Osmotic and hormonal regulation of thirst in domestic animals

Abstract

The volume and osmolality of both the intracellular and extraceilular fluids of mammals are normally maintained within a quite narrow range. For instance, if an animal becomes dehydrated, several compensatory physiological mechanisms come into operation. Vasopressin, the antidiuretie hormone, is secreted from the posterior pituitary gland and renal water losses diminish, because a more concentrated urine is formed (1, 2). In addition, the response to dehydration includes enhanced renal excretion of solutes, particularly sodium chloride (3). While these renal mechanisms can partially correct the hypertonicity and minimise further losses of body water, only intake of water can fully restore the volume and composition of body fluids. Thus, an urge or desire to drink water, termed thirst, is an important component of a series of co-ordinated responses which have the resultant homeostatic effect of maintaining body fluid volume and osmolality. Thirst is a subjective human feeling or sensation, and there can be no absolute certainty that animals experience thirst analogous to that of people. It can only be assumed that the strong urge to imbibe water, which is obvious in many species when they become depleted of water, represents the presence of a type of thirst in these animals. For the purpose of this article, the assumption will be made that when animals drink water in response to a deficit of either the intracellular or extracelular fluid or both, they experience a drive to drink water, termed thirst. Studies of water drinking in domestic animals such as dogs, goats, sheep, cattle and pigs have provided much of our knowledge of mechanisms of thirst. This article reviews the osmotic and hormonal factors which influence the thirst associated with deficits in the intracellular or extracellular fluid volume of these animals.