Fluid Metabolism and Circulation During and After Simulated Weightlessness

Fluid Metabolism and Circulation During and After Simulated Weightlessness. Orthostatic hypotension can readily be produced by whole body immersion in water lasting not longer than 8 hours. Such experiments were conducted with the view to the diuretic reflex (Gauer and Henry) which is initiated by an increase of the intrathoracic blood volume. This in turn leads to an increase in depressor signals from the baroreceptors in the circulation especially those in the heart chambers. The effect is mediated by hemodynamic changes and the excretory function of the neurohypophysis. In several sets of experiments the following results were obtained. 1 Contrary to the “normal” picture of volume conditioned diureses induced by other procedures osmotic clearance is frequently increased in addition to the usual increase in free water clearance. 2 By a single injection or a slow infusion of Vasopressin the diuresis of water immersion can be interrupted or prevented in the same manner as a water diuresis of the same strength induced by water drinking. 3 As shown by bioassay it is very likely that the diuresis of immersion is not solely due to a reduction of Vasopressin but also to the appearance of a diuretic factor in the plasma. 4 Measurements with Evans Blue revealed an average reduction in plasma volume of 14% after 8 hours. 5 Infusion of Vasopressin during water immersion in a dose too small to exert any effect on blood pressure prevented orthostatic collapse in 5 subjects. This beneficial effect may be due to a prevention of diuresis and loss of blood volume. An effect on the tone of the capacity vessels must also be considered.