Blood volume and plasma protein responses to heat acclimatization in humans

The effects of heat acclimatization on intravascular volume and protein responses to acute heat stress and exercise were studied in six male subjects. Absolute values for hematocrit and hemoglobin concentration were lower after, than before, acclimatization, indicating hemodilution. Also, after acclimatization, the magnitude of the hemoconcentration response to exercise in the heat was significantly increased. There ws no change in the concentration of plasma protein during or after acclimatization compared with before acclimatization, but there was a net increase in the total intravascular protein content. It is suggested that the hemodilution associated with heat acclimatization may be explained in terms of an increase in the intravascular oncotic pressure following an exercise-induced augmentation of protein, occurring at the expense of the interstitial compartment. It is concluded that this hemodilution is unlikely to be primarily responsible for the cardiovascular adjustment accompanying heat acclimatization and that it should be regarded as a secondary feature of adaptation to heat.