Physiology of heat loss from an extremity: the tail of the rat.

1. Hooded, conscious, male, laboratory rats were subjected to an ambient dry bulb temperature cycle between 12 degrees C and 35 degrees C over a period of approximately 5.5 h. The same procedure was followed using rats in which the temperature surrounding the tail was held constant, and using dead rats. 2. Temperature of the ambient air, and the rectum, flank skin, tail base and tail tip were measured during each ambient temperature cycle. 3. In both living and dead rats, a hysteresis loop was demonstrated for the relationship between all body temperatures measured and ambient temperature. 4. The hysteresis loops for tail temperature differed between living and dead rats in such a way as to indicate that tail temperature began to increase, as a result of an active process, at approximately 30 degrees C, but this response ceased at a lower temperature. 5. The results on rats in which tail ambient temperature was held constant agreed with the preceeding observations. 6. Clear evidence of a counter-current heat exchange system in the vasculature of the rat's tail was not obtained. 7. It is concluded that the tail of the rat resembles an on/off controller in its mode of operation.