[Physiology of orthostatic regulation].

The arterial blood pressure is integrated into a feedback control system and practically depends on three parameters, i. e. blood volume, heart dynamics, and peripheral resistance. When circulation is disturbed in the orthostatic, the correcting elements heart and vessels have to adapt themselves according to this demand. By standing up, the hydrostatic pressure in the arterial vessels increases rapidly in the lower body regions. The initial increase of the hydrostatic pressure will be stopped by the valves in the venous system. By means of a higher arteriovenous pressure gradient the blood flow into the lower body regions increases more than 100%. The blood flow fills the venous system within 1-2 min successively from downward to upward and opens the valves. The venous pressure in the feet can rise to the full hydrostatic value of 100 mm Hg. This change of pressure causes a volume increase of approximately 200 cm3 regarding both legs of healthy persons. In this way the circulatory blood quantity is reduced. The heart is able to put out shortly an increased cardiac output by loading off the central blood volume (lung and end-systolic volume), in order to compensate orthostatic demand. After a long upright position the cardiac output is falling under the starting value and heart frequency increases. By sympathetic stimulation the peripheral resistance increases as well.