Fig. 19.5 . The amount of blood in the arterioles is small; these vessels contain only approximately 3% of total blood volume (see ). Changes in the contractile state of arterioles do not significantly alter mean circulatory pressure. Thus vascular function curves that represent different peripheral resistances converge at a common point on the y-axis (see
Fig. 19.5
Pv varies inversely with total peripheral resistance (TPR) when all other factors remain constant. Physiologically, the relationship between Pv and TPR can be explained as follows: if cardiac output is held constant, a sudden increase in TPR causes a progressively greater volume of blood to be retained in the arterial system. Blood volume in the arterial system continues to increase until Pa rises sufficiently to force a flow of blood equal to cardiac output through the resistance vessels. If total blood volume does not change, this increase in arterial blood volume is accompanied by an equivalent decrease in venous blood volume. Hence, an increase in TPR diminishes Pv proportionately. This relationship between TPR and Pv, together with the inability of peripheral resistance to affect mean circulatory pressure, accounts for the clockwise rotation of the vascular function curves in response to increased arteriolar constriction (see
Fig. 19.5 ). Similarly, arteriolar dilation produces a counterclockwise rotation from the same vertical axis intercept. A higher maximal level of cardiac output is attainable when the arterioles are dilated than when they are constricted (see
Fig. 19.5).
Cardiac output and venous return are tightly linked. Except for small, transient disparities, the heart cannot pump any more blood than is delivered to it through the venous system. Similarly, because the circulatory system is a closed circuit, venous return to the heart must equal cardiac output over any appreciable time interval. The flow around the entire closed circuit depends on the capability of the pump, the characteristics of the circuit, and the total fluid volume of the system.
Thus cardiac output and venous return are simply two terms for the flow around this closed circuit. Cardiac output is the volume of blood being pumped by the heart per unit time. Venous return is the volume of blood returning to the heart per unit time. At equilibrium, these two volumes are equal. In the following section, certain techniques of circuit analysis are discussed to provide some insight into the control of flow around the circuit.
Relating the Cardiac Function Curve to the Vascular Function Curve
Coupling Between the Heart and the Vasculature
In accordance with the Frank-Starling law of the heart, cardiac output depends closely on right atrial (or central venous) pressure. Furthermore, right atrial pressure is approximately equal to right ventricular end-diastolic pressure because the normal tricuspid valve acts as a low-resistance junction between the right atrium and ventricle. Graphs of cardiac output as a function of Pv are called cardiac function curves; extrinsic regulatory influences may be expressed as shifts in such curves.
A typical cardiac function curve is plotted on the same coordinates as those for a normal vascular function curve in
Fig. 19.6 . The cardiac function curve is plotted according to the usual convention; that is, the independent variable (Pv) is plotted along the x-axis, and the dependent variable (cardiac output) is plotted along the y-axis. In accordance with the Frank-Starling mechanism, the cardiac function curve reveals that a rise in Pv increases cardiac output.
Conversely, the vascular function curve characterizes an inverse relationship between cardiac output and Pv; that is, a rise in cardiac output diminishes Pv. Pv is the dependent variable (or response) and cardiac output is the independent variable (or stimulus) for the vascular function curve. Therefore, to plot a vascular function curve in the conventional manner, Pv should be scaled along the y-axis and cardiac output along the x-axis.
To plot the cardiac and vascular function curves on the same set of axes requires a modification of the plotting convention for one of these curves. The convention for the vascular function curve is violated arbitrarily in this chapter. Note that the vascular function curve in
Fig. 19.6 is intended to reflect how Pv (scaled along the x-axis) varies in response to a change in cardiac output (scaled along the y-axis).