Thirst Mechanism

Thirst Mechanism is the driving force for water intake. An increase in plasma osmolality of only 2-3% excites the hypothalamic thirst center. Thirst is "the physiological urge to drink water". In studies, it is recognized when subjects report the conscious sensation of a desire to drink. Under normal conditions, most water intake is due not to thirst but to social and cultural factors (eg drinking with meals or at work breaks, water in food). Thirst offers a backup to these behavioral factors and to the ADH response. Both the thirst and the ADH mechanisms are regulated in the hypothalamus. Water intake can be considered to consist of two components: a regulatory component (due to thirst) and a non-regulatory component (all other fluid intake).

The 4 major stimuli to thirst are:

1.Hypertonicity: Cellular dehydration acts via an osmoreceptor mechanism in the hypothalamus

2.Hypovolaemia: Low volume is sensed via the low-pressure baroreceptors in the great veins and right atrium

3.Hypotension: The high-pressure baroreceptors in carotid sinus & aorta provide the sensors for this input

4.Angiotensin II: This is produced consequent to the release of renin by the kidney (eg in response to renal hypotension)