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Unformatted text preview: CHAPTER 4 [ARDlOVASCUlAR PHYSIOLOGY 101 Right atrium Tricuspid valve Right ventricle leer HEAR Left atrium Mitral valve Left ventricle FIGURE 4—1. A schematic diagram showing the circuitry of the cardiovascular system. The arrows Show the direction of blood flow. Percentages represent the percent (%) of cardiac output. Circled numbers correspond to the text. nism, a combination of the first two mechanisms occurs in which both cardiac output and the percent— age distribution of blood flow are altered. This third mechanism is used, for example, in the response to strenuous exercise: Blood flow to skeletal muscle increases to meet the increased metabolic demand by a combination of increased cardiac output and increased percentage distribution to skeletal mus— cle. 4. Blood flow from the organs is collected in the veins. The blood leaving the organs is venous blood and contains waste products from metabo- lism, such as carbon dioxide (C02). This mixed ve— nous blood is collected in veins of increasing size and finally in the largest vein, the vena cava. The vena cava carries blood to the right heart. 5. Venous return to the right atrium. Because the pressure in the vena cava is higher than in the right atrium, the right atrium fills with blood, the venous return. In the steady state, venous return to the right atrium equals cardiac output from the left ventricle. 6. Mixed venous blood fills the right ventri- cle. Mixed venous blood flows from the right atrium to the right ventricle through the AV valve in the right heart, the tricuspid valve. 7. Blood is ejected from the right ventricle into the pulmonary artery. When the right ventricle contracts, blood is ejected through the pulmonic valve into the pulmonary artery, which carries blood to the lungs. Note that the cardiac output ejected from the right ventricle is virtually identical pies that govern blood flow in the cardiovascular system. These basic principles of physics are the same as those applied to the movement of fluids in general. The concepts of flow, pressure, resistance, and capacitance are applied to blood flow to and from the heart and within the blood vessels. Types and Characteristics of Blood Vessels The blood vessels are the conduits through which blood is carried from the heart to the tissues and from the tissues back to the heart. In addition, some blood vessels (capillaries) are so thin walled that substances can exchange across them. The size of the various types of blood vessels varies, as does the histologic characteristics of their walls. These variations have profound effects on their resistance and capacitance properties. Figure 4—2 is a schematic drawing of a vascular bed. The direction of blood flow through the vascu— lar bed is from artery to arteriole, to capillaries, to venule, to vein. A companion figure, Figure 4—3, is a graph showing the total cross—sectional area, the- Larry ImvvuAuLuAVI 1a“, vunuuau Vi, uavuu puuLuulbu in the arteries is called the stressed volume (meaning the blood volume under high pres— sure). Arterioles. The arterioles are the smallest branches of the arteries. Their walls have an extensive development of smooth muscle, and they are the site of highest resistance to blood flow. The smooth muscle in the walls of the arte» rioles is tonically active (i.e., always con— tracted). It is extensively innervated by sympa— thetic nerve fibers, most of which are adrenergic and a few are cholinergic. (x-Adren- ergic receptors are found on the arterioles of several vascular beds (e.g., skin and splanchnic vasculature). When activated, these receptors cause contraction or constriction of the vascu- lar smooth muscle. Constriction produces a de— crease in the diameter of the arteriole, which increases its resistance to blood flow. Bz-Adren- ergic receptors and cholinergic muscarinic re- ceptors are found in arterioles of other vascular To vena cava Vein Capillaries From aorta Arteriole Artery FIGURE 4—2. Arrangement of blood vessels in the cardiovascular system. 15}? FIGURE 4—3. Area and volume contained in systemic blood ves- Sels. The blood vessels are de— scribed by the number of each type, total cross-sectional area, and percentage (%) of blood vol— ume contained. (Pulmonary blood vessels are not included in this figure.) *Total number includes veins and venules. beds (e.g., skeletal muscle). When activated, these receptors cause relaxation or dilation of the vascular smooth muscle, resulting in an in— crease in the diameter of the arteriole, which decreases its resistance to blood flow. Thus, arterioles are not only the site of highest resis- tance in the vasculature, but they also are the site where resistance can be changed by alter— ations in sympathetic nerve activity, by circulat— ing catecholamines, and by other vasoactive substances. 0 Capillaries. The capillaries are thin~walled structures lined with a single layer of endothe- lial cells, which is surrounded by a basal lam- ina. Capillaries are the site where nutrients, gases, water, and solutes are exchanged be- tween the blood and the tissues. Lipid—soluble substances (e.g., 02 and C02) cross the capillary wall by dissolving in and diffusing across the endothelial cell membranes. in contrast, water- soluble substances (e.g., ions) cross the capil— lary wall either through water~filled clefts (spaces) between the endothelial cells or through large pores present in the walls of some capillaries (e.g., fenestrated capillaries). Not all capillaries are perfused with blood at all times. Rather, there is selective perfusion of capillary beds, depending on the metabolic needs of the tissues. This selective perfusion is determined by the degree of dilation or con— striction of the arterioles and precapillary sphincters (smooth muscle bands that lie “before” the capillaries). The degree of dilation or constriction is, in turn, controlled by the sympathetic innervation of vascular smooth muscle and by vasoactive metabolites produced in the tissues. 0 Venules and veins. Like the capillaries, the ven- CHAPTER 4 CARDIOVASCULAR PHYSIOLOGY 103 Blood volume ules are thin—walled structures. The walls of the veins are composed of the usual endothelial cell layer and a modest amount of elastic tissue, smooth muscle, and connective tissue. Because the walls of the veins contain much less elastic tissue than the arteries, the veins have a very large capacitance (capacity to hold blood). In fact, the veins contain the largest percentage of blood in the cardiovascular system. The volume of blood contained in the veins is called the unstressed volume (meaning the blood volume under low pressure). The smooth muscle in the walls of the veins is, like that in the walls of the arterioles, innervated by sympathetic nerve fibers. Increases in sympathetic nerve activity cause contraction of the veins, which reduces their capacitance, and, therefore, reduces the unstressed volume. Velocity of Blood Flow The velocity of blood flow is the rate of dis- placement of blood per unit time. The blood vessels of the cardiovascular system vary in terms of diame— ter and cross—sectional area. These differences in diameter and area, in turn, have profound effects on velocity of flow. The relationship between velocity, flow, and cross—sectional area (which depends on vessel radius or diameter) is as follows: v: Q/A where v = velocity of blood flow (cm/sec) Q = flow (ml/sec) A = cross-sectional area (cmz) Velocity of blood flow (V) is linear velocity and 104’ PHYSIOLOGY refers to the rate of displacement of blood per unit time. Thus. velocity is expressed in units of distance per unit time (e.g., cm/sec). Flow (Q) is volume flow per unit time and is expressed in units of volume per unit time (6%., ml/sec). Area (A) is the cross-sectional area of a blood vessel (e.g., aorta) or a group of blood vessels (eg, all of the capillaries). Area is calculated as A =2 wrz. where r is the radius of a single blood vessel (e.g., aorta), or the total radius of a group of blood vessels (eg, all of the capillaries). - Figure 4—4 illustrates how changes in diameter alter the velocity of flow through a vessel. In this figure, three blood vessels are shown in order of increasing diameter and cross—sectional area. The as flow through each blood vessel is identical, at 10 0,62,. ml/sec. However, because of the inverse relationship between velocity and cross—sectional area, as vessel diameter increases, the velocity of flow through the ' vessel decreases. Va V‘wl This example can be extrapolated to the cardio- vascular system. lmagine that the smallest vessel represents the aorta, the medium vessel represents all of the arteries, and the largest vessel represents all of the capillaries. The flow at each level of blood vessels is identical and equal to the cardiac output (Le, the total blood flow at any level of the cardio— vascular system is the same). Because of the inverse relationship between velocity and total cross—sec- tional area, the velocity of blood flow will be highest in the aorta and lowest in the capillaries. From the standpoint of capillary function (i.e., exchange of nutrients, solutes, and water), the low velocity of blood flow is advantageous: it maximizes the time for exchange across the capillary wall. Sample Problem,” A manfhasa cardiacoutput} of I 5.5 L/min. The diameterfo his aorta isesfumatea ; togbe '20 trims and :thegtotal _surlacelarea or: his sYStem-ic capillaries is estimated tojbe 250mm; Wlitit is gt’lr'e' velocity of blood/flowlin thefaortcz" ,;__¢rp‘ss_—;seCticina1farea-(thereafter' ' FIGURE 4—4. Effect of the theme :3? __ ter of the blood vessel on the ve— til V _ QIA locity of blood flow. ' 10 ml/sec -—-———-> Area (A) 1 cm2 10 cm2 100 cm2 Flow (Q) 10 ml/sec , 10 mI/sec 10 ml/sec i i l f l l i i i . i l l i l . i ; Volume : Pre35ure * FIGURE 4-7. Capacitance of veins and arteries. Volume is plot- ted as a function of pressure. The slopes of the curves are capacitance (C). compliant and contain the unstressed volume (large volume under low pressure). The arteries are much less compliant and contain the stressed volume (low volume under high pressure). The total volume of blood in the cardiovascular system is the sum of the unstressed volume plus the stressed volume (plus Whatever volume is contained in the heart). Changes in compliance of the veins cause re- distribution of blood between the veins and the arteries (i.e., shifts blood between the unstressed and stressed volumes). For example, if the compli— ance of the veins decreases (e.g., due to constriction of the veins), there is a decrease in the volume the} veins can hold and a shift of blood from the veins to the arteries: unstressed volume decreases and stressed volume increases. If the compliance of the veins increases, there is an increase in the volume the veins can hold and a shift of blood from the arteries to the veins: unstressed VOIume increases and stressed volume decreases. Such redistributions of blood have consequences for arterial pressure, as discussed later in this chapter. Figure 4—7 also illustrates the effect of aging on compiiance of the arteries. The characteristics of the arterial walls change with increasing age: They become less distensible and less compliant. At a given arterial pressure, the arteries can hold less blood. Another way to think of the decrease in com— pliance associated with aging is that, to hold the same volume, an “old artery” must be at a higher pressure than a “young artery.” indeed, arterial pressure is increased in the elderly, which can be explained by the decreased arterial compliance. Pressures in the Cardiovascular System Blood pressures are not equal throughout the cardiovascular system. If they were equal, blood CHAPTER 4 CARDIOVASCULAR PHYSlOLOGY it)? would not flow, since flow requires a driving force (i.e., a pressure difference). The pressure differences that exist between the heart and blood vessels are the driving force for blood flow. Table 4—1 provides a summary of pressures in the systemic and pulmo— nary circulations. Pressure Profile in the Vusculuiure Figure 4-8 is a profile of pressures within the systemic vasculature. First, examine the smooth pro- file, ignoring the pulsations. The smooth curve gives mean pressure, which is highest in the arteries and decreases progressively as blood flows from the ar— teries, to the arterioles, to the capillaries, to the veins, and back to the heart, Pressure decreases as blood flows through the vasculature because energy is consumed in overcoming the frictional resist- ances. Mean pressure in the aorta is very high, averag- ing 100 mm Hg (see Table 4—1 and Figure 4—8). This high mean arterial pressure is a result of two factors: the large volume of blood pumped from the left ventricle into the aorta (cardiac output) and the low compliance of the arterial wall. (Recall that a given volume causes greater pressure when compliance of the vessel is low.) The pressure remains high in the large arteries, which branch off the aorta, because of the strong elastic recoil of the arterial walls. Thus, little energy is lost as blood flows from the aorta through the arterial tree. Beginning in the small arteries, arterial pres— sure decreases, with the most significant decrease occurring in the arterioles. At the end of the arteri— oles, mean pressure is approximately 30 mm Hg. This dramatic decrease in pressure occurs because the arterioles constitute a high resistance to flow. TABLE 4-]. Pressures in the Cardiovascular System Location Mean Pressure (mm Hg) Systemic Aorta 100 Large arteries 100 (systolic, 120; diastolic, 80) Arterioles 50 Capillaries 20 Vena cava 4 Right atrium 0—2 Pulmonary Pulmonary artery 15 (systolic, 25; diastolic, 8) Capillaries 10 Pulmonary vein 8 Left atrium* 2—5 W * Pressures on the left side of the heart are difficult to measure directly. However, left atrial pressure can be measured by the pulmonary wedge pressure. With this technique, a catheter is inserted into the pulmonary artery and advanced into a small branch of the pulmonary artery. The catheter wedges and blocks all blood flow from that branch. Once the flow is stopped, the catheter senses the pressure in the left atrium almost directly. ...
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This note was uploaded on 11/22/2011 for the course DIAG 2725 taught by Professor Williamkessel during the Winter '11 term at Life Chiropractic College West.

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KUMAR_1 - CHAPTER 4 [ARDlOVASCUlAR PHYSIOLOGY 101 Right...

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