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Brain cooling in endotherms in heat and exercise.

The upper limit of tolerable body temperatures in most land vertebrates is 40° -45°C. Although excessive temperatures have effects on virtually every organ, brain function appears to be especially vulnerable to heat (16, 17, 61). If the brain is kept cool, tolerance to elevated deep body temperature is extended (24). Many species of mammals, birds, and reptiles can keep the temperature of the brain below the temperature of the rest of the body core during heat stress. In most cases, this brain cooling is accomplished by vascular arrangements that allow venous blood cooled by evaporation to exchange heat with arterial blood supplying the brain or to cool the base of the brain conductively. The process has been studied most intensively in panting carnivores and artiodactyls in which the structure of the nasal cavity and the arrangement of the blood vessels of the nasal cavity and the brain are well-suited for brain cooling. In these animals, the extent to which the brain is cooled depends upon the rate of evaporation and the rate of blood flow through the evaporative surfaces and reaches highest levels during exercise.