Evaporative cooling in the rat: Effects of dehydration

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Abstract
Previous investigations demonstrated that the water loss of rats associated with increased salivary evaporation during heat stress is derived from both intracellular and intravascular sources. The present studies indicate that sufficient dehydration of either fluid compartment will impair temperature regulation. Salivary excretion from all dehydrated rats was virtually abolished at ambient temperatures below 38–40 °C, but temperature regulation was still possible if a large temperature gradient existed between the animals and the environment. Above these ambient temperatures, where increased evaporation is essential to survival, the rate of evaporative water loss returned to normal. However, body water reservoirs in dehydrated rats were rapidly depleted, salivary evaporation could not be maintained, and survival times were shortened. In contrast, access to drinking water significantly increased thermal tolerance. These results emphasize the importance of adequate body fluid hydration for evaporative cooling through saliva spreading by rats in the heat. In addition, they indicate that allocation of body water for evaporation takes precedence over conflicting demands for water conservation during heat stress.

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