Cardiovascular Physiology-Hemodynamics PDF

Week 4: Cardiovascular Physiology-Hemodynamics Enumerate the vascular circuitry from the heart to the pulmonary and systemic circulation. Circuitry of the Cardiovascular System Left and Right Sides of the Heart Figure 4.1 is a schematic diagram of the circuitry of the cardiovascular system. The lef...

Week 4: Cardiovascular Physiology-Hemodynamics Enumerate the vascular circuitry from the heart to the pulmonary and systemic circulation. Circuitry of the Cardiovascular System Left and Right Sides of the Heart Figure 4.1 is a schematic diagram of the circuitry of the cardiovascular system. The left and right sides of the heart and the blood vessels are shown in relation to each other. Each side of the heart has two chambers: an atrium and a ventricle, connected by one-way valves, called atrioventricular (AV) valves. The AV valves are designed so that blood can flow only in one direction, from the atrium to the ventricle. The left heart and right heart have different functions. The left heart and the systemic arteries, capillaries, and veins are collectively called the systemic circulation. The left ventricle pumps blood to all organs of the body except the lungs. The right heart and the pulmonary arteries, capillaries, and veins are collectively called the pulmonary circulation. The right ventricle pumps blood to the lungs. The left heart and right heart function in series so that blood is pumped sequentially from the left heart to the systemic circulation, to the right heart, to the pulmonary circulation, and then back to the left heart. The rate at which blood is pumped from either ventricle is called the cardiac output. Because the two sides of the heart function in series, the cardiac output of the left ventricle equals the cardiac output of the right ventricle in the steady state. The rate at which blood is returned to the atrium from the veins is called the venous return. Again, because the left and right sides of the heart operate in series, venous return to the left heart equals venous return to the right heart in the steady state. Finally, in the steady state, cardiac output from the left heart equals venous return to the right heart. Explain the function of each major class of vessels within the vasculature. Blood Vessels The blood vessels have several functions. They serve as a closed system of passive conduits, delivering blood to and from the tissues where nutrients and wastes are exchanged. The blood vessels also participate actively in the regulation of blood flow to organs. When resistance of the blood vessels, particularly of the arterioles, is altered, blood flow to an organ is adjusted. Circuitry The steps in one complete circuit through the cardiovascular system are shown in Figure 4.1. The circled numbers in the figure correspond with the steps described here: 1. Oxygenated blood fills the left ventricle. Blood that has been oxygenated in the lungs returns to the left atrium via the pulmonary vein. This blood then flows from the left atrium to the left ventricle through the mitral valve (the AV valve of the left heart). 2. Blood is ejected from the left ventricle into the aorta. Blood leaves the left ventricle through the aortic valve (the semilunar valve of the left side of the heart), which is located between the left ventricle and the aorta. When the left ventricle contracts, the pressure in the ventricle increases, causing the aortic valve to open and blood to be ejected forcefully into the aorta. (As noted previously, the volume of blood ejected from the left ventricle per unit time is called the cardiac output.) Blood then flows through the arterial system, driven by the pressure created by contraction of the left ventricle. 3. Cardiac output is distributed among various organs. The total cardiac output of the left heart is distributed among the organ systems via sets of parallel arteries. Thus simultaneously, approximately 15% of the cardiac output is delivered to the brain via the cerebral arteries, 5% is delivered to the heart via the coronary arteries, 25% is delivered Week 4: Cardiovascular Physiology-Hemodynamics to the kidneys via the renal arteries, and so forth. Given this parallel arrangement of the organ systems, it follows that the total systemic blood flow must equal the cardiac output. The percentage distribution of cardiac output among the various organ systems is not fixed, however. For example, during strenuous exercise, the percentage of the cardiac output going to skeletal and cardiac muscle increases, compared with the percentages at rest. There are three major mechanisms for achieving such changes in blood flow to an organ system. In the first mechanism, the cardiac output remains constant, but the blood flow is redistributed among the organ systems by the selective alteration of arteriolar resistance. In this scenario, blood flow to one organ can be increased at the expense of blood flow to other organs. In the second mechanism, the cardiac output increases or decreases, but the percentage distribution of blood flow among the organ systems is kept constant. Finally, in a third mechanism, a combination of the first two mechanisms occurs in which both cardiac output and the percentage distribution of blood flow are altered. This third mechanism is used, for example, in the response to strenuous exercise: Blood flow to skeletal and cardiac muscle increases to meet the increased metabolic demand by a combination of increased cardiac output and increased percentage distribution to skeletal and cardiac muscle. 4. Blood flow from the organs is collected in the veins. The blood leaving the organs is venous blood and contains waste products from metabolism, such as carbon dioxide (CO₂). This mixed venous 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, called 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 ventricle. Mixed venous blood flows from the right atrium to the right ventricle through the AV valve in the right heart, the tricuspid valve. 7. 7. Blood is ejected from the right ventricle into the pulmonary artery. When the right ventricle contracts, blood is ejected through the pulmonary valve (the semilunar valve of the right side of the heart) into the pulmonary artery, which carries blood to the lungs. Note that the cardiac output ejected from the right ventricle is identical to the cardiac output that was ejected from the left ventricle. In the capillary beds of the lungs, oxygen (O₂) is added to the blood from alveolar gas, and carbon dioxide (CO₂) is removed from the blood and added to the alveolar gas. Thus, the blood leaving the lungs has more O₂ and less CO₂ than the blood that entered the lungs. 8. 8. Blood flow from the lungs is returned to the heart via the pulmonary vein. Oxygenated blood is returned to the left atrium via the pulmonary vein to begin a new cycle. Hemodynamics The term hemodynamics refers to the principles 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. Week 4: Cardiovascular Physiology-Hemodynamics Distribution of Blood Volume The total blood volume in a 70-kg male is approximately 5 L. Of this, normally, 85% is present in the systemic circulation, 10% is present in the pulmonary circulation, and 5% is present in the cardiac chambers at the end of diastole. Of the blood volume in the systemic circulation, most (three-fourths) resides in the veins, and the remaining one-fourth resides in the arteries and capillaries; thus, the systemic veins constitute a significant reservoir for the blood volume. Characteristics of Blood Vessels Blood vessel walls have the following components. The relative amount of each component varies between arteries, arterioles, capillaries, and veins, thus conveying their different functional properties. Endothelial cells comprise a single layer that lines all blood vessels. Endothelial cells are connected by junctional complexes in arteries and, to a lesser extent, in veins. In capillaries, the “leakiness” of these junctional complexes varies, depending on the organ. Brain capillaries have narrow (“tight”) junctions that comprise the blood-brain barrier; small intestinal and glomerular capillaries have “fenestrated” capillaries, where the endothelial layer is perforated to allow passage of large volumes of fluid and solutes; liver capillaries have large gaps between endothelial cells. Elastic fibers, comprised of an elastin core covered by microfibrils, convey the elastic properties of arteries, arterioles, and veins; they are not present in capillaries. Collagen fibers are much stiffer than elastic fibers and are present in arteries, arterioles, and veins; they are not present in capillaries. Together with elastic fibers, collagen fibers are responsible for the passive tension of blood vessel walls. Vascular smooth muscle cells are present in all blood vessels except capillaries. Contraction of vascular smooth muscle is responsible for active tension in blood vessels. Types of Blood Vessels 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 and the histologic characteristics of their walls vary, as described above. These variations have profound effects on their resistance and capacitance properties. Arteries The aorta is the largest artery of the systemic circulation. Medium- and small-sized arteries branch off the aorta. The function of the arteries is to deliver oxygenated blood to the organs. The arteries are thick-walled structures with extensive development of elastic tissue, vascular smooth muscle, and connective tissue. The thickness of the arterial wall is a significant feature: The arteries receive blood directly from the heart and are under the highest pressure in the vasculature. The volume of blood contained in the arteries is called the stressed volume (meaning the blood volume under high pressure). Week 4: Cardiovascular Physiology-Hemodynamics Arterioles The arterioles are the smallest branches of the arteries. Their walls have an extensive development of vascular smooth muscle, and they are the site of highest resistance to blood flow. The smooth muscle in the walls of the arterioles is tonically active (i.e., always contracted). It is extensively innervated by sympathetic adrenergic nerve fibers. α1-Adrenergic 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 vascular smooth muscle. Constriction produces a decrease in the diameter of the arteriole, which increases its resistance to blood flow. Less common, β2-adrenergic receptors are found in arterioles of skeletal muscle. When activated, these receptors cause dilation, or relaxation, of the vascular smooth muscle, which increases the diameter and decreases the resistance of these arterioles to blood flow. Thus, arterioles are not only the site of highest resistance in the vasculature, but they also are the site where resistance can be changed by alterations in sympathetic nerve activity, by circulating catecholamines, and by other vasoactive substances. Capillaries The capillaries are thin-walled structures lined with a single layer of endothelial cells, which is surrounded by a basal lamina. Capillaries are the site where nutrients, gases, water, and solutes are exchanged between the blood and the tissues and, in the lungs, between the blood and the alveolar gas. Lipid-soluble substances (e.g., O₂ and CO₂) cross the capillary wall by dissolving in and diffusing across the endothelial cell membranes. In contrast, water-soluble substances (e.g., ions) cross the capillary wall either through water-filled clefts (spaces) between the endothelial cells or through large pores in the walls of some capillaries (e.g., fenestrated capillaries). Not all capillaries are always perfused with blood. 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 constriction 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. Venules and Veins In the vasculature, the veins are thin-walled structures. The walls of the veins are composed of a small amount of elastic tissue, smooth muscle, and connective tissue. The veins have a large capacitance (capacity to hold blood) and 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, via α1-adrenergic receptors, cause contraction of the veins, which reduces their capacitance and therefore reduces the unstressed volume. Week 4: Cardiovascular Physiology-Hemodynamics Estimate velocity of blood flow and blood flow in various parts of the vasculature. Velocity of Blood Flow The velocity of blood flow is the rate of displacement of blood per unit time. The blood vessels of the cardiovascular system vary in terms of diameter 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: "The greater the radius, the larger the cross-sectional area, resulting in a slower velocity of blood flow due to the inverse relationship between velocity and area." v = Q/A where: v = Velocity of blood flow (cm/s) Q = Flow (mL/s) A = Cross-sectional area (cm²) Velocity of blood flow (v) is linear velocity and 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/s). Flow (Q) is volume flow per unit time and is expressed in units of volume per unit time (e.g., mL/s). Area (A) is the cross-sectional area of a blood vessel (e.g., aorta) or a group of blood vessels (e.g., all of the capillaries). Area is calculated as A = πr², where r is the radius of a single blood vessel (e.g., aorta) or the total radius of a group of blood vessels (e.g., all of the capillaries). Relationships Between Blood Flow, Pressure, and Resistance Blood flow through a blood vessel or a series of vessels is determined by two key factors: the pressure difference between the two ends of the vessel (the inlet and the outlet) and the resistance of the vessel to blood flow. The pressure difference drives blood flow, while resistance opposes it. This relationship is similar to electrical circuits, where blood flow corresponds to current (I), pressure difference to voltage (V), and resistance (R) to electrical resistance, as expressed by Ohm’s law. Ohm’s law states: I = V/R, or in terms of blood flow: Q = ΔP/R. Week 4: Cardiovascular Physiology-Hemodynamics Equation for Blood Flow The equation for blood flow is: Q = ΔP / R Where: Q = Flow (mL/min) ΔP = Pressure difference (mm Hg) R = Resistance (mm Hg/mL/min) The magnitude of blood flow (Q) is directly proportional to the size of the pressure difference (ΔP) or pressure gradient. The direction of blood flow is determined by the direction of the pressure gradient and always is from high to low pressure. For example, during ventricular ejection, blood flows from the left ventricle into the aorta and not in the other direction, because pressure in the ventricle is higher than pressure in the aorta. For another example, blood flows from the vena cava to the right atrium because pressure in the vena cava is slightly higher than in the right atrium. Furthermore, blood flow is inversely proportional to resistance (R). Increasing resistance (e.g., by arteriolar vasoconstriction) decreases flow, and decreasing resistance (e.g., by arteriolar vasodilation) increases flow. The major mechanism for changing blood flow in the cardiovascular system is by changing the resistance of blood vessels, particularly the arterioles. The flow, pressure, and resistance relationship also can be rearranged to determine resistance. If the blood flow and the pressure gradient are known, the resistance is calculated as R = ΔP / Q. This relationship can be applied to measure the resistance of the entire systemic vasculature (i.e., total peripheral resistance), or it can be used to measure resistance in a single organ or single blood vessel. Total peripheral resistance. The resistance of the entire systemic vasculature is called the total peripheral resistance (TPR) or the systemic vascular resistance (SVR). TPR can be measured with the flow, pressure, and resistance relationship by substituting cardiac output for flow (Q) and the difference in pressure between the aorta and the vena cava for ΔP. Resistance in a single organ. The flow, pressure, and resistance relationship also can be applied on a smaller scale to determine the resistance of a single organ. As illustrated in the following sample problem, the resistance of the renal vasculature can be determined by substituting renal blood flow for Q and the difference in pressure between the renal artery and the renal vein for ΔP. Week 4: Cardiovascular Physiology-Hemodynamics Estimate resistance to blood flow using the Poiseuille equation. Resistance to Blood Flow The blood vessels and the blood itself constitute resistance to blood flow. The relationship between the blood vessel diameter (or radius), and blood viscosity is described by the Poiseuille equation. The total resistance offered by a set of blood vessels also depends on whether the vessels are arranged in series (i.e., blood flows sequentially from one vessel to the next) or in parallel (i.e., the total blood flow is distributed simultaneously among parallel vessels). Poiseuille Equation The factors that determine the resistance of a blood vessel to blood flow are expressed by the Poiseuille equation: R = 8ηl / πr⁴ Where: R = Resistance η = Viscosity of blood l = Length of blood vessel r⁴ = Radius of blood vessel raised to the fourth power The most important concepts expressed in the Poiseuille equation are as follows: 1. Resistance to flow is directly proportional to viscosity (η) of the blood. For example, as viscosity increases (e.g., if the hematocrit increases), the resistance to flow also increases. 2. Resistance to flow is directly proportional to the length (l) of the blood vessel. 3. Resistance to flow is inversely proportional to the fourth power of the radius (r⁴) of the blood vessel. This is a powerful relationship, indeed! When the radius of a blood vessel decreases, its resistance increases, not in a linear fashion but magnified by the fourth-power relationship. For example, if the radius of a blood vessel decreases by one- half, resistance does not simply increase twofold—it increases by 16-fold (2⁴ = 16)! Week 4: Cardiovascular Physiology-Hemodynamics Recognize the changes in resistance to flow imposed by parallel versus series flow. Series and Parallel Resistances Resistances in the cardiovascular system, as in electrical circuits, can be arranged in series or in parallel (Fig. 4.5). Whether the arrangement is series or parallel produces different values for total resistance. Series resistance is illustrated by the arrangement of blood vessels within a given organ. Each organ is supplied with blood by a major artery and drained by a major vein. Within the organ, blood flows from the major artery to smaller arteries, to arterioles, to capillaries, to venules, to veins. The total resistance of the system arranged in series is equal to the sum of the individual resistances, as shown in the following equation and in Figure 4.5. Of the various resistances in series, arteriolar resistance is by far the greatest. The total resistance of a vascular bed is determined, therefore, in large part by the arteriolar resistance. Series resistance is expressed as follows: Rtotal = Rartery + Rarterioles + Rcapillaries + Rvenules + Rvein When resistances are arranged in series, the total flow at each level of the system is the same. For example, blood flow through the aorta equals blood flow through all the large systemic arteries, equals blood flow through all the systemic arterioles, equals blood flow through all the systemic capillaries. For another example, blood flow through the renal artery equals blood flow through all the renal capillaries, equals blood flow through the renal vein (less a small volume lost in urine). Although total flow is constant at each level in the series, the pressure decreases progressively as blood flows through each sequential component (remember Q = ΔP/R or ΔP = Q × R). The greatest decrease in pressure occurs in the arterioles because they contribute the largest portion of the resistance. Parallel resistance is illustrated by the distribution of blood flow among the various major arteries branching off the aorta (see Figs. 4.1 and 4.5). Recall that the cardiac output flows through the aorta and then is distributed simultaneously, in parallel, among the various organ systems. Thus there is parallel, simultaneous blood flow through each of the circulations (e.g., renal, cerebral, and coronary). The venous effluent from the organs then collects in the vena cava and returns to the heart. As shown in the following equation and in Figure 4.5, the total resistance in a parallel arrangement is less than any of the individual resistances. Parallel resistance is expressed as follows: 1 / Rtotal = 1 / R1 + 1 / R2 + 1 / R3 + … + 1 / Rn When blood flow is distributed through a set of parallel resistances, the flow through each organ is a fraction of the total blood flow. The effects of this arrangement are that there is no loss of pressure in the major arteries and that mean pressure in each major artery will be the same and be approximately the same as mean pressure in the aorta. Week 4: Cardiovascular Physiology-Hemodynamics Recognize the benefits of laminar blood flow. Laminar Flow and Reynolds Number Ideally, blood flow in the cardiovascular system is laminar, or streamlined. In laminar flow, there is a smooth parabolic profile of velocity within a blood vessel, with the velocity of blood flow highest in the center of the vessel and lowest toward the vessel walls (Fig. 4.6). The parabolic profile develops because the layer of blood next to the vessel wall adheres to the wall and, essentially, does not move. The next layer of blood (toward the center) slips past the motionless layer and moves a bit faster. Each successive layer of blood toward the center moves faster yet, with less adherence to adjacent layers. Thus the velocity of flow at the vessel wall is zero, and the velocity at the center of the stream is maximal. Laminar blood flow conforms to this orderly parabolic profile. When an irregularity occurs in a blood vessel (e.g., at the valves or at the site of a blood clot), the laminar stream is disrupted and blood flow may become turbulent. In turbulent flow (see Fig. 4.6), the fluid streams do not remain in the parabolic profile; instead, the streams mix radially and axially. Because kinetic energy is wasted in propelling blood radially and axially, more energy (pressure) is required to drive turbulent blood flow than laminar blood flow. Laminar flow is silent, while turbulent flow is audible. For example, the Korotkoff sounds used in the auscultatory measurement of blood pressure are caused by turbulent flow. Blood vessel stenosis (narrowing) and cardiac valve disease can cause turbulent flow and often are accompanied by audible vibrations called murmurs. Reynolds Number The Reynolds number is a dimensionless number that is used to predict whether blood flow will be laminar or turbulent. It considers a number of factors including diameter of the blood vessel, mean velocity of flow, and viscosity of the blood. Thus: Nᵣ = ρdv / η where: Nᵣ = Reynolds number ρ = Density of blood d = Diameter of blood vessel v = Velocity of blood flow η = Viscosity of blood If Reynolds number (Nᵣ) is less than 2000, blood flow will be laminar. If Reynolds number is greater than 2000, there is increasing likelihood that blood flow will be turbulent. Values of Reynolds number greater than 3000 always predict turbulent flow. The major influences on Reynolds number in the cardiovascular system are changes in blood viscosity and changes in the velocity of blood flow. Decreased diameter increases Reynolds Week 4: Cardiovascular Physiology-Hemodynamics number by increasing velocity of flow. Inspection of the equation shows that decreases in viscosity (e.g., decreased hematocrit) cause an increase in Reynolds number. Likewise, narrowing of a blood vessel, which produces an increase in velocity of blood flow, causes an increase in Reynolds number. The effect of narrowing a blood vessel (i.e., decreased diameter and radius) on Reynolds number is initially puzzling because, according to the equation, decreases in vessel diameter should decrease Reynolds number (diameter is in the numerator). Recall, however, that the velocity of blood flow also depends on diameter (radius), according to the earlier equation, v = Q/A or v = Q/πr². Thus velocity (also in the numerator of the Reynolds number) increases as radius decreases, raising Reynolds number. Hence, the dependence of Reynolds number on velocity is more powerful than the dependence on diameter. Clinical Applications of Reynolds Number Several clinical examples illustrate the application of Reynolds number in predicting turbulence: Anemia is associated with a decreased hematocrit (decreased mass of red blood cells) and, because of turbulent blood flow, causes functional murmurs. Reynolds number, the predictor of turbulence, is increased in anemia due to decreased blood viscosity. A second cause of increased Reynolds number in patients with anemia is a high cardiac output, which causes an increase in the velocity of blood flow (v = Q/A). Cardiac valvular disease causes narrowing of the valve, increased velocity of blood flow, and increased Reynolds number as described above. The increased Reynolds number predicts turbulence, which causes an audible vibration or murmur. (Blood flow through normal valves is silent.) Murmurs are graded, with grade 1 murmurs being barely audible with a stethoscope and grade 6 murmurs being so loud they can be heard without a stethoscope. Atherosclerosis causes narrowing of arteries and increases blood flow velocity and Reynolds number. With advanced atherosclerotic disease, murmurs can be heard in every major artery, but are most easily detected in the carotid and femoral arteries. Arterial murmurs are called bruits. Thrombi are blood clots in the lumen of a blood vessel. Thrombi decrease blood vessel diameter, which causes an increase in blood velocity, in Reynolds number, and in turbulence. Illustrate the relationship between volume and pressure within a vessel (compliance). Shear Shear is a consequence of the fact that blood travels at different velocities within a blood vessel (see Fig. 4.6). Shear occurs if adjacent layers of blood travel at different velocities; when adjacent layers travel at the same velocity, there is no shear. Thus shear is highest at the blood vessel wall, according to the following reasoning. Right at the wall, there is a motionless layer of blood (i.e., velocity is zero); the adjacent layer of blood is moving and therefore has a velocity. The greatest relative difference in velocity of blood is between the motionless layer of blood Week 4: Cardiovascular Physiology-Hemodynamics right at the wall and the next layer in. Shear is lowest at the center of the blood vessel, where the velocity of blood is highest but where the adjacent layers of blood are essentially moving at the same velocity. One consequence of shear is that it breaks up aggregates of red blood cells and decreases blood viscosity. Therefore at the wall, where shear rate is normally highest, red blood cell aggregation and viscosity are lowest. Compliance of Blood Vessels The compliance or capacitance of a blood vessel describes the volume of blood the vessel can hold at a given pressure. Compliance is related to distensibility and is given by the following equation: C=V/P Where: C = Compliance or capacitance (mL/mm Hg) V = Volume (mL) P = Pressure (mm Hg) The equation for compliance states that the higher the compliance of a vessel, the more volume it can hold at a given pressure. Or, stated differently, compliance describes how the volume of blood contained in a vessel changes for a given change in pressure (ΔV/ΔP). Figure 4.7 illustrates the principle of compliance and shows the relative compliance of veins and arteries. For each type of blood vessel, volume is plotted as a function of pressure. The slope of each curve is the compliance. Compliance of the veins is high; in other words, the veins hold large volumes of blood at low pressure. Compliance of the arteries is much lower than that of the veins; the arteries hold much less blood than the veins, and they do so at high pressure. The difference in the compliance of the veins and the arteries underlies the concepts of unstressed volume and stressed volume. The veins are most 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 redistribution of blood between the veins and the arteries (i.e., the blood shifts between the unstressed and stressed volumes). For example, if the compliance or capacitance of the veins decreases (e.g., due to venoconstriction), there is a decrease in the volume the veins can hold and, consequently, a shift of blood from the veins to the arteries: unstressed volume decreases and stressed volume increases. If the compliance or capacitance of the veins increases, there is an increase in the volume the veins can hold and, consequently, a shift of blood from the arteries to the veins: unstressed volume increases and stressed volume decreases. Such redistributions of blood between the veins and arteries have consequences for arterial pressure, as discussed later in this chapter. Week 4: Cardiovascular Physiology-Hemodynamics Figure 4.7 also illustrates the effect of aging on compliance of the arteries. The characteristics of the arterial walls change with increasing age: The walls become stiffer, less distensible, and less compliant. (With age, increased cross-linking of collagen fibers in arterial walls stiffens their connections to other elements of the arterial wall.) At a given arterial pressure, the arteries can hold less blood. Another way to think of the decrease in compliance associated with aging is that in order for an “old artery” to hold the same volume as a “young artery,” the pressure in the “old artery” must be higher than the pressure in the “young artery.” Indeed, arterial pressures are increased in the elderly due to decreased arterial compliance.