<P> Thirst is the craving for fluids, resulting in the basic instinct of animals to drink . It is an essential mechanism involved in fluid balance . It arises from a lack of fluids or an increase in the concentration of certain osmolites, such as salt . If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, the brain signals thirst . </P> <P> Continuous dehydration can cause many problems, but is most often associated with renal problems and neurological problems such as seizures . Excessive thirst, known as polydipsia, along with excessive urination, known as polyuria, may be an indication of diabetes mellitus or diabetes insipidus . </P> <P> There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration . They signal to the central nervous system, where central processing succeeds . Some sources, therefore, distinguish "extracellular thirst" from "intracellular thirst", where extracellular thirst is thirst generated by decreased volume and intracellular thirst is thirst generated by increased osmolite concentration . Nevertheless, the craving itself is something generated from central processing in the brain, no matter how it is detected . </P> <P> It is vital for organisms to be able to maintain their fluid levels in very narrow ranges . The goal is to keep the interstitial fluid, the fluid outside the cell, at the same concentration as the intracellular fluid, fluid inside the cell . This condition is called isotonic and occurs when the same level of solutes are present on either side of the cell membrane so that the net water movement is zero . If the interstitial fluid has a higher concentration of solutes than the intracellular fluid it will pull water out of the cell . This condition is called hypertonic and if enough water leaves the cell it will not be able to perform essential chemical functions . If the interstitial fluid becomes less concentrated the cell will fill with water as it tries to equalize the concentrations . This condition is called hypotonic and can be dangerous because it can cause the cell to swell and rupture . One set of receptors responsible for thirst detects the concentration of interstitial fluid . The other set of receptors detects blood volume . </P>

When blood volume and pressure is low which part of the brain triggers the thirst response