View source for Thirst

{{Short description|Craving for potable fluids experienced by animals}}
{{Other uses}}
{{Redirect|Thirsty}}
[[File:William-Adolphe Bouguereau (1825-1905) - Thirst (1886).jpg|thumb|''Thirst'' (1886), by [[William-Adolphe Bouguereau]]]]
'''Thirst''' is the craving for potable fluids, resulting in the basic [[instinct]] of animals to [[drinking|drink]]. It is an essential mechanism involved in [[fluid balance]].<ref name="stan">{{cite journal | last1=Stanhewicz | first1=Anna E. | last2=Larry Kenney | first2=W. | title=Determinants of water and sodium intake and output | journal=Nutrition Reviews | volume=73 | issue=suppl 2 | date=2015-08-19 | issn=0029-6643 | doi=10.1093/nutrit/nuv033 | pmid=26290293 | pages=73–82|url=https://academic.oup.com/nutritionreviews/article/73/suppl_2/73/1930571| doi-access=free | url-access=subscription }}</ref> It arises from a lack of fluids or an increase in the concentration of certain [[osmolite]]s, 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.<ref name="mckinley">{{cite journal | last1=McKinley | first1=Michael J. | last2=Denton | first2=Derek A. | last3=Ryan | first3=Philip J. | last4=Yao | first4=Song T. | last5=Stefanidis | first5=Aneta | last6=Oldfield | first6=Brian J. | title=From sensory circumventricular organs to cerebral cortex: Neural pathways controlling thirst and hunger | journal=Journal of Neuroendocrinology | date=2019-03-14 | volume=31 | issue=3 | issn=0953-8194 | doi=10.1111/jne.12689 | article-number=e12689| pmid=30672620 |url=https://academic.oup.com/nutritionreviews/article/73/suppl_2/73/1930571| hdl=11343/285537 | s2cid=58947441 | hdl-access=free | url-access=subscription }}</ref>

Continuous [[dehydration]] can cause acute and chronic diseases, but is most often associated with [[kidney|renal]] and neurological disorders.<ref name=stan/> Excessive thirst, called [[polydipsia]], along with excessive urination, known as [[polyuria]], may be an indication of [[diabetes mellitus]] or [[diabetes insipidus]].

There are [[Sensory receptor|receptors]] and other systems in the body that detect a decreased volume or an increased osmolite concentration.<ref name=stan/><ref name=mckinley/> 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.<ref name=mckinley/>

==Detection==
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 called [[Tonicity#Isotonicity|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. 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 called [[Tonicity#Hypertonic solution|hypertonic]] 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 detects [[blood volume]].<ref name=mckinley/>

===Decreased volume===
{{Further|Hypovolemia}}
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.<ref name=mckinley/><ref name="Carlson">{{cite book|last=Carlson|first=Neil R.|title=Physiology of Behavior|url=https://archive.org/details/physiologybehavi00carl_811|url-access=limited|year=2013|publisher=Pearson|location=New Jersey|isbn=978-0-205-23981-8|pages=[https://archive.org/details/physiologybehavi00carl_811/page/n417 397]–400}}</ref>

====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.<ref name=Carlson /> It is also responsible for the initiation of [[drinking]] behavior and salt appetite via the [[subfornical organ]].<ref name=mckinley/>

[[File:Renin-angiotensin-aldosterone system.svg|thumb|475px|[[Renin–angiotensin system|Renin-angiotensin-aldosterone system]]]]

====Others====
* Arterial baroreceptors sense a decreased arterial pressure, and signal to the [[central nervous system]] in the [[area postrema]] and [[nucleus tractus solitarii]].<ref name=mckinley/>
* [[Cardiopulmonary receptor]]s sense a decreased blood volume, and signal to [[area postrema]] and [[nucleus tractus solitarii]].<ref name=mckinley/>

===Cellular dehydration and osmoreceptor stimulation===
{{Main|Osmoreceptor}}
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 as [[blood plasma]] and [[cerebrospinal 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, by [[osmosis]], thus causing cellular dehydration.<ref name=stan/>

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 angiotensin II in the blood. They can then activate the [[median preoptic nucleus]] which initiates water seeking and ingestive behavior.<ref name=Carlson /> 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.<ref name="Denton2006p7">{{cite book |author=Derek A. Denton |title=The primordial emotions: the dawning of consciousness |date=8 June 2006 |pages=118–19|publisher=Oxford University Press |isbn=978-0-19-920314-7|title-link=The Primordial Emotions }}</ref><ref name=boron872>{{cite book |author=Walter F. Boron |title=Medical Physiology: A Cellular And Molecular Approach |publisher=Elsevier/Saunders |year= 2005|isbn=978-1-4160-2328-9 }} Page 872</ref> In addition, there are [[viscera]]l [[osmoreceptor]]s which project to the [[area postrema]] and [[nucleus tractus solitarii]] in the brain.<ref name=mckinley/>

===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.<ref name="Carlson"/> This is also a result of the renin-angiotensin system activation.{{medical citation needed|date=February 2016}}

===Elderly===
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]].<ref>Fish LC, Minaker, KL, Rowe JW. "Altered thirst threshold during hypertonic stress in aging man". ''Gerontologist'' 1985; 25:A1189.</ref> 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.<ref>{{cite journal | last1 = Ferry | first1 = M | last2 = Hininger-Favier | first2 = I | last3 = Sidobre | first3 = B | last4 = Mathey | first4 = MF | year = 2001 | title = Food and fluid intake of the SENECA population residing in Romans, France | journal = The Journal of Nutrition, Health and Aging | volume = 5 | issue = 4| pages = 235–7 | pmid = 11753484 }}</ref><ref>{{cite journal | last1 = Haveman-Nies | first1 = A | last2 = de Groot | first2 = LC | last3 = Van Staveren | first3 = WA | year = 1997 | title = Fluid intake of elderly Europeans | journal = The Journal of Nutrition, Health and Aging | volume = 1 | issue = 3| pages = 151–5 | pmid = 10995083 }}</ref><ref>{{cite journal | last1 = Volkert | first1 = D | last2 = Kreuel | first2 = K | last3 = Stehle | first3 = P | year = 2005 | title = Fluid intake of community-living, independent elderly in Germany - a nationwide, representative study | journal = The Journal of Nutrition, Health and Aging | volume = 9 | issue = 5| pages = 305–9 | pmid = 16222395 }}</ref>
In 2009, the [[European Food Safety Authority]] (EFSA) included water as a macronutrient in its dietary reference values for the first time.<ref name="EFSA">{{cite journal |author=((EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA) )) |title=Scientific Opinion on Dietary Reference Values for water |journal=EFSA Journal |issn=1831-4732 |volume=8 |issue=3 |page=1459 |doi=10.2903/j.efsa.2010.1459 |doi-access=free |date=25 March 2010 }}</ref> 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.<ref name=EFSA/><ref>IoM (Institute of Medicine), 2004. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, Washington, D.C.</ref>

==Thirst quenching==

According to preliminary research, quenching of thirst &ndash; the [[homeostasis|homeostatic]] mechanism to stop drinking &ndash; 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.<ref name="ryan">{{cite journal|pmc=6014472|year=2018|last1=Ryan|first1=P. J|title=The neurocircuitry of fluid satiation|journal=Physiological Reports|volume=6|issue=12|article-number=e13744|pmid=29932494|doi=10.14814/phy2.13744}}</ref> The preabsorptive phase relies on sensory inputs in the mouth, [[pharynx]], [[esophagus]], and upper [[gastrointestinal tract]] to anticipate the amount of fluid needed, providing rapid signals to the brain to terminate drinking when the assessed amount has been consumed.<ref name=ryan/> The postabsorptive phase occurs via blood monitoring for [[osmolality]], fluid volume, and sodium balance, which are collectively sensed in brain [[circumventricular organs]] linked via neural networks to terminate thirst when fluid balance is established.<ref name=ryan/>

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.<ref name=ryan/>

==Neurophysiology==

The areas of the brain that contribute to the sense of thirst are mainly located in the [[midbrain]] and the [[hindbrain]]. Specifically, the [[hypothalamus]] appears to play a key role in the regulation of thirst.

The [[area postrema]] and [[nucleus tractus solitarii]] signal to the subfornical organ and to the [[lateral parabrachial nucleus]].<ref name=mckinley/> The latter signaling relies on the neurotransmitter [[serotonin]]. The signal from the lateral parabrachial nucleus is relayed to the [[median preoptic nucleus]].<ref name=mckinley/>

The median preoptic nucleus and the subfornical organ receive signals of decreased volume {{clarify|date=September 2016}} and increased osmolite concentration. Finally, the signals are received in [[Cerebral cortex|cortex]] areas of the [[forebrain]]<ref name=mckinley/> where thirst arises. The [[subfornical organ]] and the [[organum vasculosum of the lamina terminalis]] contribute to regulating the overall bodily [[fluid balance]] by signalling to the hypothalamus to form [[vasopressin]], which is later released by the [[pituitary gland]].<ref name=mckinley/>

==See also==
{{columns-list|colwidth=30em|
*[[Drought]]
*[[Hunger (motivational state)]]
*[[World Water Day]]
*[[Adipsia]]
}}
{{Portal|Food}}

==References==
{{Reflist}}

==Further reading==
*{{Cite web|title = Scientists Identify Thirst-Controlling Neurons|url = http://www.nih.gov/news-events/nih-research-matters/scientists-identify-thirst-controlling-neurons|archive-url = https://web.archive.org/web/20160105105516/http://www.nih.gov/news-events/nih-research-matters/scientists-identify-thirst-controlling-neurons|archive-date = January 5, 2016|website = National Institutes of Health (NIH)|access-date = 2016-02-11}}
{{Commons category}}
{{renal physiology}}
{{Medicine}}
{{Authority control}}

[[Category:Thirst| ]]
[[Category:Effects of external causes]]
[[Category:Body water]]