Thirst is the craving for potable 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 sodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.Continuous dehydration can cause acute and chronic diseases, but is most often associated with renal and neurological disorders. Excessive thirst, called polydipsia, along with excessive urination, known as polyuria, may be an indication of diabetes mellitus or diabetes insipidus. There are receptors and other systems in the body that detect a decreased volume or an increased osmolite concentration. Some sources 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. Detection
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, the fluid inside the cell. This condition is called isotonic and occurs when the same levels of solutes are present on either side of the cell membrane so that the net water movement is zero.
The loss of blood volume is detected by cells in the kidneys and triggers thirst for both water and salt via the renin-angiotensin system. Renin-angiotensin system
Hypovolemia leads to activation of the renin angiotensin system (RAS) and is detected by cells in the kidney. When these cells detect decreased blood flow due to the low volume they secrete an enzyme called renin. Renin then enters the blood where it catalyzes a protein called angiotensinogen to angiotensin I. Angiotensin I is then almost immediately converted by an enzyme already present in the blood to the active form of the protein, angiotensin II. Angiotensin II then travels in the blood until it reaches the posterior pituitary gland and the adrenal cortex, where it causes a cascade effect of hormones that cause the kidneys to retain water and sodium, increasing blood pressure. It is also responsible for the initiation of drinking behavior and salt appetite via the subfornical organ. Others
Arterial baroreceptors sense a decreased arterial pressure, and signal to the central nervous system in the area postrema and nucleus tractus solitarii. Cardiopulmonary receptors sense a decreased blood volume, and signal to area postrema and nucleus tractus solitarii. Cellular dehydration and osmoreceptor stimulation
Osmometric thirst occurs when the solute concentration of the interstitial fluid increases. This increase draws water out of the cells, and they shrink in volume.