Physiology 5th Ed.


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.

Types and Characteristics 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. 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 vascular bed is from artery to arteriole, to capillaries, to venule, to vein. Figure 4-3, a companion figure, is a graph showing the total cross-sectional area, the number of blood vessels at each level of the vasculature, and the percentage of the blood volume contained in each type of vessel.


Figure 4–2 Arrangement of blood vessels in the cardiovascular system.


Figure 4–3 Area and volume contained in systemic blood vessels. The blood vessels are described by the number of each type, total cross-sectional area, and percentage (%) of blood volume contained. (Pulmonary blood vessels are not included in this figure.) *Total number includes veins and venules.

image 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-walledstructures with extensive development of elastic tissue, 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).

image 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 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 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.

image 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 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 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.

image Venules and veins. Like the capillaries, the venules 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 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, via α1 adrenergic receptors, 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 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:




= Velocity of blood flow (cm/sec)


= Flow (mL/sec)


= 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/sec).

Flow (Q) is volume flow per unit time and is expressed in units of volume per unit time (e.g., mL/sec).

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).

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 flow through each blood vessel is identical, at 10 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.


Figure 4–4 Effect of the diameter of the blood vessel on the velocity of blood flow.

This example can be extrapolated to the cardiovascular system. Imagine that the smallest vessel represents the aorta, the medium-sized vessel represents all of the arteries, and the largest vessel represents allof the capillaries. The total blood flow at each level of blood vessels is the same and is equal to the cardiac output. Because of the inverse relationship between velocity and total cross-sectional 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 man has a cardiac output of 5.5 L/min. The diameter of his aorta is estimated to be 20 mm, and the total cross-sectional area of his systemic capillaries is estimated to be 2500 cm2What is the velocity of blood flow in the aorta relative to the velocity of blood flow in the capillaries?

SOLUTION. To compare the velocity of blood flow in the aorta with the velocity in the capillaries, two values are needed for each type of blood vessel: the total blood flow (Q) and the total cross-sectional area (cm2). The total flow at each level is the same and is equal to the cardiac output. The total cross-sectional area of the capillaries is given in the problem, and the cross-sectional area of the aorta must be calculated from its radius, which is 10 mm. Area = πr2 = 3.14 × (10 mm)2 = 3.14 × (1 cm)2 = 3.14 cm2. Thus,



Hence, velocity in the aorta is 800-fold that in the capillaries (1752 cm/min in the aorta compared with 2.2 cm/min in the capillaries). These calculations confirm the previous discussion concerning velocity of blood flow. The velocity of flow should be lowest in vessels with the largest total cross-sectional area (the capillaries) and highest in the vessels with the smallest total cross-sectional area (the aorta).

Relationships between Blood Flow, Pressure, and Resistance

Blood flow through a blood vessel or a series of blood vessels is determined by two 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 is the driving force for blood flow, and the resistance is an impediment to flow.

The relationship of flow, pressure, and resistance is analogous to the relationship of current (I), voltage (ΔV), and resistance (R) in electrical circuits, as expressed by Ohm’s law (Ohm’s law states that ΔV = I × R or I = ΔV/R). Blood flow is analogous to current flow, the pressure difference or driving force is analogous to the voltage difference, and hydrodynamic resistance is analogous to electrical resistance. The equation for blood flow is expressed as follows:




= Flow (mL/min)


= Pressure difference (mm Hg)


= 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 used 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.

image 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.

image 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 flow (Q) and the difference in pressure between the renal artery and the renal vein for ΔP:

SAMPLE PROBLEM. Renal blood flow is measured by placing a flow meter on a woman’s left renal artery. Simultaneously, pressure probes are inserted in her left renal artery and left renal vein to measure pressure. Renal blood flow measured by the flow meter is 500 mL/min. The pressure probes measure renal arterial pressure as 100 mm Hg and renal venous pressure as 10 mm Hg. What is the vascular resistance of the left kidney in this woman?

SOLUTION. Blood flow to the left kidney, as measured by the flow meter, is Q. The difference in pressure between the renal artery and renal vein is ΔP. The resistance to flow in the renal vasculature is calculated by rearranging the blood flow equation:


Rearranging and solving for R,


Resistance to Blood Flow

The blood vessels and the blood itself constitute resistance to blood flow. The relationship between resistance, 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:




= Resistance


= Viscosity of blood


= Length of blood vessel


= Radius of blood vessel raised to the fourth power

The most important concepts expressed in the Poiseuille equation are as follows: First, 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. Second, resistance to flow is directly proportional to the length (l) of the blood vessel. Third, and most important, 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)!

SAMPLE PROBLEM. A man suffers a stroke caused by partial occlusion of his left internal carotid artery. An evaluation of the carotid artery using magnetic resonance imaging (MRI) shows a 75% reduction in its radius. Assuming that blood flow through the left internal carotid artery is 400 mL/min prior to the occlusion, what is blood flow through the artery after the occlusion?

SOLUTION. The variable in this example is the diameter (or radius) of the left internal carotid artery. Blood flow is inversely proportional to the resistance of the artery (Q = ΔP/R), and resistance is inversely proportional to the radius raised to the fourth power (Poiseuille equation). The internal carotid artery is occluded, and its radius is decreased by 75%. Another way of expressing this reduction is to say that the radius is decreased to one fourth its original size.

The first question is How much would resistance increase with 75% occlusion of the artery? The answer is found in the Poiseuille equation. After the occlusion, the radius of the artery is one fourth its original radius; thus, resistance has increased by 1/(1/4)4, or 256-fold.

The second question is What would the flow be if resistance were to increase by 256-fold? The answer is found in the flow, pressure, resistance relationship (Q = ΔP/R). Because resistance increased by 256-fold, flow decreased to 1/256, or 0.0039, or 0.39% of the original value. The flow is 0.39% of 400 mL/min, or 1.56 mL/min. Clearly, this is a dramatic decrease in blood flow to the brain, all based on the fourth-power relationship between resistance and vessel radius.

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.


Figure 4–5 Arrangements of blood vessels in series and in parallel. The arrows show the direction of blood flow. R, Resistance (subscripts refer to individual resistances).

image 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:


When resistances are arranged in series, the total flow through 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.

image 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, on a percentage basis, 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. The subscripts 1, 2, 3, and so forth refer to the resistances of cerebral, coronary, renal, gastrointestinal, skeletal muscle, and skin circulations. Parallel resistance is expressed as follows:


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 approximately the same as mean pressure in the aorta.

Another predictable consequence of a parallel arrangement is that adding a resistance to the circuit causes total resistance to decrease, not to increase. Mathematically, this can be demonstrated as follows: Four resistances, each with a numeric value of 10, are arranged in parallel. According to the equation, the total resistance is 2.5 (1/Rtotal = 1/10 + 1/10 + 1/10 + 1/10 = 2.5). If a fifth resistance with a value of 10 is added to the parallel arrangement, the total resistance decreases to 2 (1/Rtotal = 1/10 + 1/10 + 1/10 + 1/10 + 1/10 = 2).

On the other hand, if the resistance of one of the individual vessels in a parallel arrangement increases, then total resistance increases. This can be shown by returning to the parallel arrangement of four blood vessels where each individual resistance is 10 and the total resistance is 2.5. If one of the four blood vessels is completely occluded, its individual resistance becomes infinite. The total resistance of the parallel arrangement then increases to 3.333 (1/Rtotal = 1/10 + 1/10 + 1/10 + 1/∞).

Laminar Flow and Reynolds Number

Ideally, blood flow in the cardiovascular system is laminar, or streamlined. In laminar flow, there is a 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.


Figure 4–6 Comparison of laminar flow to turbulent blood flow. The length of the arrows shows the approximate velocity of blood flow. Laminar blood flow has a parabolic profile, with velocity lowest at the vessel wall and highest in the center of the stream. Turbulent blood flow exhibits axial and radial flow.

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 energy is wasted in propelling blood radially and axially, more energy (pressure) is required to drive turbulent blood flow than laminar blood flow. Turbulent flow is often accompanied by audible vibrations called murmurs.

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,




= Reynolds number


= Density of blood


= Diameter of blood vessel


= Velocity of blood flow


= 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 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. 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 equation for Reynolds number) increases as radius decreases, raised to the second power. Hence, the dependence of Reynolds number on velocity is more powerful than the dependence on diameter.

Two common clinical situations, anemia and thrombi, illustrate the application of Reynolds number in predicting turbulence.

image 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).

image Thrombi are blood clots in the lumen of a vessel. Thrombi narrow the diameter of the blood vessel, which causes an increase in blood velocity at the site of the thrombus, thereby increasing Reynolds number and producing turbulence.


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 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:




= Compliance or capacitance (mL/mm Hg)


= Volume (mL)


= 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.


Figure 4–7 Capacitance of veins and arteries. Volume is plotted as a function of pressure. The slopes of the curves are capacitance (C).

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 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 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.

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. 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.

Pressures in the Cardiovascular System

Blood pressures are not equal throughout the cardiovascular system. If they were equal, blood 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 pulmonary circulations.

Table 4–1 Pressures in the Cardiovascular System


Mean Pressure (mm Hg)





Large arteries

100 (systolic, 120; diastolic, 80)





Vena cava


Right atrium




Pulmonary artery

15 (systolic, 25; diastolic, 8)



Pulmonary vein


Left atrium*


*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.

Pressure Profile in the Vasculature

Figure 4-8 is a profile of pressures within the systemic vasculature. First, examine the smooth profile, ignoring the pulsations. The smooth curve gives mean pressure, which is highest in the aorta and large arteries and decreases progressively as blood flows from the arteries, to the arterioles, to the capillaries, to the veins, and back to the heart. This decrease in pressure occurs as blood flows through the vasculature because energy is consumed in overcoming the frictional resistances.


Figure 4–8 Pressure profile in the vasculature. The smooth curve is the mean pressure. Pulsations, when present, are superimposed on the mean pressure.

Mean pressure in the aorta is high, averaging 100 mm Hg (see Table 4-1 and Fig. 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 high 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 pressure decreases, with the most significant decrease occurring in the arterioles. At the end of the arterioles, mean pressure is approximately 30 mm Hg. This dramatic decrease in pressure occurs because the arterioles constitute a high resistance to flow. Since total blood flow is constant at all levels of the cardiovascular system, as resistance increases, downstream pressure must necessarily decrease (Q = ΔP/R, or ΔP = Q × R).

In the capillaries, pressure decreases further for two reasons: frictional resistance to flow and filtration of fluid out of the capillaries (refer to the discussion on microcirculation). When blood reaches the venules and veins,pressure has decreased even further. (Recall that because capacitance of the veins is high, the veins can hold large volumes of blood at this low pressure.) Pressure in the vena cava is only 4 mm Hg and in the right atrium is even lower at 0 to 2 mm Hg.

Arterial Pressure in the Systemic Circulation

Further examination of Figure 4-8 reveals that although mean pressure in the arteries is high and constant, there are oscillations or pulsations of arterial pressure. These pulsations reflect the pulsatile activity of the heart: ejecting blood during systole, resting during diastole, ejecting blood, resting, and so forth. Each cycle of pulsation in the arteries coincides with one cardiac cycle.

Figure 4-9 shows an expanded version of two such pulsations in a large artery.


Figure 4–9 Systemic arterial pressure during the cardiac cycle. Systolic pressure is the highest pressure measured during systole. Diastolic pressure is the lowest pressure measured during diastole. Pulse pressure is the difference between systolic pressure and diastolic pressure. (See the text for a discussion of mean arterial pressure.)

image Diastolic pressure is the lowest arterial pressure measured during a cardiac cycle and is the pressure in the arteries during ventricular relaxation when no blood is being ejected from the left ventricle.

image Systolic pressure is the highest arterial pressure measured during a cardiac cycle. It is the pressure in the arteries after blood has been ejected from the left ventricle during systole. The “blip” in the arterial pressure curve, called the dicrotic notch (or incisura), is produced when the aortic valve closes. Aortic valve closure produces a brief period of retrograde flow from the aorta back toward the valve, briefly decreasing the aortic pressure below the systolic value.

image Pulse pressure is the difference between systolic pressure and diastolic pressure. If all other factors are equal, the magnitude of the pulse pressure reflects the volume of blood ejected from the left ventricle on a single beat, or the stroke volume.

Pulse pressure can be used as an indicator of stroke volume because of the relationships between pressure, volume, and compliance. Recall that compliance of a blood vessel is the volume the vessel can hold at a given pressure (C = V/P). Thus, assuming that arterial compliance is constant, arterial pressure depends on the volume of blood the artery contains at any moment in time. For example, the volume of blood in the aorta at a given time is determined by the balance between inflow and outflow of blood. When the left ventricle contracts, it rapidly ejects a stroke volume into the aorta, and the pressure rises rapidly to its highest level, the systolic pressure. Blood then begins to flow from the aorta into the rest of the arterial tree. Now, as the volume in the aorta decreases, the pressure also decreases. Arterial pressure reaches its lowest level, the diastolic pressure, when the ventricle is relaxed and blood is returning from the arterial system back to the heart.

image Mean arterial pressure is the average pressure in a complete cardiac cycle and is calculated as follows:


Notice that mean arterial pressure is not the simple mathematical average of diastolic and systolic pressures. This is because a greater fraction of each cardiac cycle is spent in diastole than in systole. Thus, the calculation of mean arterial pressure gives more weight to diastolic pressure than systolic pressure.

Interestingly, the pulsations in large arteries are even greater than the pulsations in the aorta (see Fig. 4-8). In other words, systolic pressure and pulse pressure are higher in the large arteries than in the aorta. It is not immediately obvious why pulse pressure should increase in the “downstream” arteries. The explanation resides in the fact that, following ejection of blood from the left ventricle, the pressure wave travels at a higher velocity than the blood itself travels (due to the inertia of the blood), augmenting the downstream pressure. Furthermore, at branch points of arteries, pressure waves are reflected backward, which also tends to augment pressure at those sites. (Given that blood flows from the aorta to the large arteries, it may seem odd that systolic pressure and pulse pressure are higher in the downstream arteries. We know that the direction of blood flow must be from high to low pressure, and not the other way around! The explanation is that the driving force for blood flow in the arteries is the mean arterial pressure, which is influenced more by diastolic pressure than by systolic pressure (because a greater proportion of each cardiac cycle is spent in diastole). Note in Figure 4-8 that while systolic pressure is higher in the large arteries than in the aorta, diastolic pressure is lower; thus, mean arterial pressure is lower downstream.)

Although systolic pressure and pulse pressure are augmented in the large arteries (compared with the aorta), from that point on, there is damping of the oscillations. The pulse pressure is still evident, but decreased, in the smaller arteries; it is virtually absent in the arterioles; and it is completely absent in the capillaries, venules, and veins. This damping and loss of pulse pressure occurs for two reasons. (1) The resistance of the blood vessels, particularly the arterioles, makes it difficult to transmit the pulse pressure. (2) The compliance of the blood vessels, particularly of the veins, damps the pulse pressure—the more compliant the blood vessel, the more volume that can be added to it without causing an increase in pressure.

Several pathologic conditions alter the arterial pressure curve in a predictable way (Fig. 4-10). As previously noted, pulse pressure is the change in arterial pressure that occurs when a stroke volume is ejected from the left ventricle into the aorta. Logically, then, pulse pressure will change if stroke volume changes, or if the compliance of the arteries changes.


Figure 4–10 Effect of arteriosclerosis and aortic stenosis on arterial pressures.


image Arteriosclerosis (see Fig. 4-10). In arteriosclerosis, plaque deposits in the arterial walls decrease the diameter of the arteries and make them stiffer and less compliant. Because arterial compliance is decreased, ejection of a stroke volume from the left ventricle causes a much greater change in arterial pressure than it does in normal arteries (C = ΔV/ΔP or ΔP = ΔV/C). Thus, in arteriosclerosis, systolic pressure, pulse pressure, and mean pressure all will be increased.

image Aortic stenosis (see Fig. 4-10). If the aortic valve is stenosed (narrowed), the size of the opening through which blood can be ejected from the left ventricle into the aorta is reduced. Thus, stroke volume is decreased, and less blood enters the aorta on each beat. Systolic pressure, pulse pressure, and mean pressure all will be decreased.

image Aortic regurgitation (not shown). When the aortic valve is incompetent (e.g., due to a congenital abnormality), the normal one-way flow of blood from the left ventricle into the aorta is disrupted. Instead, blood that was ejected into the aorta flows backward into the ventricle. Such retrograde flow can occur because the ventricle is relaxed (is at low pressure) and because the incompetent aortic valve cannot prevent it, as it normally does.

Venous Pressures in the Systemic Circulation

By the time blood reaches the venules and veins, pressure is less than 10 mm Hg; pressure will decrease even further in the vena cava and the right atrium. The reason for the continuing decrease in pressure is now familiar: The resistance provided by the blood vessels at each level of the systemic vasculature causes a fall in pressure. Table 4-1 and Figure 4-8 show the mean values for venous pressures in the systemic circulation.

Pressures in the Pulmonary Circulation

Table 4-1 also compares pressures in the pulmonary circulation with pressures in the systemic circulation. As the table shows, the entire pulmonary vasculature is at much lower pressure than the systemic vasculature. The pattern of pressures within the pulmonary circulation is analogous to the systemic circulation, however. Blood is ejected from the right ventricle into the pulmonary artery, where pressure is highest. Thereafter, the pressure decreases as blood flows through the pulmonary arteries, arterioles, capillaries, venules, and veins and back to the left atrium.

An important implication of these lower pressures on the pulmonary side is that pulmonary vascular resistance is much lower than systemic vascular resistance. This conclusion can be reached by recalling that the total flow through the systemic and pulmonary circulations must be equal (i.e., cardiac output of the left and right hearts is equal). Because pressures on the pulmonary side are much lower than pressures on the systemic side, to achieve the same flow, pulmonary resistance must be lower than systemic resistance (Q = ΔP/R). (The pulmonary circulation is discussed in more detail in Chapter 5.)