Early response of plasma contents on exposure of working men to heat.

Twelve men block-stepped (35 W) 4 h/day for 12 days and were divided into two similar groups on the basis of Vo2max. All were exposed to 33.8 degrees C dry bulb, 32.7 degrees C wet bulb for 2 h (E1) while working (30% Vo2max). Venous blood was obtained at 10-min intervals during hour 1 and at 20-min intervals during hour 2. Group 1 was acclimatized to heat. Group II continued to train. The test exposure was repeated (E2). During E1 a trend toward hemodilution was evident but not significant for either group. Protein moved into the vascular volume and a decrease in plasma osmolarity was significant only after 30 min. For both groups during E2 significant hemodilution occurred during the first 10 min. Only group I remained significantly hemodiluted for 2 h. Protein movement and osmodilution again occurred in both groups. These results support earlier suggestions as to the mechanisms of hemodilution based on 1-h blood samples. Conflicting evidence as to the pressure or absence of hemodilution upon heat exposure is noted, and a hypothesis is proposed which appears to reconcile divergent results.