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.
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.
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 intercellular 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 intercellular 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.
Decreased volume 
This is one of two types of thirst and is defined as low volume of blood plasma. This is most likely the result of extensive bleeding, but can also result from vomiting, diarrhea, or evaporation from the skin. This loss of volume is problematic because if the the total blood volume falls too low the heart cannot circulate blood effectively and the eventual result is heart failure. The vascular system responds by constricting blood vessels thereby creating a smaller volume for the blood to fill. This mechanical solution however has definite limits and usually must be supplemented with increased volume. 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.
- Further reading: Hypovolemia
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 a 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. Angitensis 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 which increase blood pressure. It is also responsible for the initiation of drinking behavior and salt appetite via the subfornical organ. 
- Arterial baroreceptors sense a decreased arterial pressure, and signals to the central nervous system in the area postrema and nucleus tractus solitarius.
- Cardiopulmonary receptors sense a decreased blood volume, and signal to area postrema and nucleus tractus solitarius as well.
Cellular dehydration and osmoreceptor stimulation 
Osmosis is the movement of liquid from one side of a semipermeable membrane with lower concentrations of dissolved substances, to the other side of the membrane with higher concentrations, resulting in dilution of the higher concentration solution. In living cells the transport of water from low concentration to high concentration solutions through the cell membrane tends to continue until equilibrium is achieved. The pressure needed to halt the movement through the membrane is the "osmotic pressure."
As the volume of extracellular fluids such as blood plasma and cerebrospinal fluid drops due to loss of water through perspiration, respiration, urination and defecation, the concentration of sodium (the major substance affecting osmosis in the extracellular fluid) and hence the osmotic pressure, rises; water migrates from the cells of the body, through their membranes, to the extracellular compartment, and cellular dehydration occurs. (Infusion of hypertonic saline solution into the blood stream has the same cellular dehydration effect.)
Clusters of cells (osmoreceptors) in the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), which lie outside of the blood brain barrier can detect the concentration of blood plasma and the presence of angeotenis II in the blood. They can then activate the median preoptic nucleus which initiates water seeking and ingestive behavior. Destruction of this part of the hypothalamus in humans and other animals results in partial or total loss of desire to drink even with extremely high salt concentration in the extracellular fluids.
Salt craving 
Because sodium is also lost from the plasma in hypovolemia, the body's need for salt proportionately increases in addition to thirst in such cases. This is also a result of the renin-angiotensin system activation.
In adults over the age of 50 years, the body’s thirst sensation reduces and continues diminishing with age, putting this population at increased risk of dehydration. Several studies have demonstrated that elderly persons have lower total water intakes than younger adults, and that women are particularly at risk of too low intake. In 2009, the European Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time. Recommended intake volumes in the elderly are the same as for younger adults (2.0 L/day for females and 2.5 L/day for males) as despite lower energy consumption, the water requirement of this group is increased due to a reduction in renal concentrating capacity.
Central processing 
The area postrema and nucleus tractus solitarius signal, by 5-HT, to lateral parabrachial nucleus, which in turn signal to median preoptic nucleus. In addition, the area postrema and nucleus tractus solitarius also signal directly to subfornical organ.
Thus, the median preoptic nucleus and subfornical organ receive signals of both decreased volume and increased osmolite concentration. They signal to higher integrative centers, where ultimately the conscious craving arises. However, the true neuroscience of this conscious craving is not fully clear. In general, the end-result is towards behavior of drinking for hydration, but this can to some degree be resisted, such as in voluntary fluid restriction.
See also 
|Look up thirst in Wiktionary, the free dictionary.|
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- Carlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.
- Carlson, Neil R. (2013). Physiology of Behavior. New Jersey: Pearson. p. 397-400. ISBN 978-0-205-23981-8.
- M.J. McKinley and A.K. Johnson (2004). "The Physiological Regulation of Thirst and Fluid Intake". News in Physiological Sciences 19 (1): 1–6. doi:10.1152/nips.01470.2003. PMID 14739394. Retrieved 2006-06-02.
- Derek A. Denton (8 June 2006). The primordial emotions: the dawning of consciousness. Oxford University Press. pp. 118–19. ISBN 978-0-19-920314-7.
- Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approach. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 872
- Fish LC, Minaker, KL, Rowe JW. Altered thirst threshold during hypertonic stress in aging man. Gerontologist 1985;25:A1189.
- Ferry M, Hininger-Favier I, Sidobre B and Mathey MF. Food and fluid intake of the SENECA population residing in Romans, France. J. Nutr. Health Aging. 2001;5:235-7.
- Haveman-Nies A, de Groot LC and Van Staveren WA. Fluid intake of elderly Europeans. J. Nutr. Health Aging. 1997;1:151-5.
- Volkert D, Kreuel K, Stehle P. Fluid intake of community-living, independent elderly in Germany - a nationwide, representative study. J Nutr Health Aging. 2005;9:305-9.
- EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA). EFSA Journal 2010;8(3):1459.
- IoM (Institute of Medicine), 2004. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, Washington DC.