Thirst

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

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 osmolytes 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 a myriad of 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[1] 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.[2]. For instance, angiotensin II, activated in RAS, is a powerful dipsogen (ie it stimulates thirst) which acts via the subfornical organ.

Increased osmolite concentration

An increase in osmotic pressure, e.g. after eating a salty meal[1] activates osmoreceptors. There are osmoreceptors already in the central nervous system, morespecifically 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.[3]

In addition, there are visceral osmoreceptors[2]. These project to the area postrema[2] and nucleus tractus solitarius[2] in the brain.

Salt craving

Because sodium is also lost the from the plasma in hypovolemia, the body's need for salt proportionately increases in addition to thirst in such cases.[1]. This is also a result of the renin-angiotensin system activation.

Central processing

The area postrema and nucleus tractus solitarius signal, by 5-HT[2], to lateral parabrachial nucleus[2], which in turn signal to median preoptic nucleus. In addition, the area postrema and nucleus tractus solitarius also signal directly to subfornical organ.[2]

Thus, the median preoptic nucleus and subfornical organ receive signals of both decreased volume and increased osmolite concentration. They signal to higher integrative centers[2], 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. 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'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.

References

  1. 1.0 1.1 1.2 Carlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 M.J. McKinley and A.K. Johnson (2004). "The Physiological Regulation of Thirst and Fluid Intake". News in Physiological Sciences. 19 (1): 1–6. Retrieved 2006-06-02.
  3. Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 872

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