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compartment. There would then be less mobilization of water from the intracellular fluid (ICF) space due to the smaller increase of extracellular osmolality. Durkot et al. (1986) reported that in rats a higher water loss occurred from the extracellular fluid (ECF) space than from the ICF space during dehydration of 10% body wt. Water movement might be linked to the electrolyte losses from each compartment, as suggested by Nose et al. (1985). These results suggest the necessity of measuring electrolyte balance to analyze water balance between fluid compartments.

It is well known that sweat [Na+] decreases during the process of heat acclimation (Kirby and Convertino, 1986; Locke et al., 1951). However, there is no experimental evidence that shows a relation between [Na+] and water mobilization from the ICF space.

The purpose of this study was to clarify the effect of sweat [Na+] on water mobilization from the ICF compartment in conditions of thermal stress. We hypothesized that a lower sweat [Na+] would be accompanied by a smaller reduction in plasma water loss during dehydration.


Ten volunteers (nine male and one female) participated in this study. Certain of their physical characteristics are shown in Table 11-1. Each subject gave informed consent and passed a physical examination to screen for medical reasons that would prevent participation. The experimental protocol was approved by the Human Investigation Committee of the Yale University School of Medicine. Experiments were performed in the spring months.

Table 11-1 Characteristics of subjects (n = 10)


Age (years)

Wt (kg)

a (−1.min−1)

Blood Volume (ml/kg)

Plasma Volume (ml/kg)













a Maximal aerobic power.

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