Transitory cardiovascular responses to rapid infusion of blood into aorta of rabbit

Blood (5 to 15 ml) was infused into aorta of the anesthetized rabbit through the carotid artery or femoral artery at a constant rate (0.55 to 8.3 ml/s). The carotid sinuses were occluded. The systemic arterial pressure (SAP) began to rise immediately at the onset of the infusion and dropped gradually in most cases during the infusion. SAP continued to decline after the infusion and in many cases became lower than the preinfusion pressure. Within seconds, SAP tapered off to its lowest point and resumed a gradual rise, leveling off higher than its initial pressure. A depressor effect caused by the rapid infusion of blood into the aorta (DRIA) was observed in every rabbit. DRIA was not suppressed much by the section of aortic or vagal nerves. DRIA was markedly suppressed by the administration of alpha-adrenergic blockade, but beta-adrenergic blockade had little effect on DRIA. By using a hydraulic model of the closed circulatory system, a theoretical analysis was made on the basis of the "theory of active fluid element," a theory developed by Hatakeyama, one of the authors, and it was demonstrated that DRIA must not be considered to be a passive hemodynamic phenomenon. The results obtained suggest that a nervous regulatory mechanism--rather than the reflex via the carotid sinus or aortic baroreceptors, or other mechanoreceptors--plays an important part in the venous and the cardiac regions.