Interaction between thermoregulation and osmoregulation in domestic mammals

The ability to maintain core temperature and volume and composition of body fluids within narrow ranges is a major characteristic of mammals. Yet, the ability to maintain a stable core temperature often relies on physiological responses that perturb the stability of body fluids. A common thermoregulatory mechanism that affects body fluid homeostasis is evaporative cooling, by sweating and/or panting, to dissipate heat from the body when core temperature is elevated. However, these responses result in a reduction of total body water thereby reducing blood volume and increasing the osmotic pressure of body fluids. While both panting and sweating are highly effective means of preventing core body temperature from increasing, unless the resultant body fluid losses are replaced (by intake of water), hypertonicity, hypovolemia and circulatory collapse can ensue. Thus, physiological control mechanisms have evolved to limit thermoregulatory body fluid losses once they have become a liability, and panting and sweating are inhibited. Thus, mammals will tolerate a higher core temperature to minimize further loss of body water. Osmoreceptors located within the lamina terminalis of the brain suppress panting and sweating when the effective osmotic pressure (tonicity) increases. Selective brain cooling (SBC) has been observed in several domesticated mammals when blood flowing to the brain in the carotid rete is cooled. Such SBC promotes reduced panting and sweating thereby preserving body water. It is also notable that the behavioural response of drinking water can rapidly invoke panting and sweating that override osmoregulatory inhibitory influences on these responses. The preoptic region of the brain has an important role in these osmoregulatory and thermoregulatory mechanisms.