Approximately 70% of the lean body mass of an individual is composed of water, with approximately two-thirds of the total body water (TBW) volume being held within the cells of the body (intracellular pool) and the remaining one-third (extracellular pool) is divided between the circulating blood plasma (intravascular pool) and the fluid-filled spaces between the cells (interstitial pool). The volume and distribution of the body fluids are mainly determined by the amounts of body water and sodium. In man, TBW content is regulated daily to within approximately 0.2% of lean body mass in normal, temperate conditions by factors that control input and output. The kidneys regulate water excretion in excess of the evaporative loss and the fecal and obligatory urine losses. Water intake occurs in the form of food and drink, with the sensation of thirst underpinning drinking behavior.
The mechanisms that monitor the body's hydration status also interact with the thirst control centers in the brain to regulate the desire to drink. This article is concerned with the physiological factors that govern the perception of thirst and how this is altered by drinking.
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