ROLE OF ACTIVE CHANGES IN VENOUS CAPACITY BY THE CAROTID BAROREFLEX -ANALYSIS WITH A MATHEMATICAL-MODEL

To elucidate the role of venous capacity active changes in short-term cardiovascular homeostasis, a mathematical model of the carotid-sinus baroreflex system has been developed. In the model the cardiovascular system is represented as the series arrangement of six lumped compartm ents, which synthesize the fundamental hemodynamic properties of the s ystemic arterial, systemic venous, pulmonary arterial, and pulmonary v enous circulations as well as of the left and right cardiac volumes. C ardiac outputs from the left and right ventricles are computed as a fu nction of both downstream arterial pressure (afterload) and upstream a trial pressure (preload). Four distinct feedback regulatory mechanisms , working on systemic arterial resistance, heart rate, systemic venous unstressed volume, and systemic venous compliance, are assumed to ope rate on the cardiovascular system in response to carotid sinus pressur e changes. All model parameters, both in the cardiovascular system and in feedback regulatory mechanisms, have been assigned on the basis of physiological data now available. The model is used here to simulate the pattern of the main hemodynamic quantities in the short time perio d (1-2 min) after acute carotid sinus activation in vagotomized subjec ts. Simulation results indicate that the model can reproduce experimen tal data quite well, with reference both to open-loop experiments and to an acute blood hemorrhage performed in closed-loop conditions. More over, computer simulations indicate that active changes in venous unst ressed volume are of primary importance in regulating cardiac output a nd systemic arterial pressure during activation of the carotid sinus b aroreflex.