The regulation of respiratory evaporative heat loss in the rabbit

1. Respiratory evaporative heat loss in the rabbit has a minimum value of 0-2-0-3 W/kg and a maximum value of about 1-1 W/kg in non-evaporatively limited environments. 2. Both skin temperature and hypothalamic temperature influence respiratory evaporative heat loss, and they do so in a multiplicative fashion. Thus, at low skin temperature the hypothalamic temperature threshold for the onset of panting is above normal hypothalamic temperature and hypothalamic thermosensitivity is high. On the other hand, at high skin temperatures, the hypothalamic temperature threshold for the onset of panting is well below normal hypothalamic temperature, but hypothalamic thermosensitivity is greatly reduced. 3. The influence of mean skin temperature (Tsk) and hypothalamic temperature on respiratory evaporative heat loss (Eres) in the rabbit can be described by the equation: Eres=1-1-0-08 (Tsk-39-7) (Thy-42-9) greater than or equal to 0-3 W/kg. 4. Thus, the ability of a lowered mean skin temperature to increase the thermosensitivity of the hypothalamus in response to local temperature changes applies to heat loss mechanisms as well as heat production mechanisms. It is suggested that the characteristics of this peripheral input into the C.N.S. are fulfilled by tonic cold fibre input originating from the peripheral cold receptors on the body surface.