Key result
Weightlessness in space induces an unexpected fluid and sodium retaining state with sympathetic nervous system activation, contrasting with findings from head-down bed rest simulations.
Understanding how weightlessness modulates fluid volume control in space provides insights into the pathophysiology of heart failure, where gravity plays a strong role in fluid and sodium retention.
Hypothesis-generating for heart failure fluid retention mechanisms; prospective validation required before clinical translation.
The human cardiovascular system and regulation of fluid volume are heavily influenced by gravity. When decreasing the effects of gravity in humans such as by anti-orthostatic posture changes or immersion into water, venous return is increased by some 25%. This leads to central blood volume expansion, which is accompanied by an increase in renal excretion rates of water and sodium. The mechanisms for the changes in renal excretory rates include a complex interaction of cardiovascular reflexes, neuroendocrine variables, and physical factors. Weightlessness is unique to obtain more information on this complex interaction, because it is the only way to completely abolish the effects of gravity over longer periods. Results from space have been unexpected, because astronauts exhibit a fluid and sodium retaining state with activation of the sympathetic nervous system, which subjects during simulations by head-down bed rest do not. Therefore, the concept as to how weightlessness affects the cardiovascular system and modulates regulation of body fluids should be revised and new simulation models developed. Knowledge as to how gravity and weightlessness modulate integrated fluid volume control is of importance for understanding pathophysiology of heart failure, where gravity plays a strong role in fluid and sodium retention.
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Peter Norsk (2005) conducted a review in Cardiovascular and fluid volume regulation in space. Weightlessness vs. Gravity / Head-down bed rest simulations was evaluated. Weightlessness in space induces an unexpected fluid and sodium retaining state with sympathetic nervous system activation, contrasting with findings from head-down bed rest simulations.
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