Changes in Fluid and Electrolyte Balance Also Occur With Heat Acclimatization

During the first week, total body water and, especially, plasma volume increase. These changes likely contribute to the cardiovascular adaptations. Later, the fluid changes seem to diminish or disappear, although the cardiovascular adaptations persist. In an unacclimatized person, sweating occurs mostly on the chest and back, but during acclimatization, especially in humid heat, the fraction of sweat secreted on the limbs increases to make better use of the skin surface for evaporation. An unacclimatized person who is sweating profusely can lose large amounts of sodium. With acclimatization, the sweat glands become able to conserve sodium by secreting sweat with a sodium concentration as low as 5 mmol/L. This effect is mediated through aldos-terone, which is secreted in response to sodium depletion and to exercise and heat exposure. The sweat glands respond to aldosterone more slowly than the kidneys, requiring several days; unlike the kidneys, the sweat glands do not escape the influence of aldosterone when sodium balance has been restored, but continue to conserve sodium for as long as acclimatization persists.

The cell membranes are freely permeable to water, so that any osmotic imbalance between the intracellular and extracellular compartments is rapidly corrected by the movement of water across the cell membranes (see Chapter

2). One important consequence of the salt-conserving response of the sweat glands is that the loss of a given volume of sweat causes a smaller decrease in the volume of the extracellular space than if the sodium concentration of the sweat is high (Table 29.3). Other consequences are discussed in Clinical Focus Box 29.1.

Heat acclimatization is transient, disappearing in a few weeks if not maintained by repeated heat exposure. The components of heat acclimatization are lost in the order in which they were acquired, the cardiovascular changes decay more quickly than the reduction in exercise core temperature and sweating changes.

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