve, an action called milking. At the same time, the distal valve in the uncompressed segment of the vein closes as some blood is pushed against it. People who are immobilized through injury or disease lack these contractions of leg muscles. As a result, their venous return is slower and they may develop circulation problems.

Figure 21.9 Action of the skeletal muscle pump in returning blood to the heart.

Milking refers to skeletal muscle contractions that drive venous blood toward the heart.

3 Just after muscle relaxation, pressure falls in the previously compressed section of vein, which causes the proximal valve to close. The distal valve now opens because blood pressure in the foot is higher than in the leg, and the vein fills with blood from the foot. The proximal valve then reopens.

The respiratory pump is also based on alternating compression and decompression of veins. During inhalation, the diaphragm moves downward, which causes a decrease in pressure in the thoracic cavity and an increase in pressure in the abdominal cavity. As a result, abdominal veins are compressed, and a greater volume of blood moves from the compressed abdominal veins into the decompressed thoracic veins and then into the right atrium. When the pressures reverse during exhalation, the valves in the veins prevent backflow of blood from the thoracic veins to the abdominal veins.

Figure 21.10 summarizes the factors that increase blood pressure through increasing cardiac output or systemic vascular resistance.

Figure 21.10 summary: This figure is a flow chart. It illustrates the various physiological mechanisms and factors that contribute to an increase in mean arterial pressure. The chart details how factors such as blood volume, muscle and respiratory pumps, and venoconstriction lead to increased venous return, which in turn increases heart rate and stroke volume, resulting in higher cardiac output. Simultaneously, it shows how an increase in red blood cells, body size, and vasoconstriction lead to higher blood viscosity, longer vessel length, and smaller vessel radii, which collectively increase systemic vascular resistance. The final outcome of both increased cardiac output and increased systemic vascular resistance is an elevation in mean arterial pressure.

Velocity of Blood Flow

Earlier we saw that blood flow is the volume of blood that flows through any tissue in a given time period (in milliliters per minute). The speed or velocity of blood flow (in cm/sec) is inversely related to the cross-sectional area. Velocity is slowest where

Clinical Connection

Syncope

Syncope sinkope, or fainting, is a sudden, temporary loss of consciousness that is not due to head trauma, followed by spontaneous recovery. It is most commonly due to cerebral ischemia, lack of sufficient blood flow to the brain. Syncope may occur for several reasons:

• Vasodepressor syncope is due to sudden emotional stress or real, threatened, or fantasized injury.

• Situational syncope is caused by pressure stress associated with urination, defecation, or severe coughing.

• Drug-induced syncope may be caused by drugs such as antihypertensives, diuretics, vasodilators, and tranquilizers.

• Orthostatic hypotension, an excessive decrease in blood pressure that occurs on standing up, may cause fainting. the total cross-sectional area is greatest (Figure 21.11). Each time an artery branches, the total cross-sectional area of all of its branches is greater than the cross-sectional area of the original vessel, so blood flow becomes slower and slower as blood moves further away from the heart, and is slowest in the capillaries. Conversely, when venules unite to form veins, the total cross-sectional area becomes smaller and flow becomes faster. In an adult, the cross-sectional area of the ay-or-tuh is only 3 to 5 centimeters², and the average velocity of the blood there is 40 centimeters/sec. In capillaries, the total cross-sectional area is 4500 to 6000 centimeters², and the velocity of blood flow is less than 0.1 centimeters/sec. In the two venae cavae combined, the cross-sectional area is about 14 centimeters², and the velocity is about 15 centimeters/sec. Thus, the velocity of blood flow decreases as blood flows from the ay-or-tuh to arteries to arterioles to capillaries, and increases as it leaves capillaries and returns to the heart. The relatively slow rate of flow through capillaries aids the exchange of materials between blood and interstitial fluid.

Figure 21.11 summary: This is a line graph. The figure illustrates the relationship between the total cross-sectional area and the blood flow velocity across different types of blood vessels, ranging from the aorta to the venae cavae. The data shows that as the total cross-sectional area increases, reaching its peak in the capillaries, the velocity of blood flow decreases to its lowest point. Conversely, as the total cross-sectional area decreases in the veins and venae cavae, the blood flow velocity begins to increase again. It can be inferred that blood flow is slowest in the capillaries, which is an adaptation that facilitates efficient exchange of materials between blood and tissues.

Circulation time is the time required for a drop of blood to pass from the right atrium, through the pulmonary circulation, back to the left atrium, through the systemic circulation down to the foot, and back again to the right atrium. In a resting person, circulation time normally is about 1 minute.

Checkpoint

Figure 21.10 Summary of factors that increase blood pressure. Changes noted within green boxes increase cardiac output; changes noted within blue boxes increase systemic vascular resistance.

Increases in cardiac output and increases in systemic vascular resistance will increase mean arterial pressure.

Figure 21.11 Relationship between velocity (speed) of blood flow and total cross-sectional area in different types of blood vessels.

Velocity of blood flow is slowest in the capillaries because they have the largest total cross-sectional area.

21.4 Control of Blood Pressure and Blood Flow

Objective

• Describe how blood pressure is regulated.

Several interconnected negative feedback systems control blood pressure by adjusting heart rate, stroke volume, systemic vascular resistance, and blood volume. Some systems allow rapid adjustments to cope with sudden changes, such as the drop in blood pressure in the brain that occurs when you get out of bed; others act more slowly to provide long-term regulation of blood pressure. The body may also require adjustments to the distribution of blood flow. During exercise, for example, a greater percentage of the total blood flow is diverted to skeletal muscles.

Role of the Cardiovascular Center

In Chapter 20, we noted how the cardiovascular (C.V) center in the medulla oblongata helps regulate heart rate and stroke volume. The C.V center also controls neural, hormonal, and local negative feedback systems that regulate blood pressure and blood flow to specific tissues. Groups of neurons scattered within the C.V center regulate heart rate, contractility (force of contraction) of the ventricles, and blood vessel diameter. Some neurons stimulate the heart (cardiostimulatory center); others inhibit the heart (cardioinhibitory center).

Still others control blood vessel diameter by causing constriction (vasoconstrictor center) or dilation (vasodilator center); these neurons are referred to collectively as the vasomotor center. Because the C.V center neurons communicate with one another, function together, and are not clearly separated anatomically, we discuss them here as a group.

The cardiovascular center receives input both from higher brain regions and from sensory receptors (Figure 21.12). Nerve impulses descend from the cerebral cortex, limbic system, and hypothalamus to affect the cardiovascular center. For example, even before you start to run a race, your heart rate may increase due to nerve impulses conveyed from the limbic system to the C.V center. If your body temperature rises during a race, the hypothalamus sends nerve impulses to the C.V center. The resulting vasodilation of skin blood vessels allows heat to dissipate more rapidly from the surface of the skin. The three main types of sensory receptors that provide input to the cardiovascular center are proprioceptors, baroreceptors, and chemoreceptors.

Figure 21.12 summary: This is a schematic diagram illustrating the neural regulation of the cardiovascular center. The figure depicts the flow of information from various input sources to the cardiovascular center in the brainstem and the subsequent output to target effectors. Inputs originate from higher brain centers, proprioceptors, baroreceptors, and chemoreceptors. The outputs are transmitted via the vagus nerves for parasympathetic control, and the cardiac accelerator and vasomotor nerves for sympathetic control. The diagram indicates that parasympathetic stimulation leads to a decreased heart rate, while sympathetic stimulation results in an increased heart rate, enhanced contractility, and vasoconstriction of blood vessels.

Proprioceptors propreoseptors monitor movements of joints and muscles and provide input to the cardiovascular center during physical activity. Their activity accounts for the rapid increase in heart rate at the beginning of exercise. Baroreceptors baroreseptors monitor changes in pressure and stretch in the walls of blood vessels, and chemoreceptors chemoreceptors monitor the concentration of various chemicals in the blood.

Output from the cardiovascular center flows along sympathetic and parasympathetic neurons of the A.N.S (Figure 21.12). Sympathetic impulses reach the heart via the cardiac accelerator nerves. An increase in sympathetic stimulation increases heart rate and contractility; a decrease in sympathetic stimulation decreases heart rate and contractility. Parasympathetic stimulation, conveyed along the vagus (10) nerves, decreases heart rate. Thus, opposing sympathetic (stimulatory) and parasympathetic (inhibitory) influences control the heart.

The cardiovascular center also continually sends impulses to smooth muscle in blood vessel walls via vasomotor nerves (va-so-Mo-tor). These sympathetic neurons exit the spinal cord through all thoracic and the first one or two lumbar spinal nerves and then pass into the sympathetic trunk ganglia (see Figure 15.2). From there, impulses propagate along sympathetic neurons that innervate blood vessels in viscera and peripheral areas. The vasomotor region of the cardiovascular center continually sends impulses over these routes to arterioles throughout the body, but especially to those in the skin and abdominal viscera. The result is a moderate state of tonic contraction or vasoconstriction, called vasomotor tone, that

Figure 21.12 Location and Function of the Cardiovascular (C.V) Center in the Medulla

oblongata. The C.V center receives input from higher brain centers, proprioceptors, baroreceptors, and chemoreceptors. Then, it provides output to the sympathetic and parasympathetic divisions of the autonomic nervous system (A.N.S).

The cardiovascular center is the main region for nervous system regulation of the heart and blood vessels. sets the resting level of systemic vascular resistance. Sympathetic stimulation of most veins causes constriction that moves blood out of venous blood reservoirs and increases blood pressure.

Neural Regulation of Blood Pressure

The nervous system regulates blood pressure via negative feedback loops that occur as two types of reflexes: baroreceptor reflexes and chemoreceptor reflexes.

Baroreceptor Reflexes Baroreceptors, pressure-sensitive sensory receptors, are located in the ay-or-tuh, internal carotid arteries (arteries in the neck that supply blood to the brain), and other large arteries in the neck and chest. They send impulses to the cardiovascular center to help regulate blood pressure. The two most important baroreceptor reflexes are the carotid sinus reflex and the aortic reflex.

Baroreceptors in the wall of the carotid sinuses initiate the carotid sinus reflex carotid, which helps regulate blood pressure in the brain. The carotid sinuses are small widenings of the right and left internal carotid arteries just above the point where they branch from the common carotid arteries (Figure 21.13). Blood pressure stretches the wall of the carotid sinus, which stimulates the baroreceptors. Nerve impulses propagate from the carotid sinus baroreceptors over sensory axons in the glossopharyngeal (9) nerves glossopharyngeal to the cardiovascular center in the medulla oblongata.

Figure 21.13 summary: This figure is an anatomical diagram illustrating the neural pathways involved in blood pressure regulation. It depicts the connection between pressure-sensitive baroreceptors located in the carotid sinus and aortic arch, the cardiovascular center within the medulla oblongata, and the autonomic nervous system's output to the heart. The diagram shows sensory pathways traveling via the glossopharyngeal and vagus nerves to the brain, and motor pathways returning via the parasympathetic vagus nerves and sympathetic cardiac accelerator nerves to target the SA node, AV node, and ventricular myocardium. The figure demonstrates a negative feedback loop where the autonomic nervous system modulates heart activity based on sensory input from baroreceptors to maintain blood pressure homeostasis.

Baroreceptors in the wall of the ascending ay-or-tuh and aortic arch initiate the aortic reflex, which regulates systemic blood pressure. Nerve impulses from aortic baroreceptors reach the cardiovascular center via sensory axons of the vagus (10) nerves.

When blood pressure falls, the baroreceptors are stretched less, and they send nerve impulses at a slower rate to the cardiovascular center (Figure 21.14). In response, the C.V center decreases parasympathetic stimulation of the heart by way of motor axons of the vagus nerves and increases sympathetic stimulation of the heart via cardiac accelerator nerves. Another consequence of increased sympathetic stimulation is increased secretion of epinephrine and norepinephrine by the suprarenal medulla. As the heart beats faster and more forcefully, and as systemic vascular resistance increases, cardiac output and systemic vascular resistance rise, and blood pressure increases to the normal level. Conversely, when an increase in pressure is detected, the baroreceptors send impulses at a faster rate. The C.V center responds by increasing parasympathetic stimulation and decreasing sympathetic stimulation.

Figure 21.14 summary: This figure is a flowchart illustrating a biological feedback loop. It depicts the process of negative feedback regulation for blood pressure, starting from a stimulus that disrupts homeostasis by lowering blood pressure. The process follows a sequence from receptors, specifically baroreceptors in the carotid sinus and aortic arch, to control centers in the medulla oblongata and suprarenal medulla, and finally to effectors including the heart and blood vessels. The diagram shows that a decrease in blood pressure leads to reduced nerve impulse rates, which triggers increased sympathetic activity and the secretion of epinephrine and norepinephrine. These actions result in an increased heart rate, stroke volume, and blood vessel constriction. The conclusion of this loop is an increase in blood pressure, which serves as a corrective response to return the system to homeostasis.

The resulting decreases in heart rate and force of contraction reduce the cardiac output. The cardiovascular center also slows the rate at which it sends sympathetic impulses along vasomotor neurons that normally cause vasoconstriction. The resulting vasodilation lowers systemic vascular resistance.

Decreased cardiac output and decreased systemic vascular resistance both lower systemic arterial blood pressure to the normal level.

Moving from a prone (lying down) to an erect position decreases blood pressure and blood flow in the head and upper part of the body. The baroreceptor reflexes, however, quickly counteract the drop in pressure. Sometimes these reflexes operate more slowly than normal, especially in the elderly, in which case a person can faint due to reduced brain blood flow after standing up too quickly.

Clinical Connection

Carotid Sinus Massage and Carotid Sinus Syncope

Because the carotid sinus is close to the anterior surface of the neck, it is possible to stimulate the baroreceptors there by putting pressure on the neck. Physicians sometimes use carotid sinus massage, which involves carefully massaging the neck over the carotid sinus, to slow heart rate in a person who has paroxysmal superventricular tachycardia, a type of tachycardia that originates in the atria. Anything that stretches or puts pressure on the carotid sinus, such as hyperextension of the head, tight collars, or carrying heavy shoulder loads, may also slow heart rate and can cause carotid sinus syncope, fainting due to inappropriate stimulation of the carotid sinus baroreceptors.

Chemoreceptor Reflexes Chemoreceptors, sensory receptors that monitor the chemical composition of blood, are located close to the baroreceptors of the carotid sinus and aortic arch in small structures called carotid bodies and aortic bodies, respectively. These chemoreceptors detect changes in blood level of O₂, C-O₂, and H⁺. Hypoxia (lowered O₂ availability), acidosis (an increase in H⁺ concentration), or hypercapnia (excess C-O₂) stimulates the chemoreceptors to send impulses to the cardiovascular center. In response, the C.V center increases sympathetic stimulation to arterioles and veins, producing vasoconstriction and an increase in blood pressure. These chemoreceptors also provide input to the respiratory center in the brain stem to adjust the rate of breathing.

Hormonal Regulation of Blood Pressure

As you learned in Chapter 18, several hormones help regulate blood pressure and blood flow by altering cardiac output, changing systemic vascular resistance, or adjusting the total blood volume:

1. Renin–angiotensin–aldosterone R.A.A system. When blood volume falls or blood flow to the kidneys decreases, juxtaglomerular cells in the kidneys secrete renin into the bloodstream. In sequence, renin and angiotensin-converting enzyme (ace) act on their substrates to produce the active hormone angiotensin I.I angiotensin, which raises blood pressure in two ways. First, angiotensin I.I is a potent vasoconstrictor; it raises blood pressure by increasing systemic vascular resistance. Second, it stimulates secretion of aldosterone, which increases reabsorption of sodium ions (Na⁺) and water by the kidneys. The water reabsorption increases total blood volume, which increases blood pressure. (See Section 21.6.)

2. Epinephrine and norepinephrine. In response to sympathetic stimulation, the suprarenal medulla releases epinephrine and norepinephrine. These hormones increase cardiac output by increasing the rate and force of heart contractions. They also cause vasoconstriction of arterioles and veins in the skin and abdominal organs and vasodilation of arterioles in cardiac and skeletal muscle, which helps increase blood flow to muscle during exercise.

3. Antidiuretic hormone (A.D.H). Antidiuretic hormone (A.D.H) is produced by the hypothalamus and released from the posterior pituitary in response to dehydration or decreased blood volume. Among other actions, A.D.H causes vasoconstriction, which increases blood pressure. For this reason A.D.H is also called vasopressin. A.D.H also promotes movement of water from the lumen of kidney tubules into the bloodstream. This results in an increase in blood volume and a decrease in urine output.

4. Atrial natriuretic peptide A.N.P. Released by cells in the atria of the heart, atrial natriuretic peptide lowers blood pressure by causing vasodilation and by promoting the loss of salt and water in the urine, which reduces blood volume.

Table 21.2 summarizes the regulation of blood pressure by hormones.

Table 21.2 summary: This table outlines how various hormones regulate blood pressure by influencing cardiac output, systemic vascular resistance, and blood volume. Hormones that enhance heart rate, contractility, or cause vasoconstriction and blood volume expansion serve to raise blood pressure. Conversely, hormones that promote vasodilation or reduce blood volume act to lower blood pressure. Certain hormones, such as epinephrine, exhibit complex roles by contributing to both increases and decreases in pressure depending on the specific physiological mechanism triggered.

Autoregulation of Blood Flow

In each capillary bed, local changes can regulate vasomotion. When vasodilators produce local dilation of arterioles and relaxation of precapillary sphincters, blood flow into capillary networks is increased, which increases O 2 level. Vasoconstrictors have the opposite effect. The ability of a tissue to automatically adjust its blood flow to match its metabolic demands is called autoregulation (aw'-tö-reg'-ü-LÄ-shun). In tissues such as the heart and skeletal muscle, where the demand for O 2 and nutrients and for the removal of wastes can increase as much as tenfold during physical activity, autoregulation is an important contributor to increased blood flow through the tissue. Autoregulation also controls regional blood flow in the brain; blood distribution to various parts of the brain changes dramatically for different mental and physical activities. During a conversation, for example, blood flow increases to your motor speech areas when you are talking and increases to the auditory areas when you are listening.

Two general types of stimuli cause autoregulatory changes in blood flow:

1. Physical changes. Warming promotes vasodilation, and cooling causes vasoconstriction. In addition, smooth muscle in arteriole walls exhibits a myogenic response myogenic—it contracts more forcefully when it is stretched and relaxes when stretching lessens. If, for example, blood flow through an arteriole decreases, stretching of the arteriole walls decreases. As a result, the smooth muscle relaxes and produces vasodilation, which increases blood flow.

2. Vasodilating and vasoconstricting chemicals. Several types of cells—including white blood cells, platelets, smooth muscle fibers, macrophages, and endothelial cells—release a wide variety of chemicals that alter blood-vessel diameter. Vasodilating chemicals released by metabolically active tissue cells include K⁺, H⁺, lactic acid, and adenosine (from A.T.P). Another important vasodilator released by endothelial cells is nitric oxide. Tissue trauma or inflammation causes release of vasodilating kinins and histamine. Vasoconstrictors include thromboxane A.2, superoxide radicals, serotonin (from platelets), and endothelins (from endothelial cells).

An important difference between the pulmonary and systemic circulations is their autoregulatory response to changes in O 2 level. The walls of blood vessels in the systemic circulation dilate in response to low O 2 . With vasodilation, O 2 delivery increases, which restores the normal O 2 level. By contrast, the walls of blood vessels in the pulmonary circulation constrict in response to low levels of O 2 . This response ensures that blood mostly bypasses those pulmonary alveoli (air sacs) in the lungs that are poorly ventilated by fresh air. Thus, most blood flows to better-ventilated areas of the lung.

Checkpoint

13. What are the principal inputs to and outputs from the cardiovascular center?

14. Explain the operation of the carotid sinus reflex and the aortic reflex.

15. What is the role of chemoreceptors in the regulation of blood pressure?

16. How do hormones regulate blood pressure?

17. What is autoregulation, and how does it differ in the systemic and pulmonary circulations?

21.5 Checking Circulation

• Define pulse, and systolic, diastolic, and pulse pressures.

Pulse

The alternate expansion and recoil of elastic arteries after each systole of the left ventricle creates a traveling pressure wave that is called the pulse. The pulse is strongest in the arteries closest to the heart, becomes weaker in the arterioles, and disappears altogether in the capillaries. The pulse may be felt in any artery that lies near the surface of the body that can be compressed against a bone or other firm structure. Table 21.3 depicts some common pulse points.

Table 21.3 summary: This table identifies various anatomical pulse points and describes their specific locations throughout the body, covering both the head and neck regions as well as the limbs and trunk.

The pulse rate normally is the same as the heart rate, about 70 to 80 beats per minute at rest. Tachycardia tachycardia; tachy-= fast) is a rapid resting heart or pulse rate over 100 beats/min. Bradycardia bradycardia; brady-= slow) is a slow resting heart or pulse rate under 50 beats/min. Endurance-trained athletes normally exhibit bradycardia.

Measuring Blood Pressure

In clinical use, the term blood pressure usually refers to the pressure in arteries generated by the left ventricle during systole and the pressure remaining in the arteries when the ventricle is in diastole. Blood pressure is usually measured in the brachial artery in the left arm (Table 21.3). The device used to measure blood pressure is a sphygmomanometer sphygmomanometer; sphygmo-= pulse; -manometer = instrument used to measure pressure). It consists of a rubber cuff connected to a rubber bulb that is used to inflate the cuff and a meter that registers the pressure in the cuff. With the arm resting on a table so that it is about the same level as the heart, the cuff of the sphygmomanometer is wrapped around a bared arm.

The cuff is inflated by squeezing the bulb until the brachial artery is compressed and blood flow stops, about 30 mmHg higher than the person's usual systolic pressure. The technician places a stethoscope below the cuff on the brachial artery, and slowly deflates the cuff. When the cuff is deflated enough to allow the artery to open, a spurt of blood passes through, resulting in the first sound heard through the stethoscope.

This sound corresponds to systolic blood pressure S.B.P, the force of blood pressure on arterial walls just after ventricular contraction (Figure 21.15). As the cuff is deflated further, the sounds suddenly become too faint to be heard through the stethoscope. This level, called the diastolic blood pressure D.B.P, represents the force exerted by the blood remaining in arteries during ventricular relaxation. At pressures below diastolic blood pressure, sounds disappear altogether.

Figure 21.15 summary: This figure consists of an anatomical diagram and a line graph. The anatomical diagram illustrates various pulse points across the human body, identifying major arteries in the head, neck, arms, and legs. The line graph depicts the relationship between blood pressure and the pressure within a cuff over time, highlighting the points where systolic and diastolic blood pressures are measured based on audible sounds. The figure demonstrates that blood pressure is measured by gradually reducing cuff pressure until blood flow resumes, with the initial sound marking the peak arterial pressure and the disappearance of sound marking the baseline arterial pressure.

The various sounds that are heard while taking blood pressure are called Korotkoff sounds korotkoff.

The normal blood pressure of an adult male is less than 120 mmHg systolic and less than 80 mmHg diastolic. For example,

Figure 21.15 Relationship of Blood Pressure Changes to Cuff Pressure.

As the cuff is deflated, sounds first occur at the systolic blood pressure; the sounds suddenly become faint at the diastolic blood pressure.

Q If a blood pressure is reported as “142 over 95,” what are the diastolic, systolic, and pulse pressures? Does this person have hypertension as defined in Disorders: Homeostatic Imbalances at the end of the chapter? “110 over 70” (written as 110/70) is a normal blood pressure. In young adult females, the pressures are 8 to 10 mmHg less. People who exercise regularly and are in good physical condition may have even lower blood pressures. Thus, blood pressure slightly lower than 120/80 may be a sign of good health and fitness.

The difference between systolic and diastolic pressure is called pulse pressure. This pressure, normally about 40 mmHg, provides information about the condition of the cardiovascular system. For example, conditions such as atherosclerosis and patent (open) ductus arteriosus greatly increase pulse pressure. The normal ratio of systolic pressure to diastolic pressure to pulse pressure is about 3:2:1.

Checkpoint

21.6 Shock and Homeostasis

Objectives

• Define shock.

• Describe the four types of shock.

• Explain how the body's response to shock is regulated by negative feedback.

Shock is a failure of the cardiovascular system to deliver enough O 2 and nutrients to meet cellular metabolic needs. The causes of shock are many and varied, but all are characterized by inadequate blood flow to body tissues. With inadequate oxygen delivery, cells switch from aerobic to anaerobic production of A.T.P, and lactic acid accumulates in body fluids. If shock persists, cells and organs become damaged, and cells may die unless proper treatment begins quickly.

Types of Shock

Shock can be of four different types: (1) hypovolemic shock hypovolemic; hypo-= low; -volemic = volume) due to decreased blood volume, (2) cardiogenic shock cardiogenic due to poor heart function, (3) vascular shock due to inappropriate vasodilation, and (4) obstructive shock due to obstruction of blood flow.

A common cause of hypovolemic shock is acute (sudden) hemorrhage. The blood loss may be external, as occurs in trauma, or internal, as in rupture of an aortic aneurysm. Loss of body fluids through excessive sweating, diarrhea, or vomiting also can cause hypovolemic shock. Other conditions—for instance, diabetes mellitus—may cause excessive loss of fluid in the urine. Sometimes, hypovolemic shock is due to inadequate intake of fluid. Whatever the cause, when the volume of body fluids falls, venous return to the heart declines, filling of the heart lessens, stroke volume decreases, and cardiac output decreases. Replacing fluid volume as quickly as possible is essential in managing hypovolemic shock.

In cardiogenic shock, the heart fails to pump adequately, most often because of a myocardial infarction (heart attack). Other causes of cardiogenic shock include poor perfusion of the heart (ischemia), heart valve problems, excessive preload or afterload, impaired contractility of heart muscle fibers, and arrhythmias.

Even with normal blood volume and cardiac output, shock may occur if blood pressure drops due to a decrease in systemic vascular resistance. A variety of conditions can cause inappropriate dilation of arterioles or venules. In anaphylactic shock anaphylactic, a severe allergic reaction—for example, to a bee sting—releases histamine and other mediators that cause vasodilation. In neurogenic shock, vasodilation may occur following trauma to the head that causes malfunction of the cardiovascular center in the medulla.

Shock stemming from certain bacterial toxins that produce vasodilation is termed septic shock. In the United States, septic shock causes more than 100,000 deaths per year and is the most common cause of death in hospital critical care units.

Obstructive shock occurs when blood flow through a portion of the circulation is blocked. The most common cause is pulmonary embolism, a blood clot lodged in a blood vessel of the lungs.

Homeostatic Responses to Shock

The major mechanisms of compensation in shock are negative feedback systems that work to return cardiac output and arterial blood pressure to normal. When shock is mild, compensation by homeostatic mechanisms prevents serious damage. In an otherwise healthy person, compensatory mechanisms can maintain adequate blood flow and blood pressure despite an acute blood loss of as much as 10% of total volume. Figure 21.16 shows several negative feedback systems that respond to hypovolemic shock.

Figure 21.16 summary: This figure is a flow chart. It illustrates the physiological feedback loop triggered by hypovolemic shock to regulate blood volume and blood pressure. The process begins with the detection of decreased blood volume and pressure by baroreceptors in the kidneys, carotid sinus, and aortic arch. These receptors send signals to control centers including the liver, lungs, hypothalamus, posterior pituitary, and the cardiovascular center in the medulla oblongata. These centers produce outputs such as angiotensin II, ADH, and increased sympathetic stimulation. These outputs act on effectors including the suprarenal cortex, kidneys, blood vessels, and the heart. The resulting actions include the liberation of aldosterone, conservation of salt and water by the kidneys, constriction of blood vessels, and an increase in heart rate and contractility. The collective responses lead to increased blood volume, increased systemic vascular resistance, and increased blood pressure, which serves to return the body to homeostasis.

1. Activation of the renin–angiotensin–aldosterone system. Decreased blood flow to the kidneys causes the kidneys to secrete renin and initiates the renin–angiotensin–aldosterone system (see Figure 18.15). Recall that angiotensin I.I causes vasoconstriction and stimulates the suprarenal cortex to secrete aldosterone, a hormone that increases reabsorption of Na⁺ and water by the kidneys. The increases in systemic vascular resistance and blood volume help raise blood pressure.

2. Secretion of antidiuretic hormone. In response to decreased blood pressure, the posterior pituitary releases more antidiuretic hormone (A.D.H). A.D.H enhances water reabsorption by the kidneys, which conserves remaining blood volume. It also causes vasoconstriction, which increases systemic vascular resistance.

3. Activation of the sympathetic division of the A.N.S. As blood pressure decreases, aortic and carotid baroreceptors initiate powerful sympathetic responses throughout the body. One result is marked vasoconstriction of arterioles and veins of the skin, kidneys, and other abdominal viscera. (Vasoconstriction does not occur in the brain or heart.) Constriction of arterioles increases systemic vascular resistance, and constriction of veins increases venous return. Both effects help maintain an adequate blood pressure. Sympathetic stimulation also increases heart rate and contractility and increases secretion of epinephrine and norepinephrine by the suprarenal medulla. These hormones intensify vasoconstriction and increase heart rate and contractility, all of which help raise blood pressure.

4. Release of local vasodilators. In response to hypoxia, cells liberate vasodilators—including K superscript plus , H superscript plus , lactic acid, adenosine, and nitric oxide—that dilate arterioles and relax precapillary sphincters. Such vasodilation increases local blood flow and may restore O 2 level to normal in part of the body. However, vasodilation also has the potentially harmful effect of decreasing systemic vascular resistance and thus lowering the blood pressure.

If blood volume drops more than 10 to 20%, or if the heart cannot bring blood pressure up sufficiently, compensatory mechanisms may fail to maintain adequate blood flow to tissues. At this point, shock becomes life-threatening as damaged cells start to die.

Signs and Symptoms of Shock

Even though the signs and symptoms of shock vary with the severity of the condition, most can be predicted in light of the responses generated by the negative feedback systems that attempt to correct the problem. Among the signs and symptoms of shock are the following:

• Systolic blood pressure is lower than 90 mmHg.

• Resting heart rate is rapid due to sympathetic stimulation and increased blood levels of epinephrine and norepinephrine.

• Pulse is weak and rapid due to reduced cardiac output and fast heart rate.

• Skin is cool, pale, and clammy due to sympathetic constriction of skin blood vessels and sympathetic stimulation of sweating.

• Mental state is altered due to reduced oxygen supply to the brain.

• Urine formation is reduced due to increased levels of aldosterone and antidiuretic hormone.

Figure 21.16 Negative Feedback Systems That Can Restore Normal Blood Pressure During Hypovolemic Shock.

Homeostatic mechanisms can compensate for an acute blood loss of as much as 10% of total blood volume.

• The person is thirsty due to loss of extracellular fluid.

• The pH of blood is low (acidosis) due to buildup of lactic acid.

• The person may have nausea because of impaired blood flow to the digestive organs from sympathetic vasoconstriction.

Checkpoint

21. Which symptoms of hypovolemic shock relate to actual body fluid loss, and which relate to the negative feedback systems that attempt to maintain blood pressure and blood flow?

22. Describe the types of shock and their causes and how a person in hypovolemic shock should be treated.

21.7 Circulatory Routes: Systemic Circulation

Objective

• Define systemic circulation and explain its importance.

Arteries, arterioles, capillaries, venules, and veins are organized into circulatory routes that deliver blood throughout the body. Now that you understand the structures of each of these vessel types, we can look at the basic routes the blood takes as it is transported throughout the body.

Figure 21.17 shows the circulatory routes for blood flow. The routes are parallel; that is, in most cases a portion of the cardiac output flows separately to each tissue of the body. Thus, each organ receives its own supply of freshly oxygenated blood.

The two basic postnatal (after birth) routes for blood flow are the systemic circulation and the pulmonary circulation. The systemic circulation includes all arteries and arterioles that carry oxygenated blood from the left ventricle to systemic capillaries, plus the veins and venules that return deoxygenated blood to the right atrium after flowing through body organs. Blood leaving the ay-or-tuh and flowing through the systemic arteries is a bright red color.

As it moves through capillaries, it loses some of its oxygen and picks up carbon dioxide, so that blood in systemic veins is dark red.

Some of the subdivisions of the systemic circulation include the coronary (cardiac) circulation (see Figure 20.8), which supplies the myocardium of the heart; cerebral circulation, which supplies the brain (see Figure 21.20c); and the hepatic portal circulation hepatic; hepat-= liver), which extends from the digestive canal to the liver (see Figure 21.28). The nutrient arteries to the lungs, such as the bronchial arteries, are also part of the systemic circulation. When blood returns to the heart from the systemic route, it is pumped out of the right ventricle through the pulmonary circulation pulmonare; pulmo-= lung) to the lungs (see Figure 21.29). In capillaries of the pulmonary alveoli (air sacs) of the lungs, the blood loses some of its carbon dioxide and takes on oxygen. Bright red again, it returns to the left atrium of the heart and reenters the systemic circulation as it is pumped out by the left ventricle.

Figure 21.28 summary: This figure is an anatomical diagram and schematic flow chart. It illustrates the venous drainage system of the lower limb, tracing the path of blood from the distal parts of the foot through various superficial and deep veins, eventually merging into the common iliac veins and flowing into the inferior vena cava. The diagram shows that the drainage system is divided into superficial and deep networks, with multiple smaller veins in the foot and leg converging into larger vessels as they move proximally toward the torso.

Another major route—the fetal circulation—exists only in the fetus and contains special structures that allow the developing fetus to exchange materials with its mother (see Figure 21.30).

Figure 21.30 summary: This figure consists of an anatomical illustration and a corresponding flow diagram. The first part provides an anterior view of the human heart and lungs, labeling key structures including the atria, ventricles, major veins, and arteries. The second part is a schematic diagram illustrating the path of pulmonary circulation, tracing the flow of blood from the heart to the lungs and back. The figure demonstrates that blood is pumped from the right ventricle through the pulmonary trunk and pulmonary arteries into the alveoli of both the right and left lungs. From there, the blood returns via the pulmonary veins to the left atrium, completing the pulmonary circuit.

The systemic circulation carries oxygen and nutrients to body tissues and removes carbon dioxide and other wastes and heat from the tissues. All systemic arteries branch from the ay-or-tuh. Deoxygenated blood returns to the heart through the systemic veins. All veins of the systemic circulation drain into the superior vena cava, inferior vena cava, or coronary sinus, which in turn empty into the right atrium.

The principal arteries and veins of the systemic circulation are described and illustrated in Sections 21.8 through 21.19 and Figures 21.18 through 21.28 to assist you in learning their names. The blood vessels are organized in the different chapter sections according to regions of the body. Figure 21.18a shows an overview of the major arteries, and Figure 21.23 shows an overview of the major veins. As you study the various blood vessels in Sections 21.8 through 21.19, refer to these two figures to see the relationships of the blood vessels under consideration to other regions of the body.

Figure 21.18 summary: This figure consists of two anatomical diagrams. The illustrations provide a detailed anterior view of the human arterial system, specifically focusing on the aorta and its principal branches. The diagrams map the distribution of blood flow from the heart through the ascending aorta, aortic arch, thoracic aorta, and abdominal aorta, extending into the major arteries of the head, neck, upper limbs, abdominal organs, and lower extremities. From these diagrams, it can be inferred that the aorta serves as the primary trunk for systemic circulation, branching into smaller specialized arteries to ensure oxygenated blood reaches all major regions of the body, including the brain via the carotid arteries and the lower limbs via the iliac and femoral arteries.

Each of the sections contains the following information:

• Overview. This provides a general orientation to the blood vessels under consideration, with emphasis on how the blood vessels are organized into various regions as well as distinguishing and/or interesting features of the blood vessels.

• Blood vessel names. Students often have difficulty with the pronunciations and meanings of blood vessels' names. To learn them more easily, study the phonetic pronunciations and word derivations that indicate how blood vessels get their names.

• Region supplied or drained. For each artery listed, there is a description of the parts of the body that receive blood from the vessel. For each vein listed, there is a description of the parts of the body that are drained by the vessel.

• Illustrations and photographs. The figures that accompany Sections 21.8 through 21.19 contain several elements. Many include illustrations of the blood vessels under consideration and flow diagrams to indicate the patterns of blood distribution or drainage. Cadaver photographs are also included in selected sections to provide more realistic views of the blood vessels.

23. What is the purpose of systemic circulation?

Figure 21.17 Circulatory routes. Long black arrows indicate the systemic circulation, short blue arrows the pulmonary circulation (detailed in Figure 21.29), and red arrows the hepatic portal circulation (detailed in Figure 21.28). Refer to Figure 20.8 for details of the coronary circulation and to Figure 21.30 for details of the fetal circulation.

Blood vessels are organized into various routes that deliver blood to tissues of the body.

21.8 The ay-or-tuh and Its Branches

Objectives

• Identify the four principal divisions of the ay-or-tuh.

• Locate the major arterial branches arising from each division.

The ay-or-tuh aorta = to lift up) is the largest artery of the body, with a diameter of 2 to 3 centimeters (about 1 in.). Its four principal divisions are the ascending ay-or-tuh, aortic arch, thoracic ay-or-tuh, and abdominal ay-or-tuh (Figure 21.18). The portion of the ay-or-tuh that emerges from the left ventricle posterior to the pulmonary trunk is the ascending ay-or-tuh (see Section 21.9). The beginning of the ay-or-tuh contains the aortic valve (see Figure 20.4a). The ascending ay-or-tuh gives off two coronary arteries that supply the myocardium of the heart. Then the ascending ay-or-tuh arches to the left, forming the aortic arch (see Section 21.10), which descends and ends at the level of the intervertebral disc between the fourth and fifth thoracic vertebrae to become the descending ay-or-tuh. As the ay-or-tuh continues to descend, it lies close to the vertebral bodies and is called the thoracic ay-or-tuh within the thorax (see Section 21.11). When the thoracic ay-or-tuh reaches the bottom of the thorax it passes through the aortic hiatus of the diaphragm to become the abdominal ay-or-tuh (see Section 21.12). The abdominal ay-or-tuh descends to the level of the fourth lumbar vertebra where it divides into two common iliac arteries.

Table summary: This table outlines the various divisions and branches of the aorta, categorizing them by anatomical section and identifying the specific regions and organs supplied by each branch. The aorta is divided into the ascending aorta, aortic arch, thoracic aorta, abdominal aorta, and common iliac arteries, with each section branching into multiple vessels that provide blood to diverse structures including the heart, head, neck, limbs, respiratory system, digestive organs, kidneys, and reproductive organs.

Figure 21.18 Continued

(see Section 21.13), which carry blood to the pelvis and lower limbs. Each division of the ay-or-tuh gives off arteries that branch into distributing arteries that lead to various organs. Within the organs, the arteries divide into arterioles and then into capillaries that service the systemic tissues (all tissues except the pulmonary alveoli of the lungs).

24. What general regions do each of the four principal divisions of the ay-or-tuh supply?

21.9 Ascending ay-or-tuh

• Identify the two primary arterial branches of the ascending ay-or-tuh.

The ascending ay-or-tuh is about 5 centimeters (2 in.) in length and begins at the aortic valve (see Figure 20.8). It is directed superiorly, slightly anteriorly, and to the right. It ends at the level of the sternal angle, where it becomes the aortic arch. The beginning of the ascending ay-or-tuh is posterior to the pulmonary trunk and right auricle; the right pulmonary artery is posterior to it. At its origin, the ascending ay-or-tuh contains three dilations called aortic sinuses. Two of these, the right and left sinuses, give rise to the right and left coronary arteries, respectively.

The right and left coronary arteries (coron-= crown) arise from the ascending ay-or-tuh just superior to the aortic valve (see Figure 21.19). They form a crownlike ring around the heart, giving off branches to the atrial and ventricular myocardium. The inferior (posterior) interventricular artery interventricular; inter-= between) of the right coronary artery supplies both ventricles, and the marginal branch supplies the right ventricle. The anterior interventricular artery, also known as the left anterior descending L.A.D artery, of the left coronary artery supplies both ventricles, and the circumflex artery circumflex; circum-= around; -flex = to bend) supplies the left atrium and left ventricle.

Figure 21.19 summary: This figure consists of anatomical diagrams and a distribution scheme. The content illustrates the arrangement of the coronary arteries and veins supplying the heart, detailing the path from the aortic arch and ascending aorta to various branches including the right and left coronary arteries, the circumflex artery, and the anterior and inferior interventricular arteries. The diagrams provide both a simplified schematic of the arterial distribution and a detailed anatomical view of the arteries and veins in relation to the heart chambers. It can be inferred that the coronary arteries originate as the primary branches of the aorta to ensure the heart muscle receives an adequate blood supply, with a complex network of branches distributing blood to the anterior and inferior regions of the ventricles.

Table summary: The table presents a specific review question regarding the anatomical supply of the left ventricle and the physiological reasoning behind its extensive blood supply.

21.10 The Aortic Arch

Objective

• Identify the three principal arteries that branch from the aortic arch.

The aortic arch is 4 to 5 centimeters (almost 2 in.) in length and is the continuation of the ascending ay-or-tuh. It emerges from the pericardium posterior to the sternum at the level of the sternal angle (Figure 21.20). The aortic arch is directed superiorly and posteriorly to the left and then inferiorly; it ends at the intervertebral disc between the fourth and fifth thoracic vertebrae, where it becomes the thoracic ay-or-tuh. Three major arteries branch from the superior aspect of the aortic arch, in order of their origination, are the following: the brachiocephalic trunk, the left common carotid, and the left subclavian.

Figure 21.20 summary: This figure is a schematic diagram and a series of anatomical illustrations. It depicts the distribution of arteries originating from the aortic arch, branching into the head, neck, and upper limbs, including the cerebral arterial circle. The diagram shows the hierarchical branching from the aortic arch to the brachiocephalic trunk, common carotid, and subclavian arteries, further dividing into specific vessels of the brain and the arm. The layout indicates that blood flow is distributed symmetrically to both sides of the body, with complex networks ensuring blood supply to the brain and extremities through a series of arterial arches and smaller branching vessels.

The first and largest branch from the aortic arch is the brachiocephalic trunk brachiocephalic; brachio-= arm; -cephalic = head). It extends superiorly, bending slightly to the right, and divides at the right sternoclavicular joint to form the right subclavian artery and right common carotid artery. The second branch from the aortic arch is the left common carotid artery carotid, which divides into the same branches with the same names as the right common carotid artery. The third branch from the aortic arch is the left subclavian artery.

Table summary: This table outlines the various branches of the aortic arch and subclavian artery, detailing their anatomical descriptions and the specific regions of the body they supply. The vessels transition from major trunk branches supplying the head and neck to more distal arteries that provide blood to the thoracic wall, brain, shoulder, and limbs.

Table summary: This table provides a detailed anatomical overview of various major arteries, describing their origins, paths, and the specific regions they supply. It categorizes vessels ranging from the forearm and hand, such as the radial and ulnar arteries and the palmar arches, to the major vessels of the head and neck, including the carotid and subclavian arteries. The descriptions highlight the complex branching patterns and anastomoses that ensure blood supply to diverse structures like the muscles of the forearm, the facial features, and the brain via the cerebral arterial circle.

Figure 21.20 Aortic arch and its branches. Note in (c) the arteries that constitute the cerebral arterial circle (circle of Willis).

The aortic arch ends at the level of the intervertebral disc between the fourth and fifth thoracic vertebrae.

Figure 21.20 Continued Q What are the three major branches of the aortic arch, in order of their origination?

subclavian, which distributes blood to the left vertebral artery and vessels of the left upper limb. Arteries branching from the left subclavian artery are similar in distribution and name to those branching from the right subclavian artery.

Checkpoint

26. What general regions do the arteries that arise from the aortic arch supply?

21.11 Thoracic ay-or-tuh

Objective

The thoracic ay-or-tuh is about 20 centimeters (8 in.) long and is a continuation of the aortic arch (Figure 21.21). It begins at the level of the intervertebral disc between the fourth and fifth thoracic vertebrae, where it lies to the left of the vertebral column. As it descends, it moves closer to the midline and extends through an opening in the diaphragm (aortic hiatus), which is located anterior to the vertebral column at the level of the intervertebral disc between the twelfth thoracic and first lumbar vertebrae.

Along its course, the thoracic ay-or-tuh sends off numerous small arteries, visceral branches visceral to viscera, and parietal branches (pa-Ri-e-tal) to body wall structures.

Table summary: This table categorizes the various branches of the thoracic aorta into visceral and parietal groups, detailing their anatomical origins, descriptions, and the specific tissues or regions they supply. The visceral branches focus on internal structures such as the pericardium, bronchial tree, esophagus, and mediastinum, while the parietal branches supply the outer boundaries and structural components including the thoracic wall, ribs, spinal structures, and the diaphragm.

Figure 21.21 summary: This anatomical diagram illustrates the branching patterns of the thoracic aorta. The figure depicts the descending aorta and its various arterial branches, categorized into visceral branches such as the bronchial, mediastinal, esophageal, and pericardial arteries, and parietal branches including the posterior intercostal, subcostal, and superior phrenic arteries. The diagram demonstrates that the thoracic aorta serves as a primary distribution hub, supplying oxygenated blood to both the internal organs of the chest cavity and the walls of the thoracic cage.

Figure 21.21 Thoracic ay-or-tuh and Abdominal ay-or-tuh and Their Principal Branches.

The thoracic ay-or-tuh is the continuation of the ascending ay-or-tuh.

21.12 Abdominal ay-or-tuh

• Identify the visceral and parietal branches of the abdominal ay-or-tuh. The abdominal ay-or-tuh is the continuation of the thoracic ay-or-tuh after it passes through the diaphragm (see Figure 21.22). It begins at the aortic hiatus in the diaphragm and ends at about the level of the fourth lumbar vertebra, where it divides into the right and left common iliac arteries. The abdominal ay-or-tuh lies anterior to the vertebral column.

As with the thoracic ay-or-tuh, the abdominal ay-or-tuh gives off visceral and parietal branches. The unpaired visceral branches arise from the anterior surface of the ay-or-tuh and include the celiac trunk and the superior mesenteric and inferior mesenteric arteries (see Figure 21.21).

Table summary: This table details the unpaired visceral branches of the abdominal aorta, specifically the celiac trunk and the superior mesenteric artery. It describes the anatomical origin, branching patterns, and the specific regions supplied by each vessel. The celiac trunk is shown to divide into three main branches—the left gastric, splenic, and common hepatic arteries—which collectively provide blood to the embryonic foregut organs, including the esophagus, stomach, spleen, liver, and parts of the pancreas and duodenum. The superior mesenteric artery arises just below the celiac trunk and supplies the remainder of the digestive canal from the duodenum through the transverse colon via several branches, including the inferior pancreaticoduodenal and the jejunal and ileal arteries.

Table summary: The table details the various branches of the inferior mesenteric artery and their respective anatomical distributions. The artery originates from the abdominal aorta and provides blood supply to a significant portion of the digestive canal, spanning from the transverse colon down to the rectum. Its branches are categorized into those supplying the left side of the large intestine, such as the left colic and sigmoid arteries, and those contributing to the distal transverse colon and the upper rectum via the superior rectal artery.

The paired visceral branches arise from the lateral surfaces of the ay-or-tuh and include the suprarenal, renal, and gonadal arteries. The lone unpaired parietal branch is the median sacral artery. The paired parietal branches arise from the posterolateral surfaces of the ay-or-tuh and include the inferior phrenic and lumbar arteries.

Checkpoint

28. Name the paired visceral and parietal branches and the unpaired visceral and parietal branches of the abdominal ay-or-tuh, and indicate the general regions they supply.

Table summary: This table outlines the paired visceral branches of the abdominal aorta, detailing their anatomical origins, descriptions, and the specific regions they supply. It describes the suprarenal arteries, which consist of multiple pairs arising from different sources to supply the suprarenal glands; the renal arteries, which arise from the lateral aspects of the aorta to supply the kidneys; and the gonadal arteries, which are differentiated by sex as either testicular or ovarian arteries to supply various reproductive organs and the ureters.

Table summary: This table categorizes the parietal branches of the abdominal aorta into unpaired and paired types, detailing their anatomical origins and the specific regions they supply. The unpaired branch consists of the median sacral artery, while the paired branches include the inferior phrenic arteries, which are the first paired branches, and multiple pairs of lumbar arteries that extend toward the abdominal wall.

Figure 21.21 summary: This figure is an anatomical diagram illustrating the distribution of the abdominal aorta and its major branches. The diagram shows the branching pattern of the aorta, including the celiac trunk, superior mesenteric artery, and inferior mesenteric artery, along with their subsequent divisions that supply various organs such as the liver, stomach, spleen, kidneys, and intestines. It can be inferred that the abdominal aorta serves as the primary arterial source for the abdominal cavity, with a complex network of visceral branches ensuring blood supply to multiple organ systems through a hierarchical distribution scheme.

Figure 21.22 Abdominal ay-or-tuh and Its Principal Branches.

The abdominal ay-or-tuh is the continuation of the thoracic ay-or-tuh.

Image summary: This is an anatomical photograph showing a dissection of the human abdominal cavity. The image displays various organs and blood vessels, including the diaphragm, inferior vena cava, kidneys, abdominal aorta, and several mesenteric and iliac arteries. The layout highlights the spatial relationships between the major vascular structures and the surrounding organs, specifically focusing on the inferior mesenteric artery and its distribution.

21.13 Arteries of the Pelvis and Lower Limbs

• Identify the two major branches of the common iliac arteries. The abdominal ay-or-tuh ends by dividing into the right and left common iliac arteries (Figure 21.23). These, in turn, divide into the internal and external iliac arteries. In sequence, the external iliacs become the femoral arteries in the thighs, the popliteal arteries posterior to the knee, and the anterior and posterior tibial arteries in the legs.

Table summary: This table details the anatomical progression and branching patterns of the arterial supply to the lower body, beginning with the common iliac arteries and descending through various stages including the internal and external iliac, femoral, popliteal, and tibial arteries. It describes how each vessel originates, its physical path through different muscle groups and joints, and the specific regions and structures it supplies, such as the pelvic organs, thighs, legs, and feet.

Figure 21.23 Arteries of the Pelvis and Right Lower Limb.

The internal iliac arteries carry most of the blood supply to the pelvic viscera and wall.

29. What general regions do the internal and external iliac arteries supply?

21.14 Veins of the Systemic Circulation

Objective

• Identify the three systemic veins that return deoxygenated blood to the heart.

As you have already learned, arteries distribute blood from the heart to various parts of the body, and veins drain blood away from the various parts and return the blood to the heart. In general, arteries are deep; veins may be superficial or deep. Superficial veins are located just beneath the skin and can be seen easily. Because there are no large superficial arteries, the names of superficial veins do not correspond to those of arteries. Superficial veins are clinically important as sites for withdrawing blood or giving injections.

Deep veins generally travel alongside arteries and usually bear the same name. Arteries usually follow definite pathways; veins are more difficult to follow because they connect in irregular networks in which many tributaries merge to form a large vein. Although only one systemic artery, the ay-or-tuh, takes oxygenated blood away from the heart (left ventricle), three systemic veins, the coronary sinus, superior vena cava S.V.C (VÉ-na KÃ-va), and inferior vena cava I.V.C, return deoxygenated blood to the heart (right atrium) (Figure 21.24). The coronary sinus receives blood from the cardiac veins that drain the heart; with few exceptions, the superior vena cava receives blood from other veins superior to the diaphragm, except the pulmonary alveoli (air sacs) of the lungs; the inferior vena cava receives blood from veins inferior to the diaphragm.

Figure 21.24 summary: This is a schematic diagram. It illustrates the drainage system of the inferior vena cava, showing the various veins that feed into it from different regions of the body, including the hepatic, renal, gonadal, lumbar, and iliac veins, ultimately leading to the heart. The diagram demonstrates that the inferior vena cava serves as the primary central conduit for returning deoxygenated blood from the lower half of the body to the heart, with blood flowing from smaller peripheral veins into larger common veins and finally into the main vena cava.

Checkpoint

30. What are the three tributaries of the coronary sinus?

Table summary: This table outlines the primary veins of the body, detailing their anatomical descriptions, tributary connections, and the specific regions they drain. It compares the coronary sinus, which collects blood from the heart tissue, with the superior vena cava, which drains the upper body, and the inferior vena cava, the largest vein in the body, which drains the lower regions. Additionally, it provides a clinical observation regarding how the inferior vena cava can be compressed during pregnancy.

Figure 21.24 Principal Veins.

Deoxygenated blood returns to the heart via the superior vena cava, inferior vena cava, and coronary sinus.

21.15 Veins of the Head and Neck

• Identify the three major veins that drain blood from the head.

Most blood draining from the head passes into three pairs of veins: the internal jugular jugular, external jugular, and vertebral veins (Figure 21.25). Within the cranial cavity, all veins drain into dural venous sinuses and then into the internal jugular veins. Dural venous sinuses are endothelial-lined venous channels between layers of the cranial dura mater.

Figure 21.25 summary: This figure consists of a schematic flow diagram and an anatomical illustration. The content depicts the venous drainage system of the head, neck, and upper thorax, tracing the flow of blood from the dural venous sinuses of the brain through various jugular and vertebral veins, eventually converging into the brachiocephalic veins and the superior vena cava. The figure illustrates that blood from the cranial sinuses drains primarily into the internal jugular veins, which then merge with external jugular and subclavian veins to facilitate the return of blood from the upper body toward the heart.

Checkpoint

31. Which general areas are drained by the internal jugular, external jugular, and vertebral veins?

Table summary: This table outlines various veins within the head and neck, detailing their anatomical descriptions, tributaries, and the specific regions they drain. The internal jugular veins are highlighted as major vessels that receive significant contributions from several dural venous sinuses, which collectively drain extensive areas including the brain, skull, and face. Other veins, such as the external jugular, vertebral, and subclavian veins, serve more localized regions like the scalp, cervical structures, and limbs, eventually contributing to the formation of the brachiocephalic veins.

Blood draining from the head passes into the internal jugular, external jugular, and vertebral veins.

21.16 Veins of the Upper Limbs

Objective

• Identify the principal veins that drain the upper limbs.

Both superficial and deep veins return blood from the upper limbs to the heart (Figure 21.26). Superficial veins are located just deep to the skin and are often visible. They anastomose extensively with one another and with deep veins, and they do not accompany arteries. Superficial veins are larger than deep veins and return most of the blood from the upper limbs.

Figure 21.26 summary: This figure is a combination of anatomical diagrams and a flow chart. The content illustrates the venous drainage system of the right upper limb, detailing the network of superficial and deep veins from the fingers up to the superior vena cava. It identifies specific veins such as the cephalic, basilic, brachial, and radial veins, as well as various venous arches and plexuses in the hand. The figure demonstrates that blood from the extremities flows through a hierarchical network of smaller veins that merge into larger vessels, eventually draining into the superior vena cava. It concludes that the venous system is organized into superficial and deep layers, with the deep veins typically paralleling the arterial system.

Deep veins are located deep in the body. They usually accompany arteries and have the same names as the corresponding arteries. Both superficial and deep veins have valves, but valves are more numerous in the deep veins.

Checkpoint

32. Where do the cephalic, basilic, median antebrachial, radial, and ulnar veins originate?

Table summary: This table categorizes the veins of the upper limb into deep and superficial systems, detailing their anatomical origins, pathways, and the specific body regions they drain. The deep veins follow a hierarchical progression from the distal extremities toward the torso, beginning with the radial and ulnar veins that unite to form the brachial veins, which then transition into the axillary and subclavian veins before ultimately forming the brachiocephalic veins. In contrast, the superficial veins, including the cephalic, basilic, and median antebrachial veins, reside within the integument and superficial muscles, providing alternative drainage routes and clinically significant sites for medical procedures.

Veins of the Thorax

• Identify the components of the azygos system of veins. Although the brachiocephalic veins drain some portions of the thorax, most thoracic structures are drained by a network of veins, called the azygos system (az-I-gus), that runs on either side of the vertebral column (Figure 21.27). The system consists of three veins—the azygos, hemiazygos, and accessory hemiazygos veins—that show considerable variation in origin, course, tributaries, anastomoses, and termination. Ultimately they empty into the superior vena cava.

Figure 21.27 summary: This figure consists of an anatomical diagram and a corresponding schematic flow chart. The content illustrates the venous drainage system of the human body, detailing the network of veins that transport blood from the head, neck, chest, and abdomen toward the heart. The diagram labels various major vessels including the jugular, subclavian, brachiocephalic, and vena cava systems, as well as the azygos and hemiazygos networks. It can be inferred that the venous system is organized hierarchically, where smaller peripheral veins merge into larger collecting vessels. The schematic confirms that blood from the upper body converges into the superior vena cava, while blood from the lower body and abdominal organs converges into the inferior vena cava, both of which serve as the primary conduits returning blood to the heart.

Table summary: This table describes several major veins, detailing their anatomical descriptions, specific tributaries, and the various body regions they drain. It highlights the differences between the paired brachiocephalic veins, which drain the upper body and head, and the asymmetric venous system of the thorax, which includes the single azygos vein on the right side and the hemiazygos and accessory hemiazygos veins on the left side.

Figure 21.27 Principal veins of the thorax, abdomen, and pelvis.

Most thoracic structures are drained by the azygos system of veins.

Which vein returns blood from the abdominopelvic viscera to the heart?

The azygos system, besides collecting blood from the thorax and abdominal wall, may serve as a bypass for the inferior vena cava, which drains blood from the lower body. Several small veins directly link the azygos system with the inferior vena cava. Larger veins that drain the lower limbs and abdomen also connect into the azygos system. If the inferior vena cava or hepatic portal vein becomes obstructed, blood that typically passes through the inferior vena cava can detour into the azygos system to return blood from the lower body to the superior vena cava.

Table summary: The table presents a single inquiry regarding the relative anatomical importance of the azygos system in comparison to the inferior vena cava.

21.18 Veins of the Abdomen and Pelvis

Chinese text. Objective

• Identify the principal veins that drain the abdomen and pelvis.

Table summary: This table outlines the major veins of the abdominal region, detailing their anatomical descriptions, tributaries, and the specific physiological areas they drain. It illustrates a complex network where various vessels, such as the hepatic, renal, and gonadal veins, collect blood from diverse organs including the digestive tract, kidneys, suprarenal glands, and reproductive organs, eventually channeling this blood toward the inferior vena cava.

Table summary: This table outlines the anatomical characteristics of the internal and external iliac veins, distinguishing them by their origins, anatomical paths, and the specific bodily regions they drain. The internal iliac veins primarily serve the pelvic and gluteal areas, while the external iliac veins drain the lower limbs and anterior abdominal regions before merging to form the common iliac veins.

Blood from the abdominal and pelvic viscera and lower half of the abdominal wall returns to the heart via the inferior vena cava. Many small veins enter the inferior vena cava. Most carry return flow from parietal branches of the abdominal ay-or-tuh, and their names correspond to the names of the arteries (see also Figure 21.27).

The inferior vena cava does not receive veins directly from the digestive canal, spleen, pancreas, and gallbladder. These organs pass their blood into a common vein, the hepatic portal vein, which delivers the blood to the liver. The superior mesenteric and splenic veins unite to form the hepatic portal vein (see Figure 21.29). This special flow of venous blood, called the hepatic portal circulation, is described shortly. After passing through the liver for processing, blood drains into the hepatic veins, which empty into the inferior vena cava.

Checkpoint

34. What structures do the lumbar, gonadal, renal, suprarenal, inferior phrenic, and hepatic veins drain?

• Identify the principal superficial and deep veins that drain the lower limbs.

As with the upper limbs, blood from the lower limbs is drained by both superficial and deep veins. The superficial veins often anastomose with one another and with deep veins along their length. Deep veins, for the most part, have the same names as corresponding arteries (Figure 21.28). All veins of the lower limbs have valves, which are more numerous than in veins of the upper limbs.

Checkpoint

35. What is the clinical importance of the great saphenous veins?

Table summary: This table outlines several deep veins located in the lower limbs, detailing their anatomical descriptions, pathways, and the specific regions they drain. It describes the progression of blood flow from the popliteal veins behind the knee, which transition into the femoral veins of the thigh, eventually leading into the iliac veins as they enter the abdominopelvic region.

Table summary: This table categorizes the veins of the lower limb into deep and superficial systems, detailing their anatomical origins, paths, and the specific regions they drain. The deep veins, including the posterior and anterior tibial veins, primarily drain the bones, muscles, and skin of the foot and leg, eventually merging to form the popliteal veins. The superficial veins, consisting of the great and small saphenous veins, drain the integumentary tissues and superficial muscles. The great saphenous vein is noted for its significant length and clinical utility in medical procedures, while the small saphenous vein is highlighted as being more prone to varicosities due to its anatomical positioning and lack of muscle support.

Figure 21.28 Principal Veins of the Pelvis and Lower Limbs.

Deep veins usually bear the names of their companion arteries.

21.20 Circulatory Routes: The Hepatic Portal Circulation

• Describe the importance of hepatic portal system. The hepatic portal circulation carries venous blood from the digestive canal organs and spleen to the liver. A vein that carries blood from one capillary network to another is called a portal vein.

The hepatic portal vein receives blood from capillaries of digestive canal organs and the spleen and delivers it to the sinusoids of the liver (Figure 21.29). After a meal, hepatic portal blood is rich in nutrients absorbed from the digestive canal. The liver stores some of them and modifies others before they pass into the general circulation. For example, the liver converts glucose into glycogen for storage, reducing blood glucose.

Figure 21.29 Hepatic portal circulation. A schematic diagram of blood flow through the liver, including arterial circulation, is shown in (b). As usual, deoxygenated blood is indicated in blue, and oxygenated blood in red.

The hepatic portal circulation delivers venous blood from the organs of the digestive canal and spleen to the liver.

Figure 21.29 Continued level shortly after a meal. The liver also detoxifies harmful substances, such as alcohol, that have been absorbed from the digestive canal and destroys bacteria by phagocytosis.

The superior mesenteric and splenic veins unite to form the hepatic portal vein. The superior mesenteric vein mesenteric drains blood from the small intestine and portions of the large intestine, stomach, and pancreas through the jejunal, ileal, ileocolic iliocolic, right colic, middle colic, pancreaticoduodenal (pan-krê-at'-i-kô-doo'-ô-DÊ-nal), and right gastrointestinal veins gastrointestinal. The splenic vein drains blood from the stomach, pancreas, and portions of the large intestine through the short gastric, left gastroomental, pancreatic, and inferior mesenteric veins. The inferior mesenteric vein, which passes into the splenic vein, drains portions of the large intestine through the superior rectal, sigmoidal, and left colic veins.

The right and left gastric veins, which open directly into the hepatic portal vein, drain the stomach. The cystic vein, which also opens into the hepatic portal vein, drains the gallbladder.

At the same time the liver is receiving nutrient-rich but deoxygenated blood via the hepatic portal vein, it is also receiving oxygenated blood via the hepatic artery, a branch of the celiac trunk. The oxygenated blood mixes with the deoxygenated blood in sinusoids. Eventually, blood leaves the sinusoids of the liver through the hepatic veins, which drain into the inferior vena cava.

Checkpoint

36. Diagram the hepatic portal circulation and describe its importance.

21.21 Circulatory Routes: The Pulmonary Circulation

Objective • Explain why pulmonary circulation is important The pulmonary circulation carries deoxygenated blood from the right ventricle to the pulmonary alveoli within the lungs and returns oxygenated blood from the pulmonary alveoli (air sacs) to the left atrium (Figure 21.30). The pulmonary trunk emerges from the right ventricle and passes superiorly, posteriorly, and to the left. It then divides into two branches: the right

Figure 21.30 Pulmonary Circulation.

The pulmonary circulation brings deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood from the lungs to the left atrium. pulmonary artery to the right lung and the left pulmonary artery to the left lung. After birth, the pulmonary arteries are the only arteries that carry deoxygenated blood. On entering the lungs, the branches divide and subdivide until finally they form capillaries around the pulmonary alveoli (air sacs) within the lungs.

C-O₂ passes from the blood into the pulmonary alveoli and is exhaled. Inhaled O₂ passes from the air within the lungs into the blood. The pulmonary capillaries unite to form venules and eventually pulmonary veins, which exit the lungs and carry the oxygenated blood to the left atrium. Two left and two right pulmonary veins enter the left atrium.

After birth, the pulmonary veins are the only veins that carry oxygenated blood. Contractions of the left ventricle then eject the oxygenated blood into the systemic circulation.

37. Explain why pulmonary circulation is important.

21.22 Circulatory Routes: The Fetal Circulation

Objective • Describe the fate of the fetal structures once postnatal circulation begins. The circulatory system of a fetus, called the fetal circulation, exists only in the fetus and contains special structures that allow the developing fetus to exchange materials with its mother (Figure 21.31). It differs from the postnatal (after birth) circulation because the lungs, kidneys, and digestive canal organs do not begin to function until birth. The fetus obtains O 2 and nutrients from the maternal blood and eliminates C-O 2 and other wastes into it.

Figure 21.31 summary: This figure consists of an anatomical diagram and a corresponding flow chart. The anatomical diagram illustrates the fetal circulatory system, highlighting specialized structures such as the ductus arteriosus, foramen ovale, ductus venosus, and umbilical vessels, while the flow chart maps the path of blood from the placenta through the heart and systemic circulation. The figure demonstrates how fetal circulation bypasses the lungs and liver using specific shunts to direct oxygenated blood from the placenta to the rest of the body, indicating that the fetal heart and vascular system are structurally modified to support development before birth.

The exchange of materials between fetal and maternal circulations occurs through the placenta placenta, which forms inside the mother's uterus and attaches to the umbilicus Figure 21.31 Fetal circulation and changes at birth. The gold boxes between parts (a) and (b) describe the fate of certain fetal structures once postnatal circulation is established.

The lungs and digestive canal organs do not begin to function until birth.

(navel) of the fetus by the umbilical cord umbilical. The placenta communicates with the mother's cardiovascular system through many small blood vessels that emerge from the uterine wall. The umbilical cord contains blood vessels that branch into capillaries in the placenta. Wastes from the fetal blood diffuse out of the capillaries, into spaces containing maternal blood (intervillous spaces) in the placenta, and finally into the mother's uterine veins.

Nutrients travel the opposite route—from the maternal blood vessels to the intervillous spaces to the fetal capillaries. Normally, there is no direct mixing of maternal and fetal blood because all exchanges occur by diffusion through capillary walls.

Blood passes from the fetus to the placenta via two umbilical arteries in the umbilical cord (Figure 21.31a, c). These branches of the internal iliac arteries are within the umbilical cord. At the placenta, fetal blood picks up O 2 and nutrients and eliminates C-O 2 and wastes. The oxygenated blood returns from the placenta via a single umbilical vein in the umbilical cord.

This vein ascends to the liver of the fetus, where it divides into two branches. Some blood flows through the branch that joins the hepatic portal vein and enters the liver, but most of the blood flows into the second branch, the ductus venosus ductus ve-Nõ-sus), which drains into the inferior vena cava.

Deoxygenated blood returning from lower body regions of the fetus mingles with oxygenated blood from the ductus venosus in the inferior vena cava. This mixed blood then enters the right atrium. Deoxygenated blood returning from upper body regions of the fetus enters the superior vena cava and also passes into the right atrium.

Most of the fetal blood does not pass from the right ventricle to the lungs, as it does in postnatal circulation, because an opening called the foramen ovale (fo-RÃ-men ovale) exists in the septum between the right and left atria. Most of the blood that enters the right atrium passes through the foramen ovale into the left atrium and joins the systemic circulation. The blood that does pass into the right ventricle is pumped into the pulmonary trunk, but little of this blood reaches the nonfunctioning fetal lungs.

Instead, most is sent through the ductus arteriosus (ar-tê-rê-Ô-sus), a vessel that connects the pulmonary trunk with the ay-or-tuh. The blood in the ay-or-tuh is carried to all fetal tissues through the systemic circulation. When the common iliac arteries branch into the external and internal iliacs, part of the blood flows into the internal iliacs, into the umbilical arteries, and back to the placenta for another exchange of materials.

As soon as a full-term fetus is delivered, there is increased uptake of oxygen by the lungs during the infant's first and subsequent inhalations. Once breathing begins, placental blood flow ceases. As a direct result of aeration of the lungs, the ductus arteriosus and ductus venosus undergo vasoconstriction and the foramen ovale closes.

1. As a result of the expansion of the infant's lungs at birth, the lungs release a kinin called bradykinin that initiates the vasoconstriction of the ductus arteriosus. The process begins 12 to 24 hours after birth and is usually completed by 21 days, at which time the ductus arteriosus atrophines into a fibrous tissue called the ligamentum arteriosum which persists throughout life. Failure of the ductus arteriosus to close is called patent ductus arteriosus. You may recall that the ligamentum arteriosum connects the trunk of the pulmonary artery to the aortic arch (See Figure 20.4). Because of this connection, it may play a role in major injuries or trauma. For example, during abrupt acceleration as occurs during a car crash, the forward motion overstretches the ligamentum arteriosum and this could result in a partial or complete tear of the ay-or-tuh (traumatic aortic tear) resulting in massive blood loss and sometimes even death.

2. Vasoconstruction of the ductus venosus results in the formation of the ligamentum venosum, a fibrous cord on the inferior surface of the liver. Although the ligamentum venosum has no known function, it does serve as a surgical landmark for removal of the left lobe of the liver.

3. The foramen ovale normally closes shortly after birth to become the fossa ovalis, a depression in the interatrial septum. This usually occurs shortly after birth and permanent closure occurs in about one year in the majority of infants (about 75%). When an infant takes its first breath, the lungs expand and blood flow to the lungs increases. The increased blood flow from the lungs to the left atrium results in a higher pressure in the left atrium than the right atrium.

The increased left atrial pressure closes the foramen ovale by pushing the valve that guards it against the interatrial septum. This closure permits the output from the left ventricle to flow entirely into systemic circulation and the output from the right ventricle to flow entirely into pulmonary circulation.

Several other vascular changes take place after birth. The fetal blood vessels (umbilical arteries and umbilical vein) within the umbilical cord are no longer needed. The umbilical arteries in the infant fill with connective tissue and become fibrous cords called the medical umbilical ligaments.

The process starts a few minutes after birth and may take up to 2 to 3 months for complete obliteration. They are located on the anterior surface of the abdominal wall and extend from the apex of the urinary bladder to the umbilicus. Functionally, the proximal portion of the ligaments forms the vesical artery and inferior iliac artery.

The ligament are also useful landmarks for surgeons performing laparoscopic inguinal hernia repair. The umbilical vein within the infant also fills with connective tissue and becomes the ligamentum teres terez or round ligament of the liver, which attaches the liver to the anterior abdominal wall. It is located inside the free edges of the falciform ligament and divides the liver into right and left lobes.

Although delivery of the placenta is not a vascular change related to birth, it is mentioned here as another example of the transition of various fetal structures to postnatal ones. The placenta is delivered as part of the last stage of labor and is often called the afterbirth. This process usually occurs in as few as 3 to 5 minutes after birth but can sometimes take up to an hour or more.

38. Discuss the anatomy and physiology of the fetal circulation. Indicate the function of the umbilical arteries, umbilical vein, ductus venosus, foramen ovale, and ductus arteriosus.

21.23 Development of Blood Vessels and Blood

Objective

• Describe the development of blood vessels and blood.

The development of blood cells and the formation of blood vessels begins outside the embryo as early as 15 to 16 days in the mesoderm of the wall of the umbilical vesicle, chorion, and connecting stalk. About 2 days later, blood vessels form within the embryo. The early formation of the cardiovascular system is linked to the small amount of yolk in the ovum and umbilical vesicle. As the embryo develops rapidly during the third week, there is a greater need to develop a cardiovascular system to supply sufficient nutrients to the embryo and remove wastes from it.

Blood vessels and blood cells develop from the same precursor cell, called a hemangioblast hemangioblast; hema-= blood; -blast = immature stage). Once mesenchyme develops into hemangioblasts, they can give rise to cells that produce blood vessels (angioblasts) or cells that produce blood cells (multipotent stem cells).

Blood vessels develop from angioblasts angioblasts, which are derived from hemangioblasts. Angioblasts aggregate to form isolated masses and cords throughout the embryonic discs called blood islands (Figure 21.32). Spaces soon appear in the islands and become the lumens of the blood vessels. Some of the angioblasts immediately around the spaces give rise to the endothelial lining of the blood vessels.

Figure 21.32 summary: This figure is a series of anatomical diagrams. It illustrates the developmental process of blood vessel formation within the wall of the umbilical vesicle, starting from the aggregation of angioblasts into blood islands and progressing through the creation of an endothelial lining and the formation of a lumen. The sequence demonstrates that blood vessels originate from clusters of cells that differentiate and hollow out to create a functional channel for blood cells, concluding that the vascular system develops through a structured transition from solid cell clusters to open tubular structures.

Angioblasts around the endothelium form the tunics (intima, media, and externa) of the larger blood vessels. Growth and fusion of blood islands form an extensive network of blood vessels throughout Blood vessel development begins in the embryo on about the 15th or 16th day. the embryo. By continuous branching, blood vessels outside the embryo connect with those inside the embryo, linking the embryo with the placenta.

Blood cells develop from multipotent stem cells multipotent derived from hemangioblasts. This development occurs in the walls of blood vessels in the umbilical vesicle, chorion, and allantois at about 3 weeks after fertilization. Blood formation in the embryo itself begins at about the fifth week in the liver and the twelfth week in the spleen, red bone marrow, and thymus.

39. What are the sites of blood cell production outside the embryo and within the embryo?

21.24 Aging and the Cardiovascular System

• Explain the effects of aging on the cardiovascular system.

General changes in the cardiovascular system associated with aging include decreased compliance (distensibility) of the ay-or-tuh, reduction in cardiac muscle fiber size, progressive loss of cardiac muscular strength, reduced cardiac output, a decline in maximum heart rate, and an increase in systolic blood pressure. Total blood cholesterol tends to increase with age, as does low-density lipoprotein L.D.L; high-density lipoprotein H.D.L tends to decrease. There is an increase in the incidence of coronary artery disease, the major cause of heart disease and death in older Americans. Congestive heart failure, a set of symptoms associated with impaired pumping of the heart, is also prevalent in older individuals. Changes in blood vessels that serve brain tissue—for example, atherosclerosis—reduce nourishment to the brain and result in malfunction or death of brain cells. By age 80, cerebral blood flow is 20% less and renal blood flow is 50% less than in the same person at age 30 because of the effects of aging on blood vessels.

Checkpoint 40. How does aging affect the heart?

To appreciate the many ways the blood, heart, and blood vessels contribute to homeostasis of other body systems, examine Focus on Homeostasis: Contributions of the Cardiovascular System.

Disorders: Homeostatic Imbalances

Hypertension

About 45 million Americans have hypertension hypertension, or persistently high blood pressure. It is the most common disorder affecting the heart and blood vessels and is the major cause of heart failure, kidney disease, and stroke. New guidelines for hypertension have been instituted because clinical studies have linked what were once considered fairly low blood pressure readings to an increased risk of cardiovascular disease. The new guidelines are as follows:

Table summary: This table outlines the blood pressure classification categories based on systolic and diastolic measurements, showing how increasing pressure levels transition from normal ranges into prehypertension and eventually stage one hypertension.

Normal blood pressure: Systolic less than 120 and diastolic less than 80. Recommendation is to maintain optimal lifestyle habits such as a healthy diet and regular exercise.

Elevated blood pressure: Systolic between 120 to 129 and diastolic less than 80. Individuals with elevated blood pressure are likely to develop high blood pressure unless there is intervention to control it.

Hypertension Stage 1: Systolic between 130 to 139 or diastolic between 80 to 89. Recommendations are to institute lifestyle changes and possibly start taking blood pressure-lowering drugs if a person has had or is at high risk for a cardiovascular event such as heart attack or stroke, has diabetes mellitus, chronic kidney disease, or is at risk for atherosclerosis.

Hypertension Stage 2: Systolic greater than 140 or diastolic greater than 90. Recommendations are to institute lifestyle changes and start taking a combination of blood pressure-lowering drugs.

Hypertensive Crisis: Systolic higher than 180 and/or diastolic higher than 120. Recommendations are to institute lifestyle changes, change blood pressure-lowering drugs in the absence of any other problems, and require immediate hospitalization if there are signs of organ damage.

Types and Causes of Hypertension Between 90

and 95% of all cases of hypertension are classified as primary hypertension, a persistently elevated blood pressure that cannot be attributed to any identifiable cause. The remaining 5 to 10% of cases are referred to as secondary hypertension, which has an identifiable underlying cause. Several disorders cause secondary hypertension:

• Obstruction of renal blood flow or disorders that damage renal tissue may cause the kidneys to release excessive amounts of renin into the blood. The resulting high level of angiotensin I.I causes vasoconstriction, thus increasing systemic vascular resistance.

• Hypersecretion of aldosterone—resulting, for instance, from a tumor of the suprarenal cortex—stimulates excess reabsorption of salt and water by the kidneys, which increases the volume of body fluids.

• Hypersecretion of epinephrine and norepinephrine may occur by a pheochromocytoma (fe-o-kro'-mo-si-To-ma), a tumor of the suprarenal medulla. Epinephrine and norepinephrine increase heart rate and contractility and increase systemic vascular resistance.

Damaging Effects of Untreated Hypertension

High blood pressure is known as the “silent killer” because it can cause considerable damage to the blood vessels, heart, brain, and kidneys before it causes pain or other noticeable symptoms. It is a major risk factor for the number-one (heart disease) and number-three (stroke) causes of death in the United States. In blood vessels, hypertension causes thickening of the tunica media, accelerates development of atherosclerosis and coronary artery disease, and increases systemic vascular resistance. In the heart, hypertension increases the afterload, which forces the ventricles to work harder to eject blood.

The normal response to an increased workload due to vigorous and regular exercise is hypertrophy of the myocardium, especially in the wall of the left ventricle. This is a positive effect that makes the heart a more efficient pump. An increased afterload, however, leads to myocardial hypertrophy that is accompanied by muscle damage and fibrosis (a buildup of collagen fibers between the muscle fibers).

As a result, the left ventricle enlarges, weakens, and dilates. Because arteries in the brain are usually less protected by surrounding tissues than are the major arteries in other parts of the body, prolonged hypertension can eventually cause them to rupture, resulting in a stroke. Hypertension also damages kidney arterioles, causing them to thicken, which narrows the lumen; because the blood supply to the kidneys is thereby reduced, the kidneys secrete more renin, which elevates the blood pressure even more.

Lifestyle Changes to Reduce Hypertension Although several categories of drugs (described shortly) can reduce elevated blood pressure, the following lifestyle changes are also effective in managing hypertension:

• Lose weight. This is the best treatment for high blood pressure short of using drugs. Loss of even a few pounds helps reduce blood pressure in overweight hypertensive individuals.

• Limit alcohol intake. Drinking in moderation may lower the risk of coronary heart disease, mainly among males over 45 and females over 55. Moderation is defined as two drinks a day for males younger than age 65, one drink a day for males 65 and older, and one drink a day for females of any age. A drink is defined as 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of 80-proof distilled spirits.

• Exercise. Becoming more physically fit by engaging in moderate activity (such as brisk walking) several times a week for 30 to 45 minutes can lower systolic blood pressure by about 10 mmHg.

• Reduce intake of sodium (salt). Roughly half the people with hypertension are “salt sensitive.” For them, a high-salt diet appears to promote hypertension, and a low-salt diet can lower their blood pressure.

• Maintain recommended dietary intake of potassium, calcium, and magnesium. Higher levels of potassium, calcium, and magnesium in the diet are associated with a lower risk of hypertension.

- Don't smoke or quit smoking. Smoking has devastating effects on the heart and can augment the damaging effects of high blood pressure by promoting vasoconstriction.

• Manage stress. Various meditation and biofeedback techniques help some people reduce high blood pressure. These methods may work by decreasing the daily release of epinephrine and norepinephrine by the suprarenal medulla.

Focus on Homeostasis

Contributions of the Cardiovascular System for All Body Systems

• The heart pumps blood through blood vessels to body tissues, delivering oxygen and nutrients and removing wastes by means of capillary exchange

• Circulating blood keeps body tissues at a proper temperature

Image summary: This figure is an anatomical illustration. It depicts the human circulatory system, showing the network of arteries and veins distributed throughout the entire body, with the heart positioned centrally in the chest. The illustration demonstrates that blood vessels extend from the heart to the extremities, including the head, arms, and legs, indicating a comprehensive system for transporting blood to all organs and tissues.

Image summary: This is an anatomical illustration. The figure depicts a full body view of a human female form. The image serves as a general representation of human anatomy and physical proportions.

Integumentary System

• Blood delivers clotting factors and white blood cells that aid in hemostasis when skin is damaged and contribute to repair of injured skin

• Changes in skin blood flow contribute to body temperature regulation by adjusting the amount of heat loss via the skin

• Blood flowing in skin may give skin a pink hue

Image summary: This figure is a medical scan. It displays a full body anatomical image showing the distribution of a tracer across the skeletal system and soft tissues. The scan indicates a widespread and systemic distribution of the tracer throughout the entire body, suggesting a generalized physiological process rather than a localized abnormality.

Skeletal System

• Blood delivers calcium and phosphate ions that are needed for building bone extracellular matrix

Image summary: This figure is an anatomical illustration. It depicts the muscular system of a human body from a frontal perspective, showing the distribution of muscles across the torso, arms, and legs. The illustration demonstrates the symmetrical arrangement of muscle groups and their relative positions across the entire body.

• Blood transports hormones that govern building and breakdown of bone extracellular matrix, and erythropoietin that stimulates production of red blood cells by red bone marrow

Muscular System

Image summary: This figure is an anatomical illustration. It depicts a human body with a focus on the internal organs located in the abdominal region. The illustration indicates that specific organs in the upper abdomen are the primary area of interest, suggesting a localized medical or biological condition affecting those internal structures.

• Blood circulating through exercising muscle removes heat and lactic acid

Nervous System

Image summary: This is an anatomical illustration. The figure depicts a human female body with various internal organs visible through the skin, including the respiratory and digestive systems. The illustration serves to show the relative positioning and arrangement of internal organs within the human torso.

• Endothelial cells lining choroid plexuses in brain ventricles help produce cerebrospinal fluid and contribute to the blood-brain barrier

Endocrine System

• Circulating blood delivers most hormones to their target tissues

• Atrial cells secrete atrial natriuretic peptide

Image summary: This figure is an anatomical diagram. It depicts a human male figure with internal organs visible, specifically highlighting the respiratory system including the lungs and trachea. The illustration indicates that the lungs occupy a significant portion of the upper chest cavity, suggesting their primary role in the body's respiratory function.

Lymphoid (Lymphatic) System and Immunity

• Circulating blood distributes lymphocytes, antibodies, and macrophages that carry out immune functions

• Lymph plasma forms from excess interstitial fluid, which filters from blood plasma due to blood pressure generated by the heart

Image summary: This is an anatomical illustration. The figure depicts a human body with internal organs and a vascular system visible through the skin. The illustration suggests a simplified representation of human anatomy, highlighting the placement of the heart, liver, and major blood vessels relative to the overall body structure.

Respiratory System

• Circulating blood transports oxygen from the lungs to body tissues and carbon dioxide to the lungs for exhalation

Image summary: This is an anatomical diagram. The figure shows a human body with specific internal organs highlighted in the abdominal and pelvic regions. The diagram indicates the location of the kidneys and the bladder within the human torso.

Digestive System

• Blood carries newly absorbed nutrients and water to the liver

• Blood distributes hormones that aid digestion

Urinary System

• Heart and blood vessels deliver 20% of the resting cardiac output to the kidneys, where blood is filtered, needed substances are reabsorbed, and unneeded substances remain as part of urine, which is excreted

Image summary: This is an anatomical diagram. The figure depicts a human silhouette with a focus on the internal organs located in the lower abdominal region. The illustration indicates that the highlighted organs are situated centrally within the pelvic area, suggesting a focus on the renal or reproductive systems.

Genital (Reproductive) Systems

• Vasodilation of arterioles in penis and clitoris causes erection during sexual intercourse

• Blood distributes hormones that regulate reproductive functions

Image summary: This figure is an illustration. It depicts a full-body human figure covered in a pattern of horizontal stripes from the neck down to the feet. The illustration shows a person standing in a neutral pose, with the striped pattern conforming to the contours of the body. The image suggests a conceptual representation of a body integrated with a repetitive linear pattern, emphasizing the form of the human figure through the distortion of the stripes.

Drug Treatment of Hypertension Drugs having several different mechanisms of action are effective in lowering blood pressure. Many people are successfully treated with diuretics diuretics, agents that decrease blood pressure by decreasing blood volume, because they increase elimination of water and salt in the urine. ace (angiotensin-converting enzyme) inhibitors block formation of angiotensin I.I and thereby promote vasodilation and decrease the secretion of aldosterone. Beta blockers (Bâ-ta) reduce blood pressure by inhibiting the secretion of renin and by decreasing heart rate and contractility.

Vasodilators relax the smooth muscle in arterial walls, causing vasodilation and lowering blood pressure by lowering systemic vascular resistance. An important category of vasodilators are the calcium channel blockers, which slow the inflow of Ca²⁺ into vascular smooth muscle fibers. They reduce the heart's workload by slowing Ca²⁺ entry into pacemaker cells and regular myocardial fibers, thereby decreasing heart rate and the force of myocardial contraction.

Medical Terminology

Aneurysm aneurysm A thin, weakened section of the wall of an artery or a vein that bulges outward, forming a balloonlike sac. Common causes are atherosclerosis, syphilis, congenital blood vessel defects, and trauma. If untreated, the aneurysm enlarges and the blood vessel wall becomes so thin that it bursts. The result is massive hemorrhage with shock, severe pain, stroke, or death. Treatment may involve surgery in which the weakened area of the blood vessel is removed and replaced with a graft of synthetic material.

Aortography aortography X-ray examination of the ay-or-tuh and its main branches after injection of a radiopaque dye.

Carotid endarterectomy carotid endarterectomy) The removal of atherosclerotic plaque from the carotid artery to restore greater blood flow to the brain.

Claudication (klaw'-di-KÃ-shun) Pain and lameness or limping caused by defective circulation of the blood in the vessels of the limbs.

Deep vein thrombosis D.V.T The presence of a thrombus (blood clot) in a deep vein of the lower limbs. It may lead to (1) pulmonary embolism, if the thrombus dislodges and then lodges within the pulmonary arterial blood flow, and (2) postphlebitic syndrome, which consists of edema, pain, and skin changes due to destruction of venous valves.

Doppler ultrasound scanning Imaging technique commonly used to measure blood flow. A transducer is placed on the skin and an image is displayed on a monitor that provides the exact position and severity of a blockage.

Femoral angiography An imaging technique in which a contrast medium is injected into the femoral artery and spreads to other arteries in the lower limb, and then a series of radiographs are taken of one or more sites. It is used to diagnose narrowing or blockage of arteries in the lower limbs. Hypotension hypotension Low blood pressure; most commonly used to describe an acute drop in blood pressure, as occurs during excessive blood loss.

Normotensive normotensive Characterized by normal blood pressure.

Occlusion occlusion The closure or obstruction of the lumen of a structure such as a blood vessel. An example is an atherosclerotic plaque in an artery.

Orthostatic hypotension orthostatic; ortho-=straight; -static=causing to stand) An excessive lowering of systemic blood pressure when a person assumes an erect or semierect posture; it is usually a sign of a disease. May be caused by excessive fluid loss, certain drugs, and cardiovascular or neurogenic factors. Also called post-tural hypotension.

Phlebitis (file-Bi-tis; phleb-= vein) Inflammation of a vein, often in a leg.

Thrombectomy thrombectomy; thrombo-= clot) An operation to remove a blood clot from a blood vessel.

Thrombophlebitis (throm'-bo-file-Bi-tis) Inflammation of a vein involving clot formation. Superficial thrombophlebitis occurs in veins under the skin, especially in the calf.

Venipuncture venipuncture; ven-= vein) The puncture of a vein, usually to withdraw blood for analysis or to introduce a solution, for example, an antibiotic. The median cubital vein is frequently used.

White coat (office) hypertension A stress-induced syndrome found in patients who have elevated blood pressure when being examined by health-care personnel, but otherwise have normal blood pressure.

2023

Chapter Review

Review

21.1 Structure and Function of Blood Vessels

1. Arteries carry blood away from the heart. The wall of an artery consists of a tunica intima, a tunica media (which maintains elasticity and contractility), and a tunica externa. Large arteries are termed elastic (conducting) arteries, and medium-sized arteries are called muscular (distributing) arteries. 2. Many arteries anastomose: The distal ends of two or more vessels unite. An alternative blood route from an anastomosis is called collateral circulation. Arteries that do not anastomose are called end arteries.

3. Arterioles are small arteries that deliver blood to capillaries. Through constriction and dilation, arterioles assume a key role in regulating blood flow from arteries into capillaries and in altering arterial blood pressure.

4. Capillaries are microscopic blood vessels through which materials are exchanged between blood and tissue cells; some capillaries are continuous and others are fenestrated. Capillaries branch to form an extensive network throughout a tissue. This network increases surface area, allowing a rapid exchange of large quantities of materials.

5. Precapillary sphincters regulate blood flow through capillaries.

6. Microscopic blood vessels in the liver are called sinusoids.

7. Venules are small vessels that continue from capillaries and merge to form veins.

8. Veins consist of the same three tunics as arteries but have a thinner tunica intima and a thinner tunica media. The lumen of a vein is also larger than that of a comparable artery. Veins contain valves to prevent backflow of blood. Weak valves can lead to varicose veins.

9. Vascular sinuses are veins with very thin walls.

10. Systemic veins are collectively called blood reservoirs because they hold a large volume of blood. If the need arises, this blood can be shifted into other blood vessels through vasoconstriction of veins. The principal blood reservoirs are the veins of the abdominal organs (liver and spleen) and skin.

21.2 Capillary Exchange

1. Substances enter and leave capillaries by diffusion, transcytosis, or bulk flow.

2. The movement of water and solutes (except proteins) through capillary walls depends on hydrostatic and osmotic pressures.

3. The near equilibrium between filtration and reabsorption in capil-laries is called Starling's law of the capillaries.

4. Edema is an abnormal increase in interstitial fluid.

21.3 Hemodynamics: Factors Affecting Blood Flow

1. The velocity of blood flow is inversely related to the cross-sectional area of blood vessels; blood flows slowest where cross-sectional area is greatest. The velocity of blood flow decreases from the ay-or-tuh to arteries to capillaries and increases in venules and veins.

2. Blood pressure and resistance determine blood flow.

3. Blood flows from regions of higher to lower pressure. The higher the resistance, however, the lower the blood flow.

4. Cardiac output equals the mean arterial pressure divided by total resistance (C-O = M.A.P ÷ R).

5. Blood pressure is the pressure exerted on the walls of a blood vessel.

6. Factors that affect blood pressure are cardiac output, blood volume, viscosity, resistance, and the elasticity of arteries.

7. As blood leaves the ay-or-tuh and flows through the systemic circulation, its pressure progressively falls to 0 mmHg by the time it reaches the right ventricle.

8. Resistance depends on blood vessel diameter, blood viscosity, and total blood vessel length.

9. Venous return depends on pressure differences between the venules and the right ventricle.

10. Blood return to the heart is maintained by several factors, including skeletal muscle contractions, valves in veins (especially in the limbs), and pressure changes associated with breathing.

21.4 Control of Blood Pressure and Blood Flow

1. The cardiovascular center is a group of neurons in the medulla oblongata that regulates heart rate, contractility, and blood vessel diameter.

2. The cardiovascular center receives input from higher brain regions and sensory receptors (baroreceptors and chemoreceptors). 3. Output from the cardiovascular center flows along sympathetic and parasympathetic axons. Sympathetic impulses propagated along cardioaccelerator nerves increase heart rate and contractility; parasympathetic impulses propagated along vagus nerves decrease heart rate.

4. Baroreceptors monitor blood pressure, and chemoreceptors monitor blood levels of O 2 , C-O 2 , and hydrogen ions. The carotid sinus reflex helps regulate blood pressure in the brain. The aortic reflex regulates general systemic blood pressure.

5. Hormones that help regulate blood pressure are epinephrine, norepinephrine, antidiuretic hormone, angiotensin I.I, and atrial natriuretic peptide.

6. Autoregulation refers to local, automatic adjustments of blood flow in a given region to meet a particular tissue's need.

O₂ level is the principal stimulus for autoregulation.

21.5 Checking Circulation

1. Pulse is the alternate expansion and elastic recoil of an artery wall with each heartbeat. It may be felt in any artery that lies near the surface or over a hard tissue.

2. A normal resting pulse and heart rate is 70 to 80 beats/min.

3. Blood pressure is the pressure exerted by blood on the wall of an artery when the left ventricle undergoes systole and then diastole. It is measured by the use of a sphygmomanometer.

4. Systolic blood pressure is the arterial blood pressure during ventricular contraction. Diastolic blood pressure is the arterial blood pressure during ventricular relaxation. Normal blood pressure is less than 120/80.

5. Pulse pressure is the difference between systolic and diastolic blood pressure. It normally is about 40 mmHg.

21.6 Shock and Homeostasis

1. Shock is a failure of the cardiovascular system to deliver enough O₂ and nutrients to meet the metabolic needs of cells.

2. Types of shock include hypovolemic, cardiogenic, vascular, and obstructive.

3. Signs and symptoms of shock include systolic blood pressure less than 90 mmHg; rapid resting heart rate; weak, rapid pulse; clammy, cool, pale skin; sweating; hypotension; altered mental state; decreased urinary output; thirst; and acidosis.

21.7 Circulatory Routes: Systemic Circulation

1. The systemic circulation carries oxygenated blood from the left ventricle through the ay-or-tuh to all parts of the body, including some lung tissue, but not the pulmonary alveoli of the lungs, and returns the deoxygenated blood to the right atrium.

2. Among the subdivisions of the systemic circulation are the coronary circulation and the hepatic portal circulation.

21.8 The ay-or-tuh and Its Branches

1. The ay-or-tuh is divided into the ascending ay-or-tuh, aortic arch, thoracic ay-or-tuh, and abdominal ay-or-tuh.

2. Each section gives off arteries that branch to supply the whole body.

21.9 Ascending ay-or-tuh

1. The ascending ay-or-tuh is the part of the ay-or-tuh that extends from the aortic valve of the heart to the aortic arch.

2. The two branches of the ascending ay-or-tuh are the right and left coronary arteries.

21.10 The Aortic Arch

1. The aortic arch is the continuation of the ascending ay-or-tuh.

2. The three branches of the aortic arch in order of origination are the brachiocephalic trunk, left common carotid artery, and left subclavian artery.

21.11 Thoracic ay-or-tuh

1. The thoracic ay-or-tuh is the continuation of the aortic arch.

2. It sends off visceral branches and parietal branches.

21.12 Abdominal ay-or-tuh

1. The abdominal ay-or-tuh is the continuation of the thoracic ay-or-tuh.

2. It gives rise to unpaired visceral branches and paired visceral branches.

21.13 Arteries of the Pelvis and Lower Limbs

1. The abdominal ay-or-tuh ends by dividing into the right and left common iliac arteries.

2. These arteries in turn branch into smaller arteries.

21.14 Veins of the Systemic Circulation

1. Blood returns to the heart through the systemic veins.

2. All veins of the systemic circulation drain into the superior or inferior venae cavae or the coronary sinus, which in turn empty into the right atrium.

21.15 Veins of the Head and Neck

1. The three major veins that drain blood from the head are the internal jugular, external jugular, and vertebral veins.

2. Within the cranial cavity, all veins drain into dural venous sinuses and then into the internal jugular vein.

21.16 Veins of the Upper Limbs

1. Both superficial and deep veins return blood from the upper limbs to the heart.

2. Superficial veins are larger than deep veins and return most of the blood from the upper limbs.

21.17 Veins of the Thorax

1. Most thoracic structures are drained by a network of veins called the azygos system.

2. The azygous system consists of the azygos, hemiazygos, and accessory hemiazygos veins.

21.18 Veins of the Abdomen and Pelvis

1. Many small veins drain blood from the abdomen and pelvis.

2. These veins in turn convey blood into the inferior vena cava.

1. Blood from the lower limbs is drained by both superficial and deep veins.

2. The superficial veins often anastomose with one another and with deep veins along their length.

21.20 Circulatory Routes: The Hepatic Circulation

1. The hepatic portal circulation directs venous blood from the digestive canal organs and spleen into the hepatic portal veins of the liver before it returns to the heart.

2. It enables the liver to utilize nutrients and detoxify harmful substances in the blood.

21.21 Circulatory Routes: The Pulmonary Circulation

1. The pulmonary circulations takes deoxygenated blood from the right ventricle to the pulmonary alveoli within the lungs and returns oxygenated blood from the pulmonary alveoli to the left atrium.

2. The pulmonary circulation includes the pulmonary trunk, pulmonary arteries, and pulmonary veins.

21.22 Circulatory Routes: The Fetal Circulation

1. Fetal circulation exists only in the fetus. It involves the exchange of materials between fetus and mother via the placenta.

2. The fetus derives O 2 and nutrients from and eliminates C-O 2 and wastes into maternal blood. At birth, when pulmonary, digestive, and liver functions begin, the special structures of fetal circulation are no longer needed.

21.23 Development of Blood Vessels and Blood

1. Blood vessels develop from mesenchyme (hemangioblasts to angioblasts to blood islands) in mesoderm called blood islands.

2. Blood cells also develop from mesenchyme (hemangioblasts arrow multipotent stem cells).

3. The development of blood cells from multipotent stem cells derived from angioblasts occurs in the walls of blood vessels in the umbilical vesicle, chorion, and allantois at about 3 weeks after fertilization. Within the embryo, blood is produced by the liver at about the fifth week and in the spleen, red bone marrow, and thymus at about the twelfth week.

21.24 Aging and the Cardiovascular System

1. General changes associated with aging include reduced compliance (distensibility) of blood vessels, reduction in cardiac muscle size, reduced cardiac output, and increased systolic blood pressure.

2. The incidence of coronary artery disease, congestive heart failure, and atherosclerosis increases with age.

Critical Thinking Questions

1. Kim Sung was told that her baby was born with a hole in the upper chambers of his heart. Is this something Kim Sung should worry about?

2. Michael was brought into the emergency room suffering from a gunshot wound. He is bleeding profusely and exhibits the following: systolic blood pressure is 40 mmHg; weak pulse of 200 beats per minute; cool, pale, and clammy skin. Michael is not producing urine but is asking for water. He is confused and disoriented. What is his diagnosis and what, specifically, is causing these symptoms?

3. Maureen's job entails standing on a concrete floor for 10-hour days on an assembly line. Lately she has noticed swelling in her ankles at the end of the day and some tenderness in her calves. What do you suspect is Maureen's problem and how could she help counteract the problem?

Answers to Figure Questions

21.1 The femoral artery has the thicker wall; the femoral vein has the wider lumen.

21.2 Due to atherosclerosis, less energy is stored in the less compliant elastic arteries during systole; thus, the heart must pump harder to maintain the same rate of blood flow.

21.3 Metabolically active tissues use O 2 and produce wastes more rapidly than inactive tissues, so they require more extensive capillary networks.

21.4 Materials cross capillary walls through intercellular clefts and fenestrations, via transcytosis in pinocytic vesicles, and through the plasma membranes of endothelial cells.

21.5 Valves are more important in arm veins and leg veins than in neck veins because, when you are standing, gravity causes pooling of blood in the veins of the limbs but aids the flow of blood in neck veins back toward the heart.

21.6 Blood volume in venules and veins is about 64% of 5 liters, or 3.2 liters; blood volume in capillaries is about 7% of 5 liters, or 350 mL.

21.7 Blood colloid osmotic pressure is lower than normal in a person with a low level of plasma proteins, and therefore capillary reabsorption is low. The result is edema.

21.8 Mean blood pressure in the ay-or-tuh is closer to diastolic than to systolic pressure.

21.9 The skeletal muscle pump and respiratory pump also aid venous return.

21.10 Vasodilation and vasoconstriction of arterioles are the main regulators of systemic vascular resistance.

21.11 Velocity of blood flow is fastest in the ay-or-tuh and arteries.

21.12 The effector tissues regulated by the cardiovascular center are cardiac muscle in the heart and smooth muscle in blood vessel walls.

21.13 Impulses to the cardiovascular center pass from baroreceptors in the carotid sinuses via the glossopharyngeal (9) nerves and from baroreceptors in the aortic arch via the vagus (10) nerves.

21.14 It represents a change that occurs when you stand up because gravity causes pooling of blood in leg veins once you are upright, decreasing the blood pressure in your upper body.

21.15 Diastolic blood pressure = 95 mmHg; systolic blood pressure = 142 mmHg; pulse pressure = 47 mmHg. This person has stage I hypertension because the systolic blood pressure is greater than 140 mmHg and the diastolic blood pressure is greater than 90 mmHg.

21.16 Almost-normal blood pressure in a person who has lost blood does not necessarily indicate that the patient's tissues are receiving adequate blood flow; if systemic vascular resistance has increased greatly, tissue perfusion may be inadequate.

21.17 The two main circulatory routes are the systemic circulation and the pulmonary circulation.

21.18 The four subdivisions of the ay-or-tuh are the ascending ay-or-tuh, aortic arch, thoracic ay-or-tuh, and abdominal ay-or-tuh.

21.19 The arteries supplying the heart are called coronary arteries because they form a crown above the ventricles of the heart.

21.20 Branches of the aortic arch (in order of origination) are the brachiocephalic trunk, left common carotid artery, and left subclavian artery.

21.21 The thoracic ay-or-tuh begins at the level of the intervertebral disc between T.4 and T.5.

21.22 The abdominal ay-or-tuh begins at the aortic hiatus in the diaphragm.

21.23 The abdominal ay-or-tuh divides into the common iliac arteries at about the level of L.4.

21.24 The superior vena cava drains regions above the diaphragm, and the inferior vena cava drains regions below the diaphragm.

21.25 All venous blood in the brain drains into the internal jugular veins.

21.26 The median cubital vein of the upper limb is often used for withdrawing blood.

21.27 The inferior vena cava returns blood from abdominopelvic viscera to the heart.

21.28 Superficial veins of the lower limbs are the dorsal venous arches and the great saphenous and small saphenous veins.

21.29 The hepatic veins carry blood away from the liver.

21.30 After birth, the pulmonary arteries are the only arteries that carry deoxygenated blood.

21.31 Exchange of materials between mother and fetus occurs across the placenta.

21.32 Blood vessels and blood are derived from mesoderm.

You have reached the end of the document.