Water and sodium balance in space

We have previously shown that fluid balances and body fluid regulation in m icrogravity (muG) differ from those on Earth (Drummer et al, Eur J Physiol 441:866-R72, 2000). Arriving in muG leads to a redistribution of body fluid -composed of a shift of fluid to the upper part of the body and an exaggera ted extravasation very early in-flight. The mechanisms for the increased va scular permeability are not known. Evaporation, oral hydration, and urinary fluid excretion, the major components of water balance, are generally dimi nished during space flight compared with conditions on Earth. Nevertheless, cumulative water balance and total body water content are stable during fl ight if hydration, nutritional energy supply, and protection of muscle mass are at an acceptable level. Recent water balance data disclose that the ph enomenon of an absolute water loss during space flight, which has often bee n reported in the past, is not a consequence of the variable muG. The handl ing of sodium, however, is considerably affected by muG. Sodium-retaining e ndocrine systems, such as renin-aldosterone and catecholamines, are much mo re activated during muG than on Earth. Despite a comparable oral sodium sup ply, urinary sodium excretion is diminished and a considerable amount of so dium is retained-without accumulating in the intravascular space. An enormo us storage capacity for sodium in the extravascular space and a mechanism t hat allows the dissociation between water and sodium handling likely contri bute to the fluid balance adaptation in weightlessness. (C) 2001 by the Nat ional Kidney Foundation, Inc.