Plasma compartment filling after exercise or heat exposure

The present study was assessed to study the restoration of the vascular compartment by rehydration after heat exposure or exercise.

Eight subjects completed four trials in a randomized order: 2.7% dehydration of body mass by passive controlled hyperthermia once with rehydration and once without rehydration during recovery, and 2.7% dehydration of body mass by treadmill exercise once with rehydration and once without rehydration during recovery. An isotonic glucose electrolyte beverage was provided twice during the recovery period for a total volume, which was equivalent to the target value of body mass loss during dehydration procedures. Plasma volume (PV) was measured using Evans Blue dilution technique, and PV changes (deltaPV) were determined using hematocrit and hemoglobin measurements.

PV was better maintained during exercise than during heat exposure, and the difference in deltaPV between the two patterns of dehydration was maintained during the first 3 h of recovery. Plasma protein seemed to be accountable for the difference in deltaPV during heat exposure and exercise but not during the 270 min of recovery. Rehydration partly restored body fluid losses, but the plasma compartment was privileged, because 26-30% of the net fluid gain was found in the plasma compartment (about 300 mL). Rehydration restored plasma osmolality and diminished the drive for arginin-vasopressin response.

The similar selective retention of water in the plasma compartment might essentially be explained by osmotic factors provided by the beverage. As PV was completely restored by rehydration after exercise and only partly restored after heat exposure, the volume of ingested beverage should be higher after heat exposure to completely restore the plasma compartment.