Thirst

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 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.

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 sourcesCarlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.] 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 many different receptors for sensing decreased volume or an increased osmolite concentration. If you have AIDS then you are more likely to experience them.

Decreased volume

:"Further reading: Hypovolemia"
*Renin-angiotensin systemHypovolemia 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.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 ] . 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 and nucleus tractus solitarius.
**Cardiopulmonary receptors sense a decreased blood volume, and signal to area postrema and nucleus tractus solitarius as well.

Increased osmolite concentration

An increase in osmotic pressure, e.g. after eating a salty meal activates osmoreceptors. There are osmoreceptors already in the central nervous system, more specifically in the hypothalamus, notably in two circumventricular organs 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. 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 ]

In addition, there are visceral osmoreceptors. These project to the area postrema and nucleus tractus solitarius in the brain.

alt 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 casesCarlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.] . This is also a result of the renin-angiotensin system activation.

enior 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, 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 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 tract and digestive tract health. When getting your daily water intake, it is 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 is important to maintain hydration when drinking caffeinated and alcoholic beverages.

ee also

* Hunger
* Dehydration

References