Effect of Cardiac Output on Circulatory Blood Volume

In anesthetized dogs venous return was drained into a blood reservoir from which blood was pumped to the right atrium at a variable perfusion rate, which was equal to cardiac output in the steady state. When cardiac output was decreased or increased by 25 or 50% of the control, the blood volume in the dog's body was changed in the same direction in the intact reflexic state as well as in the areflexic state prepared by hexamethonium and norepinephrine infusion. The volume change in the reflexic state was twice that in the areflexic state when compared 5 min after stepwise changes in cardiac output. When only the flow through the right heart and lungs was changed by-50%, with systemic flow unchanged, the decrease in blood volume was about one-fifth of that observed on a 50% decrease of cardiac output and not affected by ablation of the reflexes. It is concluded that, on a change in cardiac output, the passive change in blood volume is as large as the active or reflexic change, that the majority of the change in blood volume takes place in the systemic circulation rather than in the pulmonary circulation, and that the receptors for the reflexic change are located in the systemic circulation.