The Measurement of Human Body-Fluid Volumes: Resting Fluid Volumes Before and After Heat Acclimation

The study evaluated changes in human body-fluid volumes accompanying a 21-day heat- acclimation regime. Eight unacclimatized males, were studied for 22 consecutive days, including 19 days of heat exposure. Heat stress tests and acclimation trials were undertaken at 40 degrees C and relative humidity 60%. Total body water increased from Day 1 to 8 but a further 13 days acclimation did not elicit a further increase. This trend was reflected within the extracellular space, with the extracellular fluid volume significantly elevated after 8 days, without further expansion beyond Day 8. A plasma volume expansion of 18% was evident by Day 8, and, by Day 22, had increased to 23.6% of pre-acclimation levels. The interstitial fluid volume increase from Day 1 to Day 8 was not significant. Proportionately more of the extracellular fluid volume expansion occurred within the plasma volume and was ascribed to increases in the total plasma protein which accompanied acclimation. The intracellular volume remained stable and erythrocyte volume was similarly unaltered. Body-fluid enlargement was restricted therefore to the extracellular compartment. It was concluded that the plasma volume was expanded rapidly in response to heat acclimation, with the majority of this expansion taking place within the first 8 days. A unique observation was the tendency for the plasma volume to remain elevated from Day 8 to Day 22, indicating that further heat acclimation would maintain this expanded volume.