Venous Pressure Is Reciprocally Related To Arterial Pressure

Venous return refers to the amount of blood flowing back to the heart per unit of time. At steady state, venous return and cardiac output must be equal. Because the atria lack valves, their contraction contributes little to diastolic filling and blood flows into the heart passively as a result of the pressure in the great veins. Thus, systemic venous pressure can be considered to be the filling pressure for the right ventricle. To develop a conceptual understanding of what affects venous pressure, consider a situation in which a patient's heart is replaced by a heart-lung machine for which cardiac output can be set to any desired level and is not affected by changes in venous pressure. A simplified view of this arrangement is presented in Fig. 1. The cardiovascular system now consists of a heart-lung machine that is pumping blood out of the veins and into the aorta. Pressure in the arterial compartment forces blood from the arterial compartment across the microcirculation where most of the total peripheral resistance (TPR) is located and into the veins. Blood in the veins then returns to the heart-lung machine. A steady state will be soon be achieved wherein output of the heart-lung machine (cardiac output) equals the flow across the TPR, which, in turn, equals venous return. The system is self-regulating. If flow through the microcirculation were less than cardiac output, then blood would start to accumulate in the arterial vessels. As the additional blood stretches the arterial walls, the arterial pressure rises, which in turn increases the flow into the microcirculation until the equilibrium is again reached.

Let's assume we start with a mean arterial pressure of 102 mm Hg and a venous pressure of 2 mm Hg. This pressure difference of 100 mm Hg results in a flow of 5 L/min across the TPR (TPR in this case equals 20 mm Hg XL''1 X min'' ). If the heart-lung machine were suddenly stopped, blood would continue to flow across the microcirculation, propelled by the recoiling arterial capacitance. Arterial pressure would fall as blood leaves the arterial compartment and venous pressure would rise as blood is shifted into the venous compartment. Flow through the microcirculation will stop only when arterial and venous pressures are equal. The equilibrium pressure that is reached throughout the cardiovascular

Arteries

Arteries

FIGURE 1 The circulation can be modeled by a heart-lung machine (pump) connected to a circulatory system consisting of an arterial capacitance that is small, a peripheral resistance, and a venous capacitance that is large. All components are connected in series. See text for details.

system when cardiac output is stopped is called the mean circulatory filling pressure. Mean circulatory filling pressure is thought to be approximately 7 mm Hg and is determined by the amount of blood within the cardiovascular system and the pressure-volume relationship (capacitance) of the blood vessels.

Withdrawing blood from a patient will decrease the mean circulatory filling pressure. If blood were removed until the mean circulatory filling pressure fell to zero (no stretch on the blood vessel walls), the system would still contain an appreciable amount of blood. That volume is called the unstressed volume. The unstressed volume can be varied by the body's control systems by changing the contractile state of the smooth muscle in the walls of the veins. Venoconstriction decreases unstressed volume and thus increases mean circulatory filling pressure, whereas venodilation increases unstressed volume and decreases the mean circulatory filling pressure.

Venous Capacitance Is Much Larger Than Arterial Capacitance

In Chapter 10, we learned that the ratio of volume to pressure for a compartment defines its capacitance. The capacitance of the venous compartment is approximately 19 times greater than that of the arterial compartment. We have tried to illustrate the differences in capacitance in the shape of the arterial and venous reservoirs in Fig. 1. The arterial reservoir is tall and narrow so that a small increment in volume will greatly increase the height of fluid in the reservoir and thus its pressure. Conversely, the venous side is broad so that the same increment in volume would have much less effect on pressure. Movement of a volume of blood from the arterial side to the venous side will cause arterial pressure to decrease and venous pressure to increase. Because of the differences in capacitances, however, the arterial pressure change will be 19 times greater than that in the veins. Thus, if the output of the heart-lung machine were reduced from 5 to 2.5 L/min, arterial pressure would fall to 54.5 mm Hg and venous pressure would rise to 4.5. Although arterial pressure fell by 47.5 mm Hg, venous pressure rose by only 2.5 mm Hg, 19 times less. Note that any change in arterial pressure will be accompanied by a reciprocal but smaller change in venous pressure. Figure 2 shows the change in venous and arterial pressures that occurs when cardiac output goes from 0 to 1 L/min. At zero cardiac output, arterial and venous pressures are equal and at the mean circulatory filling pressure. As flow begins arterial pressure rises markedly, whereas venous pressure falls only slightly. Although these changes in venous pressure may seem small, they are of paramount importance because venous pressure is the filling pressure for the

Time

FIGURE 2 Arterial pressure and venous pressure are plotted as a function of time when the output of the heart-lung machine depicted in Fig. 1 is increased from 0 to 1 L/min.

Time

FIGURE 2 Arterial pressure and venous pressure are plotted as a function of time when the output of the heart-lung machine depicted in Fig. 1 is increased from 0 to 1 L/min.

heart and in the previous chapter we learned that filling pressure is a primary determinant of stroke volume.

The Relationship between Cardiac Output and Venous Pressure Is Termed the Vascular Function Curve

When the steady-state venous pressure is plotted as a function of cardiac output, one observes an inverse linear relationship (Fig. 3). Venous pressure is equal to the mean circulatory filling pressure when cardiac output is zero (point A on Fig. 3). As cardiac output is increased, venous pressure decreases linearly until zero venous pressure is attained. At that point, so much blood is sequestered in the arterial compartment that the veins are at their unstressed volume. Zero venous pressure

FIGURE 3 A vascular function curve. Venous pressure is plotted as a function of cardiac output.

Venous Pressure (mm Hg)

FIGURE 3 A vascular function curve. Venous pressure is plotted as a function of cardiac output.

represents the maximum possible cardiac output because any further fall in pressure would result in a negative pressure and the central veins would collapse. Hence, the curves become flat in the range of negative venous pressures. The plot in Fig. 3 is known as the vascular function curve. By convention, plots of the vascular function curve have cardiac output (the independent variable) on the y axis and venous pressure (the dependent variable) on the x axis. Thus, any point (e.g., B) on the vascular function curve is associated with a specific cardiac output and a venous pressure. This convention facilitates a graphic analysis that is useful in determining cardiac output, as explained later.

Periphery Interacts with the Heart through Changes in Venous Pressure

In Chapter 13, we saw that venous filling pressure was an important determinant of stroke volume. If arterial pressure, heart rate, and contractility are held constant, cardiac output will be a positive function of venous pressure. The relationship between venous pressure and cardiac output for the heart is shown by the cardiac function curve in Fig. 4. The cardiac function curve presented here is similar to the ventricular function curve described in Chapter 13 except that cardiac output rather than stroke work is plotted on the vertical axis. We chose cardiac output so that the curve can be co-plotted with the vascular function curve. Keep in mind that unlike the ventricular function curve, the cardiac output/venous pressure plot is affected by changes in both contractility and aortic pressure.

Although venous pressure determines the output of the heart, venous pressure is, in turn, inversely related to cardiac output, as shown by the vascular function curve in the same figure. Thus, a constant interplay occurs between the heart and blood vessels at the level of venous pressure. As cardiac output tries to increase, venous pressure decreases, thus bringing cardiac output back to its original value in a self-regulating fashion. To calculate the equilibrium cardiac output, we must treat the two relationships as simultaneous equations and solve for the one cardiac output and venous pressure pair that would satisfy both curves. In Fig. 4, we have solved the simultaneous equations graphically by co-plotting them. Where the two curves cross is their equilibrium point. If the heart described by the cardiac function curve in Fig. 4 is connected to the vascular system depicted by the venous function curve in Fig. 4, then the cardiac output will be 5.5 L/min and the venous pressure will be 1 mm Hg. It is important for the student to understand that the system cannot operate off the equilibrium point. The only way that cardiac output can be altered is to change one or both of the curves.

FIGURE 4 Simultaneous plots on the same axis of the vascular function and cardiac function curves. The intersection of the two curves reveals the steady-state cardiac output and venous pressure that must exist in the system.

Venous Pressure (mm Hg)

FIGURE 4 Simultaneous plots on the same axis of the vascular function and cardiac function curves. The intersection of the two curves reveals the steady-state cardiac output and venous pressure that must exist in the system.

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