Cardio-renal responses to a simulated gravity-free state induced by water immersion

HOI induces a significant increase in the cardiac output, which is accompanied by increases in the urinary excretion of water, Na and K. However, the renal responses are not always associated with the cardiac response, suggesting a possible dissociation of the two events. Although the urinary excretion of both ADH and aldosterone decreases during HOI, the mechanism for these changes is not clearly understood. The renal response to HOI, especially the natriuresis, is considerably attenuated in endurance-trained athletes, which may be related to the more moderate inhibition of both ADH and renin-aldosterone systems. The exact role of the cardiac receptors in inducing the renal responses to HOI is still not defined, although the latter receptors are involved in inducing the natriuresis. Finally, important diurnal variations have been demonstrated for the renal responses to HOI, and elucidation of the mechanism for this interesting phenomenon may help understand the overall mechanisms for the development of cardiorenal-endocrine responses to HOI.