Movement of water, protein and crystalloids between vascular and extra-vascular compartments in heat-exposed men during dehydration and following limited relief of dehydration.

1. Five male subjects were exposed to a hot environment (43-44 degrees C dry bulb, 28-29 degrees C wet bulb) and allowed to dehydrate for 4 hr. Water was then provided ad libitum for drinking during a 20-min period; thereafter the subjects continued at rest in the hot room for an additional 140 min.2. Through periodic samples of venous blood, water and protein movement into or out of the intravascular compartment were assessed during dehydration and during and following water ingestion that partially relieved dehydration.3. Consideration of changes in blood osmolarity, plasma protein content and haematocrit values led to the conclusion that protein was added to plasma during heat exposure and that plasma, as previously supposed, did not lose water at rates greater than that of the whole body under such conditions.4. Haemodilution occurred when dehydration was partially relieved by drinking water and this dilution was maintained even when sweat loss exceeded the amount of water ingested.5. For similar levels of bodily dehydration before and after water ingestion, body temperatures were lower when haemodilution was present.6. A hypothesis that relates changes in plasma volume and constituents with levels of exercise is presented.