as the large systemic veins and the right atrium. These receptors, through the vagal system, stimulate thirst and drinking. Because of the compensatory activation of the renin-angiotensin-aldosterone system, preservation of body fluid is also achieved through a reduction in urinary output. Triggering of thirst through hypovolemia requires more than small changes in blood volume. The role of various thirst mechanisms with altered hydration status has been reviewed in detail elsewhere (Mack and Nadel, 1996; Stricker and Sved, 2000). However, in almost all situations where smaller volumes are lost over time (such as 2 to 3 L of sweat over 6 hours due to high temperatures or exercise), thirst mechanisms come into play over the ensuing 24 hours to trigger replacement of fluids lost; thus, in general, normal hydration is maintained by thirst mechanisms and normal drinking behavior. Such replacement is enhanced by consuming beverages at meals and in other social situations (Engell, 1995; Szlyk et al., 1990), which may be a necessary component to achieve adequate rehydration within a short period of time due to minor fluid deficits induced by exercise or heat strain.
Dehydration can adversely influence cognitive function and motor control. Dehydration and poor mental function have been reported to be associated in physically ill older people (Seymour et al., 1980). Table 4-9 summarizes studies that examined the effects of dehydration on cognitive performance and motor function in healthy individuals.
Interpretation of these reports is difficult because the experimental designs often do not allow discrimination of confounding factors, such as effect of thermal (or exercise) stress and that of dehydration per se (Epstein et al., 1980; Hancock, 1981; Leibowitz et al., 1972; Sharma et al., 1983). For example, a degradation in mental alertness, associative learning, visual perception, and reasoning ability were noted when healthy men exercised while exposed to a high climatic heat stress (Sharma et al., 1983). Although the subjects drank water ad libitum, they may not have consumed enough fluids over the 4-hour session and thus became dehydrated due to the exercise and heat stress. However, the possible effect of dehydration on the above mental functions was not addressed. In another study, men and women exercised in the heat for 6 hours to elicit dehydration levels of 2.5 and 5 percent (Leibowitz et al., 1972).