Regulation of Thirst

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Another appetitive mechanism under hypothalamic control is thirst. Drinking is regulated by plasma osmolality and ECF volume in much the same fashion as vasopressin secretion. Water intake is increased by increased effective osmotic pressure of the plasma , by decreases in ECF volume, and by psychologic and other factors.Osmolality acts via osmoreceptors, receptors that sense the osmolality of the body fluids.These osmoreceptors are located in the anterior hypothalamus.

Decreases in ECF volume also stimulate thirst by a pathway independent of that mediating thirst in response to increased plasma osmolality. Thus, hemorrhage causes increased drinking even if there is no change in the osmolality of the plasma.

The effect of ECF volume depletion on thirst is mediated in part via the renin–angiotensin system.Renin secretion is increased by hypovolemia and results in an increase in circulating angiotensin II. The angiotensin II acts on the subfornical organ, a specialized receptor area in the diencephalon  to stimulate the neural areas concerned with thirst.Some evidence suggests that it acts on the organum vasculosum of the lamina terminalis (OVLT) as well. These areas are highly permeable and are two of the circumventricular organs located outside the blood–brain barrier.

However, drugs that block the action of angiotensin II do not completely block the thirst response to hypovolemia, and it appears that the baroreceptors in the heart and blood vessels are also involved. The intake of liquids is increased during eating (prandial drinking). The increase has been called a learned or habit response, but it has not been investigated in detail. One factor is an increase in plasma osmolality that occurs as food is absorbed. Another may be an action of one or more gastrointestinal hormones on the hypothalamus.

When the sensation of thirst is obtunded, either by direct damage to the diencephalon or by depressed or altered states of consciousness, patients stop drinking adequate amounts of fluid. Dehydration results if appropriate measures are not instituted to maintain water balance. If the protein intake is high, the products of protein metabolism cause an osmotic diuresis , and the amounts of water required to maintain hydration are large.

Most cases of hypernatremia are actually due to simple dehydration in patients with psychoses or hypothalamic disease who do not or cannot increase their water intake when their thirst mechanism is stimulated. Lesions of the anterior communicating artery can also obtund thirst because branches of this artery supply the hypothalamic areas concerned with thirst.

Other Factors Regulating Water Intake
•       A number of other well-established factors contribute to the regulation of water intake.
•       Psychologic and social factors are important.
•       Dryness of the pharyngeal mucous membrane causes a sensation of thirst.
•       Patients in whom fluid intake must be restricted sometimes get appreciable relief of thirst by sucking ice chips or a wet cloth.