RESTORATION OF PLASMA-VOLUME AFTER 16 DAYS OF HEAD-DOWN TILT INDUCED BY A SINGLE BOUT OF MAXIMAL EXERCISE

Seven healthy men performed maximal exercise 24 h before the end of 16 days exposure to 6 degrees head-down tilt (HDT) to test the hypothesi s that such an exercise technique could restore plasma volume (PV) at the end of a simulated space mission. Exercise consisted of supine cyc ling with graded work rates increasing by 16 W/min to volitional fatig ue and required an average of 16 min. The experimental protocol was a standard cross-over design in which the order of treatment (exercise o r control) was counterbalanced across all seven subjects. PV, fluid in take (ad libitum), urine output, renal function, and hormones associat ed with fluid homeostasis were measured before HDT, 24 h before the en d of HDT just prior to exercise, and at the end of HDT 24 h after exer cise. HDT reduced PV by 16% in both control and exercise conditions. M aximal exercise completely restored plasma volume within 24 h to 3.9 /- 3.2% of pre-HDT levels despite continued HDT. Compared with control , exercise induced a 660-ml larger positive fluid balance because of g reater fluid intake and reduced urine volume during the 24 h after exe rcise. These results suggest that one bout of maximal leg exercise bef ore return from 16 days of spaceflight may be completely effective in stimulating thirst and restoring plasma volume to preflight levels.