Graphic format for teaching long-term control of systemic arterial pressure

Circulatory homeostasis is a difficult notion. The graphic format presented here facilitates the teaching of long-term control of systemic arterial blood pressure and cardiac output. It is based on the view that the following four "function curves" cooperate in long-term regulation: the relation between blood volume and ventricular filling pressure, the relation between ventricular filling pressure and cardiac output, the relation between cardiac output and peripheral resistance, and the relation between arterial pressure and natriuresis. Positioning the function curves in the format presented here clarifies their cooperativity. The distinction between a nonsteady state and a steady state deserves emphasis. Long-term pathophysiology of the circulation is most easily taught on the basis of the assumption that, generally, there will be a steady state. The format clarifies why some known physiological relations are almost impossible to demonstrate in the intact organism, and it discourages explanations of pathophysiology that are not firmly based on physiology.