Chapter 20 Cardiovascular system: Vessels and Circulation
Venous Drainage - Three primary pairs of veins drain the neck and head (figure 20.21a). On each side of the head is a vertebral vein and an external jugular vein, both of which empty into the subclavian vein. The third vein is an internal jugular vein, which joins with the subclavian vein to form the brachiocephalic vein. The external jugular primarily drains superficial head and neck structures, while the internal jugular drains blood from the cranial cavity. The right and left brachiocephalic veins join to form the superior vena cava
(venous drainage.jpeg) Venous Blood Flow from the Head and Neck. (a) Right lateral view shows the major veins and their tributaries that drain blood from the head and neck. (b) Venous drainage of the cranium from a superior anterolateral view. The dural venous sinuses are labeled in bold.
Blood Vessels - ***Increased peripheral resistance. More blood vessels are stimulated to vasoconstrict than vasodilate because there are more blood vessels that house smooth muscle with α1 receptors than β2 receptors; consequently, peripheral resistance is increased, raising blood pressure.^^ Larger circulating blood volume. Vasoconstriction of veins shifts blood from venous reservoirs (see section 20.1d) and circulating blood volume increases, thus increasing blood pressure. Redistribution of blood flow. More blood flow reaches skeletal muscles and the heart, and less blood flow goes to most other structures. Thus, organs requiring additional nutrients and oxygen have adequate perfusion.
***Inhibition of the sympathetic division reverses these changes: Peripheral resistance decreases, blood shifts to venous reservoirs, and blood flow distribution returns toward its previous levels.^^
Baroreceptors - ***Baroreceptors are specialized sensory nerve endings that respond to stretch (see section 16.1d). Here the baroreceptors detect stretch of the blood vessel wall that occurs as the blood volume within them changes. The two main baroreceptors for the cardiovascular system are those located within the aortic arch and carotid sinuses, specifically within the tunica externa. The aortic arch baroreceptors transmit nerve signals to the cardiovascular center through the vagus nerve (CN X; see table 13.5).^^ The aortic arch baroreceptors are important in regulating systemic blood pressure.
***The carotid sinus baroreceptors are located within each internal carotid artery, near the artery's initial bifurcation (branching) from the common carotid artery. They transmit nerve signals to the cardiovascular center through the glossopharyngeal nerve (CN IX; see table 13.5). These baroreceptors within the carotid sinuses monitor blood pressure changes in the head and neck—thus, they are important in monitoring blood pressure affecting the brain. Baroreceptors in the carotid sinuses are more sensitive to blood pressure changes than baroreceptors in the aortic arch—a condition that is not surprising, given the importance of delivering sufficient blood to the brain. Baroreceptors continuously transmit nerve signals along sensory neurons within the vagus nerves and glossopharyngeal nerves to the cardiovascular center at a particular rate. Their firing rate changes when the stretch in the blood vessel wall changes.^^
Vessels called true capillaries branch from the metarteriole and make up the bulk of the capillary bed. At the origin of each true capillary, a smooth muscle ring called the precapillary sphincter controls blood flow into the true capillaries. Sphincter relaxation permits blood to flow into the true capillaries, whereas sphincter contraction causes blood to flow directly from the metarteriole and thoroughfare channel into the postcapillary venule with blood bypassing the capillary bed. The precapillary sphincters go through cycles of contracting and relaxing at a rate of about 5 to 10 cycles per minute. This cyclical process is referred to as vasomotion.
At any given time, only about one-quarter of the capillary beds are open, because there are over 60,000 miles of capillaries and only about 250 to 300 milliliters (mL) of blood (about 5% of the total blood volume) moving through the capillaries. There is simply not enough blood available to fill all capillaries at the same time. The specific amount of blood entering capillaries per unit time per gram of tissue is called perfusion, typically expressed in milliliters per minute per gram (mL/min/g) (see section 19.1a).
Capillaries are the smallest blood vessels. They connect arterioles to venules (the smallest veins)
The average capillary is approximately 1 mm in length with a diameter of 8 to 10 micrometers, just slightly larger than the diameter of a single erythrocyte. The narrow vessel diameter means erythrocytes must move in single file (termed rouleau) through each capillary (see section 18.3b). Capillaries consist solely of an endothelial layer (of simple squamous cells) resting on a basement membrane. The narrow vessel diameter and the thin wall are optimal for exchange of substances between blood and body tissues.
*Vessel walsl are composed of layers called "tunics" (tunica = coat). The tunic surround the "lumen", or inside space, of the vessel through which blood flows. The three tunics are the "tunica intima", "tunica media", and "tunica externa". Arteries, capillaries, and veins differin both the specific composition of their tunics and their functions.
("Folder CH20, capp.jpeg) Both arteries and veins have a tunica intima, tunica media and externa. However, an artery has a thicker tunica media and a relatively smaller lumen, whereas a vein's thickest layer is the tunica externa, and it has a larger lumen. Larger veins also have valves. Capilalries typically have only a tunica intima (basement membrane and endothelium), but they do not have a subendothelial layer. (*Different types of arteries vary in the distribution of elastic fibers within the tunica media. The array of elastic fibers depicted here, which are labeled elsatic laminae, is the arrangement within a muscular artery.)
Arterial Supply - Most of the blood to the head and neck is supplied by the common carotid arteries (figure 20.20a). The common carotid arteries are positioned parallel immediately lateral to either side of the trachea. At the superior border of the thyroid cartilage of the larynx, each artery divides into an external carotid artery, which supplies structures external to the skull, and an internal carotid artery, which supplies internal skull structures. Recall that the carotid sinus, a receptor that helps regulate blood pressure, is within the internal carotid artery near its bifurcation from the common carotid artery (see sections 19.5b and 20.6a).
(arterial supply.jpeg) Arterial Blood Flow to the Head and Neck. (a) Right lateral view shows the branches that supply blood to the head and neck. (b) An inferior view of the brain shows the branches of the internal carotid and vertebral arteries that supply the brain. An inset shows an enlarged view of the cerebral arterial circle, which is an anastomosis of blood vessels that surrounds the hypophyseal fossa of the sphenoid bone, which houses the pituitary gland.
Thoracic and Abdominal Walls - Arterial Supply - The thoracic and abdominal walls are both supplied by several pairs of arteries (figure 20.22a). A left and right internal thoracic artery emerge from each subclavian artery to supply the anterior thoracic wall and mammary gland. Each internal thoracic artery is located lateral to the sternum and has the following branches: the first six anterior intercostal arteries and a musculophrenic (mŭs′kū-lō-fren′ik; phren = diaphragm) artery that divides into anterior intercostal arteries 7-9. The intercostal arteries supply the contents of the intercostal spaces. The internal thoracic artery then becomes the superior epigastric (ep-i-gas′trik; epi = upon, gastric = stomach) artery, which transports blood to the superior abdominal wall.
(thoracic 123.jpeg) Figure 20.22Circulation to the Thoracic and Abdominal Body Walls. (a) The arteries that supply the thoracic and abdominal wall are shown. Notice the anastomoses of the superior and inferior epigastric arteries. (b) The venous drainage of the thoracic and abdominal wall is more complex than its arterial supply. (c) A schematic of the venous drainage.
Aldosterone and Antidiuretic Hormone - Aldosterone is released from the adrenal cortex in response to several stimuli, including angiotensin II. Aldosterone increases the absorption of sodium ion (Na+) and water in the kidney, decreasing their loss in the urine; this helps maintain blood volume and blood pressure. Aldosterone is discussed in sections 17.9a and 25.4c. Antidiuretic hormone (ADH) is released from the posterior pituitary in response to nerve signals from the hypothalamus (see section 17.7b). The hypothalamus stimulates the posterior pituitary following either detection of increased concentration of blood (typically correlated with low blood volume) or stimulation of the hypothalamus by angiotensin II. ADH increases the absorption of water in the kidney, decreasing its loss in the urine; this helps maintain blood volume and blood pressure.
***ADH also stimulates the thirst center so that there is fluid intake, and blood volume increases. During extreme cases of low blood volume, as might occur with hemorrhaging, extensive release of ADH occurs, which causes vasoconstriction.^^ This vasoconstriction increases peripheral resistance and blood pressure. This is why ADH is also referred to as vasopressin. Antidiuretic hormone is discussed in detail in section 25.4b.
Integration of Variables That Influence Blood Pressure - ***Homeostatic mechanisms to maintain a normal blood pressure are dependent on three primary variables: cardiac output, resistance, and blood volume. All three of these variables are directly related to blood pressure. An increase in any of the three increases blood pressure, and a decrease in any of the three decreases blood pressure.^^ The relationship of these primary variables is summarized in figure 20.16.
***Figure 20.16Factors That Regulate Blood Pressure. Three primary factors influence blood pressure: (a) cardiac output, (b) peripheral resistance, and (c) blood volume.^^
Arterial Flow to the Upper Limb - ***A subclavian artery supplies blood to each upper limb. The left subclavian artery emerges directly from the aortic arch, while the right subclavian artery is a division of the brachiocephalic trunk (see figure 20.22a). Before extending into the arm, the subclavian artery extends multiple branches to supply parts of the body's upper region: the vertebral artery, the thyrocervical trunks, the costocervical trunk, and the internal thoracic artery, as described in section 20.10a.^^ ***After the subclavian artery passes over the lateral border of the first rib, it is renamed the axillary (ak′sil-ār-ē) artery (figure 20.27). The axillary artery extends many branches to the shoulder and thoracic region. When the axillary artery passes the inferior border of the teres major muscle, it is renamed the brachial (brā′kē-ăl) artery. One of its branches is the deep brachial artery, which supplies blood to most brachial (arm) muscles. In the antecubital region, the brachial artery divides into a radial artery and an ulnar artery. Both arteries supply the forearm and wrist before they anastomose and form two arterial arches in the palm: the deep palmar arch (formed primarily from the radial artery) and the superficial palmar arch (formed primarily from the ulnar artery). Digital arteries emerge from the arches to supply the fingers.^^
***Figure 20.27Vascular Supply to the Upper Limb. The subclavian artery transports oxygenated blood to the upper limb; veins merge to return deoxygenated blood to the heart. (a) Arteries that supply the upper limb. (b) Superficial and deep veins that return blood from the upper limb.^^
Fetal Circulation ***The fetal circulatory route is listed here and shown in figure 20.29:^^ *** 1.) Oxygenated blood from the placenta enters the body of the fetus through the umbilical vein. *** 2.)The blood from the umbilical vein is shunted away from the liver and directly toward the inferior vena cava through the ductus venosus (dŭk′tŭs vē-nō′sŭs). *** 3.)Oxygenated blood in the ductus venosus mixes with deoxygenated blood in the inferior vena cava. *** 4.)Blood from the superior and inferior venae cavae empties into the right atrium. *** 5.)Because pressure is greater on the right side of the heart than on the left side, most of the blood is shunted from the right atrium to the left atrium via the foramen ovale. This blood flows into the left ventricle and then is pumped out through the aorta. *** 6.)A small amount of blood enters the right ventricle and then the pulmonary trunk, but much of this blood is shunted from the pulmonary trunk to the aorta through a vessel detour called the ductus arteriosus (ar-tēr′ē-ō′sŭs). *** 7.)Blood is transported to the rest of the body, and the deoxygenated blood returns to the placenta through a pair of umbilical arteries. *** 8.) Nutrient and gas exchange occurs at the placenta (see section 29.2e), and the cycle repeats.
***Figure 20.29Fetal Circulation. Structural changes in both the heart and blood vessels accommodate the different needs of the fetus and the newborn. The pathway of blood flow is indicated by black arrows. The chart at the bottom of the drawing summarizes the fate of each fetal cardiovascular structure following birth.^^
Systematic Veins as Blood reservoirs - The percentage of the total blood that is moving through each of the different components of the cardiovascular system while at rest is illustrated in figure 20.6. Relatively small amounts of blood are within the pulmonary circulation (about 18%) and the heart (about 12%). The largest percentage of blood is within the systemic circulation (about 70%), with the greatest amount (about 55%) within the body's systemic veins. The relatively large amount of blood within veins allows veins to function as blood reservoirs. Blood may be shifted from venous reservoirs into circulation through vasoconstriction of veins, when more blood is needed with increased physical exertion—and shifted back into venous reservoirs through vasodilation of veins, when less blood is needed at rest
***The largest percentage of blood is within the systemic circulation (about 70%), with the greatest amount (about 55%) within the body's systemic veins. The relatively large amount of blood within veins allows veins to function as blood reservoirs
General Arterial Flow Out of the Heart - Oxygenated blood is pumped out of the left ventricle of the heart and enters the ascending aorta. The left and right coronary arteries emerge immediately from the wall of the ascending aorta and supply the heart wall (figure 20.19 inset; also see figure 19.13a). Page 819 The ascending aorta curves toward the left side of the body and becomes the aortic arch (also called the arch of the aorta). Recall from section 20.6a that the aortic bodies for regulating blood pressure are within the tunica externa of the aortic arch.
***Three main arterial branches emerge from the aortic arch: ***1.)The brachiocephalic (brā-kē-ō-se-fal′ik) trunk, which bifurcates into the right common carotid (ka-rot′id) artery, supplying arterial blood to right side of the head and neck, and the right subclavian (sŭb-klā′vē-an; sub = beneath) artery, supplying the right upper limb and some thoracic structures ***2.)The left common carotid artery, supplying the left side of the head and neck ***3.)The left subclavian artery, supplying the left upper limb and some thoracic structures ***The aortic arch curves posterior to the heart and projects inferiorly as the descending thoracic aorta, several branches of which supply the thoracic wall and internal organs. As this artery extends inferiorly through the aortic opening (hiatus) in the diaphragm, it is renamed the descending abdominal aorta, where it supplies the abdominal wall and internal organs.^^
Single Pathway - In the simple pathway, one major artery delivers blood to the organ or body region and then branches into smaller and smaller arteries to become arterioles. Each arteriole feeds into a single capillary bed. A venule drains blood from the capillaries and merges with other venules to form one major vein that drains blood from the organ or body region. Thus, the simple pathway includes one artery, a capillary bed, and one vein associated with an organ or a body region. Blood transported to and from the spleen is an example of a simple blood flow pathway. A single splenic artery delivers oxygenated blood to the spleen with the exchange made in a capillary bed of the spleen, and a single splenic vein drains deoxygenated blood from the spleen (see figure 21.7a). Arteries that provide only one pathway through which blood can reach an organ are referred to as end arteries (see section 19.4a).
***Thus, the simple pathway includes one artery, a capillary bed, and one vein associated with an organ or a body region. Blood transported to and from the spleen is an example of a simple blood flow pathway.
Blood Vessels: classified into three primary types based on function:
-Arteries: transport blood away from the heart to the capillaries. -Capillaries:are microscopic, relatively porous blood vessels for the exchange of substances between blood and tissues. -Veins:drain blood from the capillaries, transporting it back to the heart.
Autoregulation and Changing Metabolic Activity - Autoregulation is most noticeable when blood supply is temporarily disrupted and then restored. When the blood flow is temporarily disrupted, the tissue is deprived of needed oxygen and nutrients, and metabolic wastes accumulate. When the local blood flow is restored, there is a marked increase in blood flow to the affected tissue—a condition called reactive hyperemia. The additional blood is required to resupply the oxygen and nutrients and eliminate the accumulated wastes.
An example of reactive hyperemia occurs when you enter a warm room after being outside in the cold, and your cheeks turn bright red. When you were outside in the cold, the blood vessels in the dermis constricted, forcing more blood away from the dermis through an arteriovenous shunt to conserve heat (see sections 1.6b, 6.1d and 27.8b). When you leave the cold, the reddish color of your cheeks is due to increased blood flow in the dermis. After a short period of time, the reddish color subsides, as normal blood flow in the skin resumes.
Arterial Supply to the Abdomen - ***Three unpaired arteries emerge from the anterior wall of the descending abdominal aorta to supply the GI tract. From superior to inferior, these arteries are the celiac trunk, superior mesenteric artery, and inferior mesenteric artery (figure 20.24).^^ The specific regions that each arterial branch supplies blood to are included in figure 20.24b.
Arterial Supply to the Gastrointestinal Tract and Abdominal Organs. The celiac trunk, superior mesenteric artery, and inferior mesenteric artery supply most of the abdominal organs. (a) Branches of the celiac trunk supply part of the esophagus, stomach, spleen, pancreas, liver, and gallbladder. (b) Branches of the superior mesenteric and inferior mesenteric arteries primarily supply the intestines.
*Elastic arteries are the largest arteries, with diameters ranging from 2.5 to 1 centimeters. They are also called conducting arteries because they conduct blood—from the heart to the smaller muscular arteries.
As their name suggests, these arteries have a large proportion of elastic fibers; these are present throughout all three tunics, especially in the tunica media. The abundant elastic fibers allow the artery to stretch and accommodate the blood when a heart ventricle ejects blood into it during ventricular systole (contraction) and then recoil, which helps propel the blood through the arteries during ventricular diastole (relaxation). The largest arteries close to the heart (e.g., aorta, pulmonary trunk, brachiocephalic, common carotid, subclavian) and the common iliac arteries are examples of elastic arteries. Elastic arteries branch into muscular arteries.
Blood Flow Distribution During Exercise - During exercise, there is an increase in total blood flow due to a faster and stronger heartbeat and because blood is removed from the "reservoirs" of the veins to the active circulation. There is also a redistribution of blood. Both of these changes help ensure that the most metabolically active tissues are receiving adequate blood flow to meet the needs of the tissue cells.
Blood flow to the coronary arteries of the heart increases approximately three-fold (from 250 mL/min to 750 mL/min), a change that helps to ensure that sufficient oxygen reaches the cardiac muscle within the heart wall (see section 19.4a). Skeletal muscle blood flow increases an amazing 11-fold (from 1100 mL/min to 12,500 mL/min)—which is approximately 70% of the total cardiac output—a change needed to meet the high metabolic demands experienced by skeletal muscle during exercise (see section 10.4a). The percentage of blood flow to the skin increases to almost five times its resting level (from 400 mL/min to 1900 mL/min) to dissipate heat (see sections 1.6b and 6.1d).
The cross-sectional area of a vessel is the diameter of the vessel's lumen. The total cross-sectional area is estimated as the aggregate lumen diameter across the total number of a given type of vessel (artery, capillary, or vein) if they were all positioned side by side. You may be surprised to learn that, although the cross-sectional area of an individual artery is relatively large, the total cross-sectional area of arteries is relatively small. This observation is also accurate for veins, which have a relatively small total cross-sectional area. See the blue line on the graph in figure 20.9. In comparison, an individual capillary has a very small cross-sectional area; however, the total cross-sectional area of capillaries—of which there are approximately 60,000 miles—is the largest with a value of approximately 4500 square centimeters (cm2). The total cross-sectional area is physiologically significant because of its influence on blood flow velocity.
Blood flow velocity is the rate of blood transported per unit time and typically measured in centimeters per second. Observe the graph in figure 20.9 and notice that there is an inverse relationship between the total cross-sectional area and blood flow velocity (red line on the graph). Blood flow velocity in both arteries and veins, with their relatively small total cross-sectional area, is relatively fast. In comparison, blood flow velocity in capillaries, with their relatively large total cross-sectional area, is relatively slow. Thus, blood flow velocity changes as it moves through the different types of vessels: Velocity of blood flow is relatively fast in the arteries, slowest in the capillaries, and relatively fast again through the veins. Consider the analogous phenomenon of water flow in a river. In regions where the river is narrow, the river moves more quickly, and where the river is wider, the river moves more slowly. Of course, the amount of water flow is the same in these different regions and always moves toward the ocean. Likewise, blood flow velocity is altered as it moves through the different portions of the vasculature, but it always moves along a blood pressure gradient (described in section 20.5a) as it is transported through the vasculature of the cardiovascular system.
***Blood pressure is the force per unit area that blood exerts against the inside wall of a vessel (as described earlier in section 20.3b). A blood pressure gradient is the change in blood pressure from one end of a blood vessel to its other end. A blood pressure gradient exists in the vasculature because blood pressure is highest in the arteries as the heart rhythmically contracts, and it is lowest in the veins.^^
Blood pressure gradients are both clinically and physiologically significant because they are the driving force that propels blood through the vessels. Please refer to figure 20.11 as you read through this section.
Capillary beds - Capillaries do not function independently; rather, a group of capillaries (10 to 100) function together and form a capillary bed (figure 20.5). A capillary bed is fed by a metarteriole (met′ar-tēr′ē-ōl; meta = between), which is a branch of an arteriole. The proximal part of the metarteriole is encircled by scattered smooth muscle cells, whereas the distal part of the metarteriole (called the thoroughfare channel) has no smooth muscle cells. The thoroughfare channel connects to a postcapillary venule (ven′ūl, vē′nūl), which drains the capillary bed.
Capillary Bed Structure and Perfusion Through the Bed. A capillary bed originates from a metarteriole. The metarteriole continues as a thoroughfare channel that merges with the postcapillary venule. True capillaries branch from the metarteriole, and blood flow into these true capillaries is regulated by the precapillary sphincters. (a) A well-perfused capillary bed, with all of the precapillary sphincters relaxed, and (b) a capillary bed where most blood bypasses the capillary bed due to contracted precapillary sphincters.
Types of Capillaries - Capillaries are differentiated based on their relative degree of permeability and include continuous capillaries, fenestrated capillaries, and sinusoids
Continuous capillaries are the most common type of capillary. The endothelial cells form a complete, continuous lining around the lumen that rests on a complete basement membrane. Tight junctions (see section 4.6d) secure endothelial cells to one another; however, they do not form a complete "seal." The gaps between the endothelial cells are called intercellular clefts. Materials can move into or out of the blood either through endothelial cells by membrane transport processes (e.g., diffusion, pinocytosis; see section 4.3) or between endothelial cells through intercellular clefts by diffusion and bulk flow (see section 20.3).
Capillary blood pressure - Capillary blood pressure must be sufficient for exchange of substances between the blood and surrounding tissue, but not so high that it would damage the relatively fragile capillaries. Blood pressure on the arterial end of the capillary is about 40 mm Hg and drops quickly (along the approximately 1-millimeter length of a capillary) to below 20 mm Hg on the venous end of the capillary. These blood pressure values are used to determine net filtration pressure for capillary exchange, as previously described in section 20.3c. Recall that the relatively high blood pressure on the arteriole end of the capillary accounts for filtration and the relatively low blood pressure on the venous end allows for reabsorption as colloid osmotic pressure pulls fluids back into the blood.
Deep vein thrombosis - Deep vein thrombosis (throm-bō′sis; a clotting) (DVT) refers to a thrombus (blood clot) in a vein. The most common site for the thrombus is a vein in the sural region (calf). DVT typically occurs in individuals with heart disease or those who are inactive or immobile for a long period of time, such as bedridden patients. Even healthy individuals who have been on a long airline trip may develop DVT.
The size of intercellular clefts prevents the movement of large substances, including formed elements and plasma proteins, while allowing the movement of fluid containing small substances (smaller than about 5 nanometers), such as glucose, amino acids, and ions. Continuous capillaries are found, for example, in muscle, the skin, the thymus, the lungs, and the central nervous system.
Fenestrated (fen′es-trā′ted; fenestra = window) capillaries are also composed of a complete, continuous lining of endothelial cells and a complete basement membrane. However, small regions of the endothelial cells (typically 10 to 100 nanometers in diameter) are extremely thin; these thin areas are called fenestrations (or pores). Fenestrations are small enough to prevent formed elements from passing through the wall yet large enough to allow the movement of some smaller plasma proteins. Fenestrated capillaries are seen where a great deal of fluid transport between the blood and interstitial tissue occurs. Examples of structures that contain fenestrated capillaries include the small intestine for the absorption of nutrients (see section 26.3b), the ciliary process of the eye in the production of aqueous humor (see section 16.4b), the choroid plexus of the brain in the production of cerebrospinal fluid (see section 13.2c), most of the endocrine glands to facilitate the absorption of hormones into the blood (see section 17.1a), and the kidneys for the filtering of blood (see section 25.5c).
*Arteries and veins that supply the same body region and tend to lie next to one another are called companion vessels. **Compared to their venous companions, arteries have a thicker tunica media, a narrower lumen, and more elastic and collagen fibers. These differences mean that arterial walls can spring back to shape and are more resilient and resistant to changes in blood pressure than are veins. In addition, an artery remains patent (open) even without blood in it. **In contrast, veins have a thicker tunica externa, a wider lumen, and less elastic and collagen fibers than a companion artery. The wall of a vein is typically collapsed if no blood is in the vessel.
Figure 20.2 is a histologic image of a companion artery and vein. (CH20, figure202.jpeg)
Chemoreceptor Reflexes - ***The two main peripheral chemoreceptors are the aortic bodies and carotid body. The aortic bodies are located in the arch of the aorta, whereas a carotid body is located at the bifurcation of each common carotid as it splits into an external carotid and an internal carotid artery. Both aortic and carotid bodies send sensory input to the cardiovascular center; the aortic bodies send nerve signals via the vagus nerve, and the carotid body along the glossopharyngeal nerve.^^
High carbon dioxide levels, low pH, and very low oxygen levels stimulate the chemoreceptors, and their increased firing primarily stimulates the vasomotor center. The vasomotor center responds by increasing nerve signals along sympathetic pathways to blood vessels, which increases resistance and shifts blood from venous reservoirs to increase venous return. The changes raise blood pressure and increase blood flow, including blood flow to the lungs, which allows for an accompanying change in respiratory gas exchange. As a result, blood gas levels return to normal. (The reason high carbon dioxide, low blood pH, and very low oxygen levels function in stimulating chemoreceptors is discussed in detail in section 23.5c.)
*Tunica externa, or tunica adventitia, is the outer most layer of the blood vessel wall.
It is composed of areolar connective tissue that contains elastic and colalgen fibers. The tunica externa helps anchor the vessel to the other structures. Very large blood vessels require their own blood supply to the tunica externa in the form of a network of small arteries called the vasa vasorum (vessels of vessels). The vasa vasorum extends through the tunica externa
*Tunica media is the middle layer of the vessel wall.
It is composed predominantly of circularly arranged layers of smooth muscle cells that are supported by elastic fibers. Contraction of smooth muscle in the tunica media results in vasoconstriction, or narrowing of the blood vessel lumen; relaxation of the smooth muscle causes vasodilation, or widening of the blood vessel lumen.
Arterioles - Arterioles are the smallest arteries, with diameters ranging from 0.3 millimeters to 10 micrometers; these vessels are not named. In general, arterioles have fewer than six layers of smooth muscle in their tunica media.
Larger arterioles have all three tunics, whereas the smallest arterioles may have a tunica intima surrounded by a single layer of smooth muscle cells. Smooth muscle in the arterioles is slightly contracted (just as your skeletal muscles often are in a partial state of contraction; see section 10.7a). This contracted state is called vasomotor tone and is regulated by the vasomotor center in the brainstem (see section 13.5c). Sympathetic motor tone results in vasoconstriction, which allows for varying degrees of change from this slightly contracted state (see section 15.7a). Blood vessels can be either vasoconstricted to a greater degree to decrease blood flow or vasodilated to allow more blood into an area. Arterioles have a significant role in regulating systemic blood pressure and blood flow to the different areas of the body.
*Etiology - Although the etiology (cause) of atherosclerosis is not completely understood, the response-to-injury hypothesis is the most widely accepted. This proposal states that injury to the endothelium of an arterial wall, especially repeated injury caused by infection, trauma, or hypertension (high blood pressure), results in an inflammatory reaction, eventually leading to the development of an atheroma. The injured endothelium becomes more permeable, which encourages leukocytes and platelets to adhere to the lesion and initiate an inflammatory response
Low-density lipoproteins and very-low-density lipoprotens (LDLs and VLDLs) enter the tunica intima, combine with oxygen, and remain stuck to the vessel wall. This oxidation of these lipoproteins attracts monocytes, which adhere to the endothelium and migrate into the wall. As the monocytes migrate into the wall, they digest the lipids and develop into structures called foam cells. Eventually, smooth muscle cells from the tunica media migrate into the atheroma and proliferate, causing further enlargement of the atheroma and narrowing of the lumen of the blood vessel, thereby restricting blood flow to the regions the artery supplies.
Mean arterial pressure (MAP) is the average (or mean) measure of the blood pressure forces on the arteries. Because diastolic pressure usually lasts slightly longer than systolic pressure, MAP is not simply an average of these two pressures.
Mean arterial pressure is clinically significant because it provides a numeric value for how well body tissues and organs are perfused. A MAP of 70 to 110 mm Hg typically indicates good perfusion. A MAP lower than 60 mm Hg may indicate insufficient blood flow, and a very high MAP could indicate the delivery of too large of blood flow to body tissues with the possibility of causing edema (swelling) in the tissues (see Clinical View 20.5: "Cerebral Edema"). High blood pressure is also a risk factor for Page 806the development of atherosclerosis (see Clinical View 20.1: "Atherosclerosis").
Blood Pressure Gradient in the Systemic Circulation - We now know the normal range of blood pressure values in the various portions of the vasculature. We can use the average blood pressure in the arteries close to the heart and the blood pressure in the inferior vena cava to calculate the total blood pressure gradient in the systemic circulation. This can be determined by viewing figure 20.11. The average, or mean, arterial blood pressure (MAP) in the arteries is 93 mm Hg. The blood pressure in the inferior vena cava is 0 mm Hg. Thus, the blood pressure gradient established by the pumping action of the heart is 93 mm Hg.
Most importantly, this blood pressure gradient is the driving force to move blood through the vasculature. Changes in the blood pressure gradient are directly correlated with changes in total blood flow. An increase in the blood pressure gradient increases total blood flow, whereas a decrease in the blood pressure gradient decreases total blood flow.
Treatment Options - If an artery is occluded (blocked) in one or just a few areas, one form of treatment is an angioplasty (an′jē-ō-plas-tē; angeion = vessel, plastos = formed). A physician inserts a balloon-tip catheter into an artery and positions it at the site where the lumen is narrowed. Then the balloon is inflated, forcibly expanding the narrowed area, and a stent is placed in the vessel. A stent is a piece of wire-mesh that springs open to keep the vessel lumen open. For occluded coronary arteries, sometimes a much more invasive treatment known as coronary bypass surgery may be needed. A vein (e.g., the great saphenous vein) or artery (e.g., the internal thoracic artery) is detached from its original location and grafted from the aorta to the coronary artery system, thus bypassing the area(s) of atherosclerotic narrowing.
Muscular arteries have a proportionately thicker tunica media, with multiple layers of smooth muscle cells. Unlike in elastic arteries, the elastic fibers in muscular arteries are confined to two circumscribed sheets: The internal elastic lamina (lam′i-nă) separates the tunica media from the tunica intima, and the external elastic lamina separates the tunica media from the tunica externa. The relatively greater amount of muscle and lesser amount of elastic tissue result in a better ability to vasoconstrict and vasodilate, although with a lessened ability to stretch in comparison to elastic arteries. Most named arteries (e.g., the brachial, anterior tibial, coronary, and inferior mesenteric arteries) are examples of muscular arteries. Muscular arteries branch into arterioles.
Short-Term Regulation Due to Damaged Tissue or as Part of the Body's Defense System - Local blood flow is also regulated when vasoactive chemicals are released from damaged tissue, leukocytes, and platelets in response to tissue damage or as part of the body's defense. This process is referred to as inflammation and is discussed in detail in section 22.3d. For example, histamine and bradykinin are inflammatory mediators, which are released in response to a trauma, an allergic reaction, an infection, or even exercise. These chemicals cause vasodilation by either directly stimulating arterioles or indirectly by stimulating endothelial cells of the vessel to release nitric oxide. Nitric oxide is a very powerful, but short-lived, vasodilator.
Other vasoactive substances, such as prostaglandins and thromboxanes, are local hormones released with tissue injury that can cause vasoconstriction (see description of local hormones in section 17.3b). Recall from section 18.4a that if endothelial cells are damaged, they release an array of chemicals (including endothelin) that are powerful vasoconstrictors to help prevent blood loss from the damaged vessel. Systemic hormones also alter blood flow, and their effects are described in section 20.6b. See table 20.3 for a list of vasodilators and vasoconstrictors.
Local Blood Flow - Recall that there is simply not enough blood in the body to fill all capillaries at the same time. Blood must therefore be directed to organs and tissues where it is most needed and away from areas where it is not. Local blood flow is the blood delivered locally to the capillaries of a specific tissue and is measured in milliliters per minute. Recall that the specific amount of blood entering capillaries per unit time per gram of tissue is called perfusion. The ultimate function of the cardiovascular system is adequate perfusion of all tissues (see section 19.1a).
The amount of blood delivered to a specific organ or tissue is dependent upon several factors, including (1) the degree of vascularization of the tissue, (2) the myogenic response, (3) local regulatory factors that alter blood flow, and (4) total blood flow.
Degree of Vascularization and Angiogenesis - The degree of vascularization, or the extent of blood vessel distribution within a tissue, determines the potential ability of blood delivery. Organs that are very active metabolically, such as the brain, skeletal muscle, the heart, and the liver, generally are highly vascularized. In comparison, some structures, such as tendons and ligaments, have little vascularization; blood delivery to these tissues is limited. Additionally, some structures contain no capillaries (are avascular); these include epithelial tissue, cartilage, and the cornea and lens of the eye.
The amount of vascularization in a given tissue may change over time through the process of angiogenesis. Angiogenesis (an′jē-ō-jen′ĕ-sis; angio = vessel, genesis = production) is the formation of new blood vessels in tissues that require them. This process helps provide adequate perfusion through long-term anatomic changes that occur over several weeks to months. For example, angiogenesis is stimulated in skeletal muscle in response to aerobic training; in adipose tissue, angiogenesis occurs in adipose connective tissue when an individual gains weight in the form of fat deposits. Angiogenesis also occurs in response to a slow, gradual occlusion (blockage) of coronary vessels, thus potentially providing alternative routes to deliver blood to the heart wall.
Blood Flow Through the Pulmonary Circulation - In the pulmonary circulation, deoxygenated blood is pumped out of the right ventricle into the pulmonary trunk (figure 20.18). This vessel bifurcates into a left and right pulmonary artery that go to the corresponding lungs. The pulmonary arteries divide into smaller arteries that continue to subdivide to form arterioles. These arterioles finally branch into pulmonary capillaries, where gas exchange occurs. ***Carbon dioxide diffuses from the blood and enters the alveoli (air sacs) of the lungs, while oxygen moves in the opposite direction, from the alveoli into the blood.^^
The capillaries merge to form venules and then the pulmonary veins. Typically, two left and two right pulmonary veins transport the newly oxygenated blood to the left atrium of the heart.
Pelvis and Perineum - The aorta divides at its inferior end into the right and left common iliacs. Each common iliac further divides into an internal iliac artery and an external iliac artery. The internal iliac artery is the primary arterial supply to the pelvis and perineum (figure 20.26b). Some branches of the internal iliac artery include the superior and inferior gluteal (glū′tē-ăl) arteries, the superior vesical artery, the middle rectal artery, the vaginal artery and uterine artery (in females), the internal pudendal (pū-den′dăl; pudeo = to feel ashamed) artery, and the obturator (ob′tū-rā-tŏr) artery. Remnant vessels of fetal circulation are the medial umbilical ligaments—previously, umbilical arteries that transported blood from the fetus to the placenta (see section 20.12).
The pelvis and perineum are drained by veins with the same names as the supplying arteries (see figure 20.22b). The veins merge with the internal iliac vein that merges with the common iliac vein, which subsequently drains into the inferior vena cava.
*Arteries progressively branch into smaller vessels as they extend from the heart to the capillaries.
There is both a corresponding decrease in lumen diameter and a change in the composition of the tunic wall that includes both a decrease in the relative amount of elastic fibers and an increase in the relative amount of smooth muscle. Arteries may be classified into three basic types: elastic arteries, muscular arteries, and arterioles (CH20, arteries.jpeg, arteries2.jpeg) Arteries may be classified into three basic types: elastic arteries, muscular arteries, and arterioles
Capillary Exchange - The function of capillaries is to allow for the exchange of substances (e.g., respiratory gases, nutrients, wastes, and hormones) between the blood and the surrounding tissues. Exchange processes include diffusion, vesicular transport, and bulk flow. Diffusion and Vesicular Transport - ***Within systemic capillaries, substances such as oxygen, hormones, and nutrients move by diffusion (see section 4.3a) from their relatively high concentration in the blood into the interstitial fluid and then into the tissue cells, where the concentration of these materials is lower. Conversely, carbon dioxide and waste products diffuse from the higher concentration in the tissue cells to the lower concentration in the interstitial fluid and then to the blood. Very small solutes (e.g., O2, CO2, glucose, ions) and fluids may diffuse via the endothelial cells or intercellular clefts, whereas larger solutes, such as small proteins, must pass through the fenestrations in fenestrated capillaries or gaps in sinusoids.
Vesicular transport occurs when endothelial cells use pinocytosis (see section 4.3c) to form fluid-filled vesicles, which are then transported to the other side of the cell and released by exocytosis. Substances can be moved either from the blood into the interstitial fluid or from the interstitial fluid into the blood. Solutes that are relatively large (e.g., insulin) are transported across the endothelial cells by this method.
*Tunics - The innermost layer of a blood vessel wall is the tunica intima, or tunica interna.
it is composed of an endothelium that is adjacent to the blood vessel lumen and a thin subendothelial layer of areolar connective tissue. The endothelium both provides a smooth surface as the blood moves through the lumen of the blood vessel and releases substances to regulate contraction and relaxation of smooth muscle within the tunica media. Recall that the endothelium is continuous with the endocardium, which is the inner lining of the heart.
Relationship of Blood Flow to Blood Pressure Gradients and Resistance - Total blood flow is the amount of blood that moves through the cardiovascular system per unit time and is influenced by both blood pressure gradients and resistance, as previously described. This relationship is expressed mathematically as follows:
where F = blood flow, ΔP = pressure gradient (P1 − P2), and R = resistance.
Vessel Radius - How specifically does vessel radius influence resistance? Blood tends to flow fastest in the center of the vessel lumen, whereas blood near the sides of the vessels slows, because it encounters resistance from the nearby vessel wall. This difference in flow rate within a blood vessel (or in any conduit) is called laminar flow. You can see evidence of laminar flow by studying a river: The water flow near the banks, or edges, of the river is a bit slower or sluggish, whereas the water flow near the center of the river is quite fast in comparison. So, if vessel diameter increases, relatively less blood flows near the edges and overall blood flow increases. In contrast, if vessel radius decreases, then relatively more blood flows near the edges and overall blood flow decreases.
where F = flow and r = radius of the lumen of a vessel (The radius is 1/2 the diameter of the lumen.). This mathematical expression reflects that flow is directly proportional to the fourth power of a radius. If a vessel vasodilates and its radius increases from 1 millimeter (mm) to 2 mm, the overall change in flow is 16 times greater: If r = 1 mm, then r 4 = 1, and F = 1 mm per second; and when r = 2 mm, then r 4 = 16, and F = 16 mm per second. In contrast, if a vessel vasoconstricts and its radius decreases from 2 mm to 1 mm, the overall change in flow is 16 times less.
