Fluid shifts during thermal stress with and without fluid replacement.

Six unacclimatized men rested for 4 h in a hot, dry environment (Ta 50 degrees C, Tw 26 degrees C; wind speed 0.5 m/s) without fluid replacement (DH). Another group of six men were exposed to the same thermal stress, replacing evaporative fluid loss with warm 0.1% NaCl solution (FRP). Total grams of circulating hemoglobin, determined by CO immediately prior to and again during the last minutes of heat exposure, increased an insignificant 1.6 and 1.3% during DH and FRP, respectively. With DH, body weight loss of 2.6% was accompanied by a 7.8% reduction in calculated plasma volume (PV). Even when body weight was maintained (FRP), PV decreased 2.9% during the heat exposure. Total circulating serum protein did not change as a result of the heat stress with either DH or FRP. In a test-retest series of experiments on four men, DH was not detrimental to sweat rate which averaged 251 g/m2-h compared with 239 g/m2-h with FRP. It was concluded that hemodilution is not a general response to acute heat exposure, and the disproportionately large reduction in PV during thermal dehydration was confirmed.