[Physiologic mechanisms in extremity veins supporting chronic orthostatic tolerance]

As a consequence of chronic orthostatic load of the organism several long-term antigravitation defense mechanism are activated in the lower extremity veins and microvessels. The following adaptive mechanisms are brought into focus: acute pressure-induced myogenic tone and its augmentation which promote the venous capacity autoregulation, potassium channels counterregulating the depolarization of the venous smooth muscle membrane, functional and structural remodeling of sympathetic innervation in the vascular wall, changes in venous network properties, decreased capillary supply in oxidative muscles of the extremities, and probably transendothelial homeostatic water transport in the wall of veins.