Characterization of renal response to prolonged immersion in normal man

During the initial phase of spaceflight, there is a translocation of fluid from the lower parts of the body to the central vascular compartment with a resultant natriuresis, diuresis, and weight loss. Whether this natriuresis and diuresis result in the attainment of a new steady state or whether the circulatory adjustment is incomplete is the subject of continuing controversy. Because water immersion is regarded as an appropriate model for studying the redistribution of fluid that occurs in weightlessness, we carried out an immersion study of relatively prolonged duration in order to characterize the temporal profile of the renal adaptation to central hypervolemia. Twelve normal male subjects underwent an immersion study of 8-h duration in the sodium-replete state. Immersion resulted in marked natriuresis and diuresis which were sustained throughout the immersion period. The failure of that natriuresis and diuresis of immersion to abate or cease despite marked extrcellular fluid volume contraction as evidenced by a mean weight loss of -2.2 +/- 0.3 kg suggests that central blood volume was not restored to normal and that some degree of central hypervolemia probably persisted.