Physiological thirst is the sensation that produces a conscious drive to replace depleted body fluids. The sensation is most often described as a dryness of the mouth and throat, accompanied by a craving for liquid. Thirst is a critical component in the regulation of body fluids and electrolyte concentrations (e.g., sodium).

Body fluid balance, or hydration, is dependent on the difference between water intake and output each day. A great deal of water is lost each day via evaporation from the skin, expulsion and evaporation in respiratory air, and excretion by the kidneys. A balancing water intake--in whatever drink form it is delivered--is needed to prevent dehydration.

The thirst regulation center of the brain is located in a localized area of the hypothalamus. Physiological experimentation in animals establishes that electrical stimulation of this area produces drinking behavior associated with thirst. Neural cells in the thirst center are...