Journal of Applied Physiology, Vol 42, Issue 2 252-256, Copyright © 1977 by American Physiological Society
Fluid shifts during thermal stress with and without fluid replacement
L. G. Myhre and S. Robinson
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.
