The cardiac and vascular interactions in the regulation of cardiac output

Abstract

Cardiac output is determined by the following four factors: preload, afterload, heart rate, and myocardial contractility. The first two of these factors may be called coupling factors, because their magnitudes are determined by the characteristics of both the cardiac and vascular components of the cardiovascular system. The nature of the coupling between these two components can be analyzed by means of two characteristic curves. The first curve, the so-called cardiac function curve, reflects the Frank-Starling mechanism. It describes the cardiac output as a function of the cardiac filling pressure (or preload). The second curve, the so-called vascular function curve, reflects certain vascular characteristics, namely the peripheral resistance, arterial and venous compliances, and total blood volume. This curve describes the cardiac filling pressure as a function of the cardiac output. The equilibrium values of cardiac output and cardiac filling pressure are defined by the point of intersection of these two curves. The various humoral, neural, and pharmacological factors that affect the cardiovascular system will alter one or both of these curves. The effects of such factors on cardiac output and preload are defined by the point of intersection of the new cardiac and vascular function curves.