In man, daily fluid intake in the form of food and drink (plus that formed from substrate oxidation) is usually in excess of the obligatory water loss (trans-cutaneous, pulmonary, and renal output), with renal excretion being the main mechanism regulating body water content. However, conservation of water or electrolytes by the kidneys can only reduce the rate of loss; it cannot restore a deficit. The sensation of thirst, which underpins drinking behavior, indicates the need to drink and hence is critical in the control of fluid intake and water balance. While thirst appears to be a poor indicator of acute hydration status in man, the overall stability of the total water volume of an individual indicates that the desire to drink is a powerful regulatory factor over the long term.
The act of drinking may not be a direct consequence of a physiological need for water intake but can be initiated by habit, ritual, taste, or desire for nutrients, stimulants, or a warm or cooling effect. A number of the sensations associated with thirst are learned, with signals such as dryness of the mouth or throat inducing drinking, while distention of the stomach can stop ingestion before a fluid deficit has been restored. However, the underlying regulation of thirst is controlled separately by the osmotic pressure and volume of the body fluids and as such is governed by the same mechanisms that affect water and solute reabsorption in the kidneys and control central blood pressure.
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