Cardiac Output and Venous Return as Interdependent and Independent Variables

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Abstract
Under steady states the heart pumps whatever it receives and receives whatever it pumps. Cardiac output (CO) and venous return (VR) are equal; the distinction between the 2 seems unnecessary. Under nonsteady states the 2 are temporarily unequal and the distinction becomes significant. VR varies directly with the difference in pressure between the end of systemic capillaries and the right ventricle during filling and inversely with the total resistance of the venous system. The energy for VR is derived from CO. In some transient states VR becomes an independent variable and CO dependent until a new steady state is reached (e.g., exercise, hemorrhage, fevers, hyperthyroidism, severe anemia, etc.). In other conditions the opposite is true (e.g., myocardial infarction, altered ventricular contractility, etc.) Explanation of changes in CO in various perturbations of circulation are based on the identification of the independent variable (VR or CO) in a given physiologic or pathologic condition during the period that a nonsteady state exists.

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