Initial and delayed circulatory responses to orthostatic stress in normal humans and in subjects with orthostatic...

Gravitational stresses, which are common daily event for humans, result in a diminution in central blood volume, due to displacement of blood to the lower parts of the body. They demand complex adjustments in the cardiovascular system to offset the decrease in cardiac filling pressure. Such changes are necessary to sustain arterial blood pressure at an appropriate level so that there is adequate perfusion of vital organs, especially the brain. These adjustments must compensate for both the initial and sustained orthostatic stress. The rapid short-term adaptations are mediated primarily by the cardiovascular reflexes with humoral agents reinforcing these reflexes during severe and prolonged orthostatic stress. Understanding these complex reflex adjustments in normal humans is necessary in order to appreciate subjects with orthostatic intolerance.