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FLUID REPLACEMENT AND HEAT STRESS
plasma sodium and osmolality, subjective ratings of thirst, and water intake. The main sensations associated with the infusions were a dry and unpleasant-tasting mouth. The increase in plasma sodium (4.2 meq/liter) associated with thirst and drinking was well within the physiological range and was similar to the changes seen after exercise, thermal dehydration, and water deprivation.
The evidence for a role for hypovolemia in dehydration-induced drinking is more equivocal. Fluid deprivation decreases plasma volume, and such volume changes would be detected by receptors in and around the heart and kidneys. When the kidneys detect hypovolemia, renin is released, and it acts on a substrate in the plasma to increase the formation of angiotensin. Angiotensin II has been found to be a potent thirst stimulus in most species (Rolls and Rolls, 1982), but we have found that it stimulates drinking in humans only at supraphysiological levels (Phillips et al., 1985a). More experiments are needed to define the situations in which hypovolemia is a thirst stimulus in humans and to determine its mechanism of action.
SENSATIONS ASSOCIATED WITH DEHYDRATION
The idea that thirst is associated with unpleasant oral sensations dates back to the ancient Greeks. However, only recently have there been systematic attempts to characterize these sensations. Visual analog scales, which are 100 mm lines on which subjects indicate how they currently feel in relation to a very specific question (e.g., how dry is your mouth now?) have been critical in this assessment. We found, for example, that after 24 h without fluids, healthy young men showed a marked increase in ratings of thirst, how pleasant it would be to drink water, dryness of the mouth, and unpleasantness of the taste in the mouth. The changes in the sensations were significantly correlated with subsequent water intake (Rolls et al., 1980). Recently, there has been a detailed examination of the sensations accompanying graded levels of dehydration induced by restricting food and fluid intake and imposing a regimen of moderate heat-exercise stress. The number and intensity of unpleasant sensations reported increased with the level of dehydration. Sensations that showed a significant linear trend in intensity were a dry and irritated mouth; a bad and chalklike taste in the mouth; a dry, scratchy, and warm throat; chapped lips; feeling weary, dizzy, lightheaded, sleepy, tired, irritable, and thirsty; having a headache and loss of appetite; and thinking of drinking (Engell et al., 1987). It is not clear from this study whether the more general sensations are associated with a lack of caffeine that would result from fluid restriction (i.e., no coffee or tea). This