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THE CARDIOVASCULAR system is a closed circuit. At any given moment, the rate at which blood returns to the heart from the venous system (venous return) may differ considerably from the rate at which the heart pumps it out into the arterial system (cardiac output). Under steady state conditions, however, cardiac output (CO) and venous return (VR) are virtually equal. Therefore, it probably is better not to distinguish between CO and VR under such conditions, but simply to consider the total blood flow (OJ around the circuit. There has been a tendency on the part of some investigators to explain steady state changes in CO in response to certain conditions (e.g., blood loss, exercise, vasoactdve drugs) by invoking changes in VR. Such an explanation is meaningless, however. These conditions affect Q. for reasons that merit analysis, and such changes in Q are, of course, attended by equal changes in CO and VR at equilibrium. However, to explain the steady state change in CO on the basis of a change in VR is a patent example of circular reasoning; it is tantamount to explaining a change in Q on the basis of a change in Q.

The cardiac and vascular factors that determine systemic blood flow.