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Craving for potable fluids experienced by animals

Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids or an increase in the concentration of certain osmolites, such as sodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst. Continuous dehydration can cause acute and chronic diseases, but is most often associated with renal and neurological disorders.

Hypovolemia leads to activation of the renin angiotensin system (RAS) and is detected by cells in the kidney. When these cells detect decreased blood flow due to the low volume they secrete an enzyme called renin. Renin then enters the blood where it catalyzes a protein called angiotensinogen to angiotensin I. Angiotensin I is then almost immediately converted by an enzyme already present in the blood to the active form of the protein, angiotensin II. Angiotensin II then travels in the blood until it reaches the posterior pituitary gland and the adrenal cortex, where it causes a cascade effect of hormones that cause the kidneys to retain water and sodium, increasing blood pressure. In addition, there are visceral osmoreceptors which project to the area postrema and nucleus tractus solitarii in the brain. Several studies have demonstrated that elderly persons have lower total water intakes than younger adults, and that women are particularly at risk of too low an intake. In 2009, the European Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time. Recommended intake volumes in the elderly are the same as for younger adults (2.0 L/day for females and 2.5 L/day for males) as despite lower energy consumption, the water requirement of this group is increased due to a reduction in renal concentrating capacity.

According to preliminary research, quenching of thirst – the homeostatic mechanism to stop drinking – occurs via two neural phases: a "preabsorptive" phase which signals quenched thirst many minutes before fluid is absorbed from the stomach and distributed to the body via the circulation, and a "postabsorptive" phase which is regulated by brain structures sensing to terminate fluid ingestion. The preabsorptive phase relies on sensory inputs in the mouth, pharynx, esophagus, and upper gastrointestinal tract to anticipate the amount of fluid needed, providing rapid signals to the brain to terminate drinking when the assessed amount has been consumed. The postabsorptive phase occurs via blood monitoring for osmolality, fluid volume, and sodium balance, which are collectively sensed in brain circumventricular organs linked via neural networks to terminate thirst when fluid balance is established. Thirst quenching varies among animal species, with dogs, camels, sheep, goats, and deer replacing fluid deficits quickly when water is available, whereas humans and horses may need hours to restore fluid balance.

The areas of the brain that contribute to the sense of thirst are mainly located in the midbrain and the hindbrain. Specifically, the hypothalamus appears to play a key role in the regulation of thirst. The area postrema and nucleus tractus solitarii signal to the subfornical organ and to the lateral parabrachial nucleus. The latter signaling relies on the neurotransmitter serotonin. The signal from the lateral parabrachial nucleus is relayed to the median preoptic nucleus. The median preoptic nucleus and the subfornical organ receive signals of decreased volume and increased osmolite concentration. Finally, the signals are received in cortex areas of the forebrain where thirst arises. The subfornical organ and the organum vasculosum of the lamina terminalis contribute to regulating the overall bodily fluid balance by signalling to the hypothalamus to form vasopressin, which is later released by the pituitary gland.