Thirst 509632 224119894 2008-07-07T12:00:48Z Barutazaru 5913914 /* Salt craving */ {{otheruses}} [[Image:William-Adolphe Bouguereau (1825-1905) - Thirst (1886).jpg|thumb|[[William-Adolphe Bouguereau]]'s ''Thirst'' (1886)]] '''Thirst''' is the craving for liquids, resulting in the basic [[instinct]] of humans or animals to [[drink]]. It is an essential mechanism involved in [[fluid balance]]. It arises from a lack of fluids and/or an increase in the concentration of certain [[osmolite]]s 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. Continuous [[dehydration]] can cause myriad problems, but is most often associated with neurological problems such as seizures, and renal problems. Excessive thirst, known as [[polydipsia]], along with excessive urination, known as [[polyuria]], may be an indication of [[diabetes]]. 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<ref name="Carlson">Carlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.</ref> 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. ==Detection== There are different receptors for sensing decreased volume or an increased osmolite concentration. ===Decreased volume=== :''Further reading: [[Hypovolemia]]'' *'''Renin-angiotensin system''' Hypovolemia leads to activation of the [[renin angiotensin system]] (RAS) and a decrease in [[atrial natriuretic peptide]]. These mechanisms, along their other functions, contribute to elicit thirst, by affecting the [[subfornical organ]].<ref name=McKinley>{{cite journal | author=M.J. McKinley and A.K. Johnson | title=The Physiological Regulation of Thirst and Fluid Intake | journal=News in Physiological Sciences | volume=19 | issue=1 | year=2004 | pages=1–6 | url=http://physiologyonline.physiology.org/cgi/content/full/19/1/1 | accessdate=2006-06-02 | pmid=14739394 | doi=10.1152/nips.01470.2003 }}</ref>. For instance, [[angiotensin II]], activated in RAS, is a powerful [[dipsogen]] (ie it stimulates thirst) which acts via the subfornical organ. *'''Other''' **[[Arterial baroreceptors]] sense a decreased arterial pressure, and signals to the [[central nervous system]] in the [[area postrema]]<ref name=McKinley/> and [[nucleus tractus solitarius]]<ref name=McKinley/>. **[[Cardiopulmonary receptor]]s sense a decreased blood volume, and signal to [[area postrema]]<ref name=McKinley/> and [[nucleus tractus solitarius]]<ref name=McKinley/> as well. ===Increased osmolite concentration=== {{Main|Osmoreceptor}} An increase in [[osmotic pressure]], e.g. after eating a salty meal<ref name="Carlson"/> activates [[osmoreceptor]]s. There are osmoreceptors already in the [[central nervous system]], more specifically in the [[hypothalamus]], notably in two [[circumventricular organ]]s that lack an effective [[blood-brain barrier]], the [[organum vasculosum of the lamina terminalis]] (OVLT) and the [[subfornical organ]] (SFO). However, although located in the same parts of the brain, these osmoreceptors that evoke thirst are distinct from the neighbouring osmoreceptors in the OVLT and SFO that evoke [[arginine vasopressin]] release to decrease [[fluid output]].<ref name=boron872> {{cite book |author=Walter F., PhD. Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |location= |year= |pages= |isbn=1-4160-2328-3 |oclc= |doi=}} Page 872 </ref> In addition, there are [[visceral osmoreceptor]]s<ref name=McKinley/>. These project to the [[area postrema]]<ref name=McKinley/> and [[nucleus tractus solitarius]]<ref name=McKinley/> in the brain. ===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">Carlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.</ref>. This is also a result of the renin-angiotensin system activation. ===Senior citizens=== For adults over age 50, the body’s [[thirst]] sensation diminishes and continues diminishing with age, causing many to suffer symptoms of [[dehydration]]. ==Central processing== The [[area postrema]] and [[nucleus tractus solitarius]] signal, by [[5-HT]]<ref name=McKinley/>, to [[lateral parabrachial nucleus]]<ref name=McKinley/>, which in turn signal to median preoptic nucleus. In addition, the area postrema and nucleus tractus solitarius also signal directly to subfornical organ.<ref name=McKinley/> Thus, the median preoptic nucleus and subfornical organ receive signals of both decreased volume and increased osmolite concentration. They signal to higher integrative centers<ref name=McKinley/>, where ultimately the conscious craving arises. However, the true [[neuroscience]] of this conscious craving is not fully clear. In addition to thirst, the [[organum vasculosum of the lamina terminalis]] and the [[subfornical organ]] contribute to [[fluid balance]] by [[vasopressin]] release. ==Preventing subtle dehydration== For optimal health, experts recommend that humans get 8-10 servings of about 8-ounces of [[water]] (in total, approximately 2 litres) per day to maintain hydration.{{fact|date=April 2008}} This figure does vary according to ambient temperature, movement and physical size. Being that water is essential to the general function of the human and all animal bodies, eight servings is widely regarded as the minimum for the body to function optimally. However, water can be obtained from many sources, such as [[foods]] and other [[beverages]] containing water. Getting enough water from your diet and staying hydrated is key to your overall health, including [[urinary|urinary tract]] and [[digestive tract]] health. When getting your daily water intake, it's important to not rely heavily on caffeinated beverages, as they actually work as a [[diuretic]]. Further, moderate or excessive [[alcohol]] consumption can lead to [[dehydration]], thus it's important to maintain hydration when drinking caffeinated and alcoholic beverages. ==See also== {{Wiktionary}} * [[Hunger]] * [[Dehydration]] ==References== {{reflist}} [[Category:Food and drink]] [[gn:Uhéi]] [[de:Durst]] [[es:Sed]] [[fr:Soif]] [[ko:목마름]] [[he:צמא]] [[la:Sitis]] [[nl:Drinken#Dorst]] [[pl:Pragnienie (fizjologia)]] [[qu:Yakunayay]] [[ru:Жажда]] [[fi:Jano]] [[sv:Törst]] [[uk:Спрага]]