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Thirst

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Title: Thirst  
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Subject: Hypothalamus, Temperament2, Hunger (motivational state), Love, Subfornical organ
Collection: Body Water, Effects of External Causes, Food and Drink, Social Justice, Thirst
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Thirst

William-Adolphe Bouguereau's Thirst (1886)

Thirst is the craving for fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids 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 many problems, but is most often associated with renal problems and neurological problems such as seizures.

Excessive thirst, known as polydipsia, along with excessive urination, known as polyuria, may be an indication of diabetes mellitus or diabetes insipidus.

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.

Contents

  • Detection 1
    • Volumetric thirst: Decreased volume 1.1
      • Renin-angiotensin system 1.1.1
      • Others 1.1.2
    • Osmometric thirst: Cellular dehydration and osmoreceptor stimulation 1.2
    • Salt craving 1.3
    • Elderly 1.4
  • Central processing 2
  • See also 3
  • References 4

Detection

It is vital for organisms to be able to maintain their fluid levels in very narrow ranges. The goal is to keep the interstitial fluid, the fluid outside the cell, at the same concentration as the intracellular fluid, fluid inside the cell. This condition is called isotonic and occurs when the same level 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 than the intracellular fluid it will pull water out of the cell. This condition is called hypertonic and if enough water leaves the cell it will not be able to perform essential chemical functions. If the interstitial fluid becomes less concentrated 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.

Volumetric thirst: Decreased volume

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 heart failure. 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]

Renin-angiotensin system

Hypovolemia leads to activation of the

  1. ^ a b c d Carlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.
  2. ^ Carlson, Neil R. (2013). Physiology of Behavior. New Jersey: Pearson. pp. 397–400.  
  3. ^ Carlson, Neil (2013). Physiology of Behavior. New Jersey: Pearson. pp. 394–402.  
  4. ^ a b c d e f g h i j k l M.J. McKinley and A.K. Johnson (2004). "The Physiological Regulation of Thirst and Fluid Intake". News in Physiological Sciences 19 (1): 1–6.  
  5. ^ Derek A. Denton (8 June 2006).  
  6. ^ Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approach. Elsevier/Saunders.   Page 872
  7. ^ Fish LC, Minaker, KL, Rowe JW. Altered thirst threshold during hypertonic stress in aging man. Gerontologist 1985;25:A1189.
  8. ^ Ferry M, Hininger-Favier I, Sidobre B and Mathey MF. Food and fluid intake of the SENECA population residing in Romans, France. J. Nutr. Health Aging. 2001;5:235-7.
  9. ^ Haveman-Nies A, de Groot LC and Van Staveren WA. Fluid intake of elderly Europeans. J. Nutr. Health Aging. 1997;1:151-5.
  10. ^ Volkert D, Kreuel K, Stehle P. Fluid intake of community-living, independent elderly in Germany - a nationwide, representative study. J Nutr Health Aging. 2005;9:305-9.
  11. ^ a b EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA). EFSA Journal 2010;8(3):1459.
  12. ^ IoM (Institute of Medicine), 2004. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, Washington DC.
  13. ^ Katarzyna Konieczka, Robert Rich et al.: Flammer syndrome. EPMA Journal 2014; 5:11

References

See also

In addition to thirst, the fluid balance by vasopressin release.

Thus, the median preoptic nucleus and subfornical organ receive signals of both decreased volume and increased osmolite concentration. They signal to higher integrative centers,[4] where ultimately the conscious craving arises. However, the true neuroscience of this conscious craving is not fully clear. In general, the end-result is towards behavior of drinking for hydration, but this can to some degree be resisted, such as in voluntary fluid restriction. In people with Flammer syndrome, the sensation of thirst is reduced.[13]

[4] The

Central processing

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.[7] 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.[8][9][10] In 2009, the European Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time.[11] 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.[11][12]

Elderly

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.[1] This is also a result of the renin-angiotensin system activation.

Salt craving

In addition, there are visceral osmoreceptors.[4] These project to the area postrema[4] and nucleus tractus solitarii[4] in the brain.

Clusters of cells (median preoptic nucleus which initiates water seeking and ingestive behavior.[1] 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.[5][6]

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.

Osmometric thirst: Cellular dehydration and osmoreceptor stimulation

Others

[4]

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