Journal of Applied Physiology, Vol 60, Issue 1 60-62, Copyright © 1986 by American Physiological Society
Mechanism for negative water balance during weightlessness: an hypothesis
J. E. Greenleaf
The mechanism for the apparent decrease in body fluid volume in astronauts
during spaceflight remains obscure. The widespread postulate that the
hypohydration is the result of the Henry-Gauer reflex, a diuresis caused by
inhibition of vasopressin secretion resulting from increased left and
perhaps right atrial (central) venous pressure, has not been established
with direct measurements on astronauts. An hypothesis is proposed to
account for fluid-electrolyte shifts during weightlessness. A moderate but
transient increase in central venous pressure occurs when orbit is entered
that is insufficient to activate the Henry-Gauer reflex but sufficient to
stimulate the release of atrial natriuretic peptides. Increased sodium
excretion would facilitate some increased urinary water loss. The resulting
relatively dilute plasma and interstitial fluids would cause fluid to shift
into the cellular space, resulting in edema in the head and trunk and
inhibition of thirst and drinking. Thus the negative water balance in
astronauts would be caused by a gradual natriuresis and diuresis coupled
with reduced fluid intake.
