When fluid intakes are insufficient to maintain normal body water content (approximately 60% for males and 50-55% for females), deficits arise in all fluid compartments, with the reduction in plasma volume being of particular concern. Dehydration decreases plasma volume and increases tonicity. Plasma hypertonicity signals the circulatory system to conserve plasma volume for internal organs at the expense of skin blood flow. Reduction in skin blood flow decreases evaporative cooling. Additionally, decreased plasma volume reduces stroke volume and cardiac output, which impairs cooling capacity and exercise performance. The effects of dehydration on heart rate, body temperature, and endurance are shown in Figure 4. Consuming water to replace sweat loss while cycling for 6h at 55% VO2 max in the heat was associated with lower heart rates and core temperatures compared to a trial in which no water was ingested. The increase in heart rate while cycling without water replacement is a compensatory mechanism to maintain cardiac output in response to reduced plasma volume. Elevated core temperatures in cyclists not consuming water
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Figure 4 Consequences of consuming or not consuming water during cycle ergometer work (50% VO2 max, 30 °C, 50% relative humidity). Subjects drank water to replace that lost during exercise (water group) or did not drink during exercise (no water group). Note that the no water group could not exercise as long as the water group. (Adapted from Barr SI, Costill DL and Fink WJ (1991) Fluid replacement during prolonged exercise: Effects of water, saline, or no fluid. Medicine and Science in Sports and Exercise 23: 811-817.)
resulted from reduced skin blood flow and ultimately forced the cessation of exercise.
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