As mean arterial pressure changes, both short- and long-term changes are initiat

Hypotension, a decrease in mean arterial pressure, stimulates the sympathetic nervous system to increase heart rate, stroke volume (ventricular contractility), venous tone (decreasing compliance), and total peripheral resistance (arteriole constriction). At the same time, increases in renin elevate aldosterone concentration and, thereby, blood volume in an attempt to provide a long-term solution to the hypotension. However, the increase in total peripheral resistance does not affect every vascular bed equally. While an increase in total peripheral resistance decreases blood flow to most organs, this is not the case for the heart and the brain. Each of these vascular beds are poorly influenced by sympathetic activity and, thus, resistance in these organs remains unaffected by sympathetic activity. This maintenance of blood flow to the brain and heart are indicative of the importance of oxygen delivery to these tissues for survival. If hypotension is maintained for more than an hour or two due to an inadequate expansion of blood volume (or continued hemorrhaging), the reduction in blood flow to the remainder of the organs of the body begins to reverse; the reduced blood flow causes the tissue to become ischemic, thereby inducing a local reflex vasodilation of the arterioles within those tissues and increasing blood flow. This further decreases total peripheral resistance and mean arterial pressure until the vasculature collapses (a dilated state). This is known as circulatory shock.