In the steady state, blood flow out of the heart must be the same as blood flow into the heart. Thus, flow in the cardiovascular system should depend not only on cardiac determinants of flow-pericardial pressure, afterload, heart rate, and contractility-but also on systemic venous determinants of flow as well. We studied the venous determinants of flow in nine baboons. The mean systemic pressure, the upstream driving pressure for venous return, was 10 mm Hg higher than the right atrial pressure. Flow generated by this upstream pressure was modulated by a venous resistance. This resistance acted as if it were interposed between the upstream capacitance vessels and the right atrium. These determinants of venous return in the baboon would be inadequately described by classical concepts of cardiovascular physiology. We suspect that classical concepts would be inadequate in man as well.
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