Orthostatic hypotension and the role of changes in venous capacitance.

We suggest a mechanism by which blood volume changes might explain the hypotension that, after space flight, often accompanies the return to normal gravity. Upon entering microgravity, peripheral veins may collapse and, because of volume redistribution, raise the pressure in the central venous compartment. After some time in space, homeostatic mechanisms may cause volume excretion and reduce the pressure in the central venous compartment to normal values. Upon return to normal gravity, peripheral veins may re-expand and distribute a reduced blood volume into an enlarged space, thus lowering pressure in the central venous compartment. This would reduce cardiac preload, output, and arterial pressure. To prevent this sequence of events, leg cuffs might be inflated before the end of the space flight to allow homeostatic mechanisms to increase blood volume to normal levels.