cardiovascular disorders
Orthostatic Hypotension
Pathophysiology of Orthostatic Hypotension
Normally, the gravitational stress of suddenly standing causes blood (½ to 1 L) to pool in the veins of the legs and trunk. The subsequent transient decrease in venous return reduces cardiac output and thus BP. In response, baroreceptors in the aortic arch and carotid sinus activate autonomic reflexes to rapidly return BP to normal. The sympathetic nervous system increases heart rate and contractility and increases vasomotor tone of the capacitance vessels. Simultaneous parasympathetic (vagal) inhibition also increases heart rate. In most people, changes in BP and heart rate upon standing are minimal and transient, and symptoms do not occur. With continued standing, activation of the renin-angiotensin-aldosterone system andvasopressin(antidiuretic hormone [ADH]) secretion cause sodium and water retention and increase circulating blood volume. 