Control of Cardiac Output — Response of the Organism

Abstract

Cardiac Output is uniquely determined by factors described by the intersection of a cardiac function curve and a venous return curve. Algebraically this is CO = VR = (Pms – Pra)/RVR where CO is cardiac Output, VR is venous return, Pms is the mean systemic pressure, Pra is the right atrial pressure relative to athmospheric pressure, and RVR is the resistance to venous return. To influence cardiac Output the organism must vary these parameters. However, it is known that the organism under conditions of stress not only increases the cardiac Output, but also redistributes the cardiac Output. The goal of the organism seems to be to keep the delivery of oxygen to the tissues as normal as possible; or, if this fails, to redistribute the Output to those organs with the more necessary functions. To accomplish this there are many reflexes and hormones which come into play. Some act systematically and some locally, and more and more agents and pathways are discovered all the time. However this dicussion will be limited to mechanical adjustments to the circulation.