The Arterial and Venous Systems Roland Pittman, Ph.D. OBJECTIVES: 1. State the primary characteristics of the arterial and venous systems. 2. Describe the elastic properties of arteries in terms of pressure, volume and capacitance. 3. Define systolic, diastolic, pulse and mean arterial pressures. 4. Describe the relationship among arterial pulse pressure, stroke volume and arterial capacitance. 5. State the characteristics of the venous system in terms of blood volume and capacitance. 6. Describe the ways in which mean circulatory (or systemic) pressure can be changed in terms of blood volume and venous contraction. SUGGESTED READING ASSIGNMENT: R. M. Berne, B. M. Koeppen, M. N. Levy, and B. A. Stanton, Physiology, 5th Ed., St. Louis: Mosby, pp. 355-365, 2004 L.S. Costanzo, Physiology, 3rd Ed., Philadelphia: W.B. Saunders, pp. 120-125; 152, 2006. I. THE WHOLE SYSTEM A. Overall schematic diagram showing: 1. Parallel architecture of the vascular network 2. Unidirectional movement of blood (valves in veins and heart) Figure 1.
B. Some facts about the arteries and veins 1. About 80% of the resistance to blood flow is located on the arterial side of the circulation. 2. About 70% of the blood volume is located on the venous side of the circulation. 3. The veins and heart have one-way valves to ensure the unidirectional flow of blood. 4. The arteries and veins are in a state of partial constriction due to the continuous release of norepinephrine, a vasoconstrictor, from the sympathetic nerve terminals located in the walls of these vessels. In addition, the vasodilator nitric oxide (NO) is continuously released from the endothelial cell lining of all blood vessels. II. THE ARTERIAL SYSTEM A. Elastic recoil of arteries 1. The heart ejects blood only during about 1/3 of the cardiac cycle. 2. The potential energy stored in the elastic arteries maintains the pressure that keeps blood flowing during the rest of the cardiac cycle. B. Arterial elasticity 1. Static pressure/volume relationship a. The volume of blood in the arteries determines arterial pressure. An increase (decrease) in arterial blood volume produces an increase (decrease) in arterial pressure. This general principle, that the blood pressure is determined by the blood volume, is true for any segment of the vascular system. b. The distensibility of the arterial wall generally decreases with increasing age (i.e., the arteries of older individuals are generally stiffer than those of younger persons).
Figure 2. Pressure-volume relationships for aortas obtained at autopsy from humans in the different age groups (denoted by the numbers at the right end of each of the curves). (Redrawn from Hallock P. Benson IC: J Clin Invest 16:595, 1937). 2. Definition of capacitance (also compliance or distensibility) C a = dv/dp or ΔV/ΔP This equation quantifies the passive elastic properties of the arteries by defining the capacitance of the arteries (or any other vessel) as the change in volume divided by the associated change in pressure, i.e., it is the slope of the above pressure - volume relationship. C. Determinants of arterial pressure 1. Pressures depend on the volume of blood in the large arteries at each instant of time during the cardiac cycle. a. Diastolic pressure: P d (min art. press. during cardiac cycle) b. Systolic pressure: P s (max art. press. during cardiac cycle) c. Mean arterial pressure: P a P d + (P s - P d )/3 d. Pulse pressure: P p = P s - P d
Figure 3. Arterial systolic, diastolic, pulse and mean pressures. The mean arterial pressure (P a ) represents the area under the arterial pressure curve (shaded area) divided by the cardiac cycle duration (t 2 - t 1 ). D. There are two determinants of the volume of blood in the large arteries (and hence the pressure in the large arteries) at any given instant during the cardiac cycle. These are: 1. The rate at which blood enters the large arteries from the left ventricle of the heart (i.e., the cardiac output); 2. The rate at which blood flows through all of the organs of the body (i.e., blood flow through the total peripheral resistance, sometimes called vascular runoff). 3. Thus, the rate of change of blood volume in the large arteries is just the rate at which blood flows into the arteries minus the rate at which blood flows out of the arteries. Symbolically this can be represented as: dv a /dt = Inflow from heart (t) - Outflow through organs (t) Thus, as P a increases during systole, blood must be entering the arteries faster than it is flowing through the organs; during diastole, the opposite is true. E. Determinants of pulse pressure 1. Stroke volume: P p stroke volume/c a This relationship is fairly accurate (we ignore vascular runoff for the moment) and comes from (1) rearranging the equation that defines C a and (2) identifying the change in volume as the stroke volume of the
B left ventricle and the resulting change in pressure as the pulse pressure. For simplicity, the graph below shows a linear pressure - volume relationship, a simplifying approximation that retains the essential feature of the elastic property of the arteries. Figure 4. Effect of a change in stroke volume on pulse pressure in a system in which arterial compliance remains constant over the range of pressures and volumes involved. A larger volume increment (V 4 - V 3 as compared with V 2 - V 1 ) results in a greater mean pressure (P B as compared with PA) and a greater pulse pressure (P 4 - P 3 as compared with P 2 - P 1 ). 2. Arterial capacitance: C a The effect on pulse pressure of a change in arterial capacitance can be seen by noting that P p is inversely proportional to C a. Thus, increases in C a (i.e., increasing distensibility) produce lower pulse pressure, whereas decreases in C a (i.e., decreasing distensibility) produce higher pulse pressure. Figure 5. For a given volume increment (V 2 - V 1 ) a reduced arterial compliance (curve B as compared with curve A) results in an increased pulse pressure (P 4 - P 1 as compared with P 3 - P 2 ).
3. Total peripheral resistance: TPR = (P a - P v or ra )/CO The dependence of pulse pressure on total peripheral resistance (TPR) is less obvious than the dependence on stroke volume and capacitance, because TPR does not explicitly appear in the equation for pulse pressure. What happens when TPR increases? By rearranging Ohm's law for the flow of blood through the total peripheral resistance, one sees that: P a = P v or ra + TPR CO Since P v (central venous pressure or blood pressure in the large veins) is close to atmospheric pressure (i.e., about 0 mm Hg), then we see that: P a TPR CO to a good approximation. At a constant CO, increasing TPR will result in an elevated P a (this could be called hypertension if P a is elevated high enough). Panel A of the figure below shows a linear pressure - volume relationship and an elevated P a does not produce any change in pulse pressure for the same stroke volume (recall that we are keeping CO constant) that we had at the lower P a. However, when we consider the more accurate nonlinear pressure -volume relationship shown to the right in panel B, the elevated P a puts us on a portion of the curve that has a smaller slope (and hence lower distensibility and C a ). So, since a lower value of C a is associated with the elevated arterial pressure, pulse pressure will be higher than at the lower mean arterial pressure. Thus, one would expect a hypertensive individual to have a higher pulse pressure for a given stroke volume than would a person with normal arterial pressure. Since hypertension is often more common in older individuals, the elevated mean arterial pressure, combined with their less compliant large arteries, exacerbates the condition of higher pulse pressures, as well as higher systolic and diastolic pressures.
Figure 6. Effect of a change in TPR (volume increment remaining constant) on pulse pressure when the pressure-volume curve for the arterial system is rectilinear, A, or curvilinear, B. III. THE VENOUS SYSTEM A. Distensibility Veins are about 20 times as distensible as arteries! Thus, the capacitance or compliance for veins is about 20 times its value for the arteries. Symbolically this is expressed as: C v 20 C a B. Most (about 70%) of the blood volume resides in the veins. It is convenient to define the terms unstressed volume and stressed volume for our future discussions of the vascular function curve. 1. Unstressed volume can be thought of as the "slack" volume in the cardiovascular system under a given level of vascular tone (or contraction). It is the amount of blood you would have to introduce into a previously empty vascular system before there would be elastic recoil of the vessels. Thus, the blood pressure would be 0 mm Hg in an unperfused circulatory system until the blood volume exceeded the unstressed volume. 2. Stressed volume can be thought of as the difference between total blood volume and the unstressed volume. It is the increment in blood volume that produces the recoil pressure within the circulatory system.
3. Blood volume (V B ) = unstressed volume (V U ) + stressed volume (V S ) C. Static pressure - volume relationship for the isolated, unperfused circulatory system 1. The figure below shows the relationship between the blood pressure (P mc, mean circulatory filling pressure) in an isolated, unperfused circulatory system, as a function of the volume of blood contained in it. Note that this pressure is produced by the elastic recoil of the arteries and veins against the blood contained within them. P mc does not rise above 0 mm Hg until the unstressed volume has been filled. One can think of this recoil pressure as reporting the degree to which the circulatory system is filled with blood. Figure 7. 2. The two most common ways to change P mc (by changing the stressed volume) are to change: a. Blood volume Increases (decreases) in blood volume produce increases (decreases) in P mc, because of the elastic recoil of the stressed volume. b. Venous contraction Recall that most of the blood volume resides in the veins and that the veins are in a constant state of partial constriction due to their stimulation by the sympathetic nerves. Increasing
venous smooth muscle contraction produces constriction, resulting in a smaller diameter and, hence, smaller unstressed volume. A smaller unstressed volume with no change in overall blood volume will lead to an increase in stressed volume and, hence, in P mc. Graphically, this corresponds to a shift to the left in the static pressure - volume relationship. A decrease in venous contraction will lead to the opposite results. The reason why venous, rather than arterial, contraction is the important factor here is because most of the blood volume is located in the veins, so that any change in their contractile state will have a larger impact on the distribution of blood within the circulatory system. STUDY QUESTIONS 1. If the volume of blood in an isolated nonflowing vascular system is 6 liters, the unstressed volume is 4.5 liters and the capacitance of the stressed volume is 150 ml/mm Hg, the mean circulatory pressure will be: A. 5 mm Hg B. 6 mm Hg C. 7 mm Hg D. 10 mm Hg E. 14 mm Hg ANSWER: D You need to remember the graph relating mean circulatory pressure to blood volume and that only the volume of blood identified as being in the "stressed volume" contributes to the vascular recoil that produces the mean circulatory pressure. The stressed volume in this question is 6 minus 4.5 liters, or 1.5 liters. You also need to remember that the capacitance is defined as the change in volume divided by the associated change in pressure. In this case, the change in volume is the stressed volume and the change in pressure (change above the pressure existing when blood volume equals unstressed volume) is the mean circulatory pressure. Thus, P mc = 1500/150 = 10 mm Hg. 2. If heart rate is increased, but total peripheral resistance, stroke volume and compliance remain unchanged, which of the following will be true? (Assume a linear pressure-volume relation for the arteries.)
1. Diastolic pressure will increase. 2. Pulse pressure will be unchanged. 3. Systolic pressure will increase. 4. Cardiac output will increase. ANSWER: E (1, 2, 3, & 4) This question is challenging, but you should be able to reason out the answer. You need to remember that the factors which can change pulse pressure are stroke volume of the left ventricle, compliance of the large arteries and total peripheral resistance. Thus, response 2 is correct. If heart rate increases and stroke volume is constant, then cardiac output must also increase, so that response 4 is correct. Deciding whether responses 1 and/or 3 are true is harder for most people to determine. Since cardiac output is increased while total peripheral resistance remains constant, mean arterial blood pressure will increase. Since pulse pressure remains constant (see reasoning above), both systolic and diastolic pressure must increase. This means that there must be a new and increased steady state volume of blood in the large arteries. 3. Against the advice of his physician, a student takes too much of the experimental drug Loosenup, resulting in increased distensibility of the aorta. Assuming that stroke volume and heart rate are unchanged in the presence of the drug, which of the following would be expected to decrease? 1. Systolic pressure in the aorta. 2. Diastolic pressure in the aorta. 3. Pulse pressure in the aorta. 4. Mean arterial pressure. ANSWER: B (1 & 3) If the aorta becomes more distensible (i.e., more compliant), then the recoil pressure on the same stroke volume will be less, so pulse pressure and systolic pressure both fall. Mean arterial pressure will not change since cardiac output and total peripheral resistance remain unchanged (no mention is made of a change in TPR, so you should assume that it remained the same; also the resistance vessels would have to be affected by the drug and it is stated that only the aorta is affected). Since mean arterial pressure remains unchanged and pulse pressure falls, diastolic pressure must rise. 4. Pulse pressure: A. is the difference between systolic and diastolic pressures. B. decreases when stroke volume increases. C. increases when arterial compliance increases. D. decreases when total peripheral resistance increases.
E. increases when heart rate increases at constant cardiac output. ANSWER: A Pulse pressure is defined as the difference between systolic and diastolic pressures. Pulse pressure increases when stroke volume increases (more stretch and elastic recoil of the large arteries); it decreases when arterial compliance increases (large arteries would be more distensible and thus accommodate the stroke volume with less recoil); and it increases when TPR increases (the large arteries would be operating at the higher pressure end of their static pressure-volume relationship). An increased heart rate at constant cardiac output implies that stroke volume must have decreased, thereby lowering pulse pressure.