Thirst is the craving for potable fluids, resulting in the basicinstinct of animals todrink. It is an essential mechanism involved influid balance.[1] It arises from a lack of fluids or an increase in the concentration of certainosmolites, such assodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in thebrain detect changes in blood constituents and signal thirst.[2]
There arereceptors and other systems in the body that detect a decreased volume or an increased osmolite concentration.[1][2] 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.[2]
It is vital for organisms to be able to maintain their fluid levels within 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 calledisotonic and occurs when the same levels of solutes are present on either side of thecell membrane so that the net water movement is zero. If the interstitial fluid has a higher concentration of solutes (or a lower concentration of water) than the intracellular fluid, it will pull water out of the cell. This condition is calledhypertonic and if enough water leaves the cell, it will not be able to perform essential chemical functions. The animal will then become thirsty in response to the demand for water in the cell. After the animal drinks water, the interstitial fluid becomes less concentrated of solutes (more concentrated of water) than the intracellular fluid and 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 detectsblood volume.[2]
This is one of two types of thirst and is defined as thirst caused by loss of blood volume (hypovolemia) without depleting the intracellular fluid. This can be caused by blood loss, vomiting, and diarrhea. This loss of volume is problematic because if the total blood volume falls too low the heart cannot circulate blood effectively and the eventual result is hypovolemic shock. 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.[2][3]
Hypovolemia leads to activation of therenin 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 calledrenin. Renin then enters the blood where it catalyzes a protein calledangiotensinogen toangiotensin 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 theposterior pituitary gland and theadrenal cortex, where it causes a cascade effect ofhormones that cause the kidneys to retain water and sodium, increasing blood pressure.[3] It is also responsible for the initiation ofdrinking behavior and salt appetite via thesubfornical organ.[2]
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. The solute concentration of the interstitial fluid increases by high intake of sodium in diet or by the drop in volume of extracellular fluids (such asblood plasma andcerebrospinal fluid) due to loss of water through perspiration, respiration, urination and defecation. The increase in interstitial fluid solute concentration causes water to migrate from the cells of the body, through their membranes, to the extracellular compartment, byosmosis, thus causing cellular dehydration.[1]
Clusters of cells (osmoreceptors) in theorganum vasculosum of the lamina terminalis (OVLT) andsubfornical organ (SFO), which lie outside of the blood brain barrier can detect the concentration of blood plasma and the presence of angiotensin II in the blood. They can then activate themedian preoptic nucleus which initiates water seeking and ingestive behavior.[3] 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.[4][5] In addition, there arevisceralosmoreceptors which project to thearea postrema andnucleus tractus solitarii in the brain.[2]
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.[3] This is also a result of the renin-angiotensin system activation.[medical citation needed]
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 ofdehydration.[6] 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 an intake.[7][8][9]In 2009, theEuropean Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time.[10] 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.[10][11]
According to preliminary research, quenching of thirst – thehomeostatic mechanism to stop drinking – occurs via two neural phases: a "preabsorptive" phase which signals quenched thirst many minutes before fluid is absorbed from the stomach and distributed to the body via the circulation, and a "postabsorptive" phase which is regulated by brain structures sensing to terminate fluid ingestion.[12] The preabsorptive phase relies on sensory inputs in the mouth,pharynx,esophagus, and uppergastrointestinal tract to anticipate the amount of fluid needed, providing rapid signals to the brain to terminate drinking when the assessed amount has been consumed.[12] The postabsorptive phase occurs via blood monitoring forosmolality, fluid volume, and sodium balance, which are collectively sensed in braincircumventricular organs linked via neural networks to terminate thirst when fluid balance is established.[12]
Thirst quenching varies among animal species, with dogs, camels, sheep, goats, and deer replacing fluid deficits quickly when water is available, whereas humans and horses may need hours to restore fluid balance.[12]
The areas of the brain that contribute to the sense of thirst are mainly located in themidbrain and thehindbrain. Specifically, thehypothalamus appears to play a key role in the regulation of thirst.
^Walter F. Boron (2005).Medical Physiology: A Cellular And Molecular Approach. Elsevier/Saunders.ISBN978-1-4160-2328-9. 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; Mathey, MF (2001). "Food and fluid intake of the SENECA population residing in Romans, France".The Journal of Nutrition, Health and Aging.5 (4):235–7.PMID11753484.
^Haveman-Nies, A; de Groot, LC; Van Staveren, WA (1997). "Fluid intake of elderly Europeans".The Journal of Nutrition, Health and Aging.1 (3):151–5.PMID10995083.
^Volkert, D; Kreuel, K; Stehle, P (2005). "Fluid intake of community-living, independent elderly in Germany - a nationwide, representative study".The Journal of Nutrition, Health and Aging.9 (5):305–9.PMID16222395.
^IoM (Institute of Medicine), 2004. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, Washington, D.C.
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