Physiology of the upright posture

Orthostatic stresses are common daily events for humans. They result in a shift of blood away from the chest to the distensible venous capacitance system below the diaphragm. Venous pooling and enhanced capillary filtration of blood in the dependent limbs contribute to a rapid diminution of central blood volume, that is, the volume of blood directly available to the cardiac ventricles, which is of paramount importance for the beat-to-beat adjustment of blood pressure. Unless compensatory adjustments are promptly instituted, blood pressure falls, and the person faints within minutes. This chapter provides a general overview of the available regulatory mechanisms that serve to maintain blood pressure in the upright posture. For details of the difference between the initial (first 30 s) cardiovascular adjustments between active and passive changes of posture, refer to Wieling et al. (2007).