Effects of head-down bed rest & microgravity on renal fluid excretion.

The significance of transverse gravity stress on fluid volume regulation in supine humans was studied on the D2 Spacelab mission, a Mir mission, and in a head-down tilt bed rest study. A new hypothesis is presented which states that on the ground, the transverse G-stress in supine or slightly head-down tilted humans has a more pronounced effect than anticipated on the interaction of central cardiovascular, endocrine, and renal variables. Also, total abolishment of the G-stress during space flight induces a more pronounced decrease in intravascular volume than the decrease induced by prolonged head-down bed rest, which results in stimulation of fluid retaining mechanisms.