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FLUID REPLACEMENT AND HEAT STRESS
been biased by the presence of glucose in their rehydration solution because taste of glucose-electrolyte solution may have influenced drinking behavior. More recently, Nose et al. (1985, 1986) demonstrated that the degree of involuntary dehydration was reduced in rats supplied with water containing 0.45 or 0.9% NaCl to compensate for the loss of electrolytes during thermal dehydration phenomenon.
There has been other evidence demonstrating the importance of the plasma volume change in involuntary dehydration. Nose et al. (1986) reported that in rats 17-20% of the ingested water remained in the vascular space, which is twice as much as expected, assuming that ingested fluid is distributed proportionally among the body compartments. These results also suggested to us that the high retention of ingested fluid in the vascular space might diminish volume-dependent dipsogenic stimulation despite the incomplete restoration of the total water deficit.
The purpose of this study was to assess the involuntary dehydration phenomenon in humans. We wished to examine the distribution and fate of the water ingested during rehydration to determine the mechanisms that contribute to the high retention of ingested fluids in the vascular space. Our hypothesis was that the disproportionately high recovery of plasma volume, with respect to total body water, contributes to the removal of the dipsogenic drive. Furthermore, removal of the osmotic stimulus accompanying plasma volume dilution limits the rate of body fluid restitution.
Six male volunteers were studied. Their physical characteristics are shown in Table 12-1. With a few exceptions, to be described below, the procedures and analytic techniques were the same as in the preceding chapter and as in Nose and colleagues (1988a). We induced a dehydration of 2.3% body weight by exposing subjects for 90-110 min to simultaneous heat [36°C, <30% relative humidity (rh)] and exercise (40% maximal aerobic power) stress in the seated position.