Many basic concepts of cardiac output and arterial pressure control have changed considerably in the past few years. In general, each tissue controls its own local resistance and blood flow regardless of the level of arterial pressure; the sum of the local flows then determines the venous return and cardiac output. However, the arterial pressure is normally controlled by separate mechanisms that do not significantly alter the cardiac output. During acute circulatory stresses, such as exercise, the arterial pressure is controlled almost entirely by nervous reflex mechanisms; but over long periods, there reflex mechanisms fade away because they adapt. The arterial pressure is then controlled mainly by a renal-volume-endocrine pressure control system, in which the blood volume and total peripheral resistance are manipulated slowly to adjust the pressure.