ABSTRACT
First, some physical processes, in particular the circulation of the blood, require the maintenance of volume and pressure within a narrow range. In order for ingested water to influence the intracellular fluid, it has to be absorbed from the gastrointestinal tract into the blood stream, and from there into the interstitial fluid. There are two sensory mechanisms involved in the control of water: volumetric thirst, based on loss of water volume, and osmometric thirst, based on changes in electrolyte concentrations. Blood-pressure receptors (baro-receptors) located in the walls of the atria of the heart detect reduced pressure in the blood returning to the heart, and this also results in drinking. Loss of water alone changes electrolyte concentrations in cells, particularly in osmoreceptors located in the circumventricular organs in the center of the brain, where they can be rapidly affected by electrolytes and hormones in the blood.
