Correction of changes in fluid-electrolyte metabolism in manned space flights.

In order to prevent and correct hypohydration and negative electrolyte balance, the effects of exercises, lower body negative pressure (LBNP) and water-salt supplements (WSS) were investigated in more than 100 test subjects during 14-, 49- and 182-d headdown tilt tests (-4 degrees). A combined use of WSS and LBNP during regular exercises led to a distinct water and sodium retention. These changes were mainly determined by the stimulation of the renin-angiotension-aldosterone and antidiuretic systems. After these countermeasures were tested in simulation studies, they were used by 12 cosmonauts during the 63- and 185-d space flights. The detailed analysis of the postflight examinations of the crewmembers suggests that these countermeasures may exert a beneficial effect on fluid-electrolyte balance.