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Thirst

thirst trap, thirsty thursday
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

  • 1 Detection
    • 11 Decreased volume
      • 111 Renin-angiotensin system
      • 112 Others
    • 12 Cellular dehydration and osmoreceptor stimulation
    • 13 Salt craving
    • 14 Elderly
  • 2 Neurophysiology
  • 3 See also
  • 4 References
  • 5 Further reading

Detectionedit

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 volumemedical citation needed

Decreased volumeedit

Further information: 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 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 system23

Renin-angiotensin systemedit

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 which increase blood pressure1 It is also responsible for the initiation of drinking behavior and salt appetite via the subfornical organ4

Renin-angiotensin-aldosterone system

Othersedit

  • Arterial baroreceptors sense a decreased arterial pressure, and signals to the central nervous system in the area postrema4 and nucleus tractus solitarii4
  • Cardiopulmonary receptors sense a decreased blood volume, and signal to area postrema4 and nucleus tractus solitarii4 as well

Cellular dehydration and osmoreceptor stimulationedit

Main article: 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 dehydrationmedical citation needed

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 behavior1 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 fluids56

Imidazolium salt

In addition, there are visceral osmoreceptors4 These project to the area postrema4 and nucleus tractus solitarii4 in the brain

Salt cravingedit

Because sodium is also lost from the plasma in hypovolemia, the body's need for salt proportionately increases in addition to thirst in such cases1 This is also a result of the renin-angiotensin system activationmedical citation needed

Elderlyedit

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 dehydration7 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 intake8910 In 2009, the European Food Safety Authority EFSA included water as a macronutrient in its dietary reference values for the first time11 Recommended intake volumes in the elderly are the same as for younger adults 20 L/day for females and 25 L/day for males as despite lower energy consumption, the water requirement of this group is increased due to a reduction in renal concentrating capacity1112

Neurophysiologyedit

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 nucleus4 The latter signaling relies on the neurotransmitter serotonin4 The signal from the lateral parabrachial nucleus is relayed to the median preoptic nucleus4

The median preoptic nucleus and the subfornical organ receive signals of decreased volume clarification needed and increased osmolite concentration Finally, the signals are received in cortex areas of the forebrain4 where ultimately the conscious craving 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 glandadditional citation needed

See alsoedit

  • Drought
  • Hunger motivational state
  • World Water Day
  • Adipsia
  • Food portal

Referencesedit

  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 ISBN 978-0-205-23981-8 
  3. ^ Carlson, Neil 2013 Physiology of Behavior New Jersey: Pearson pp 394–402 ISBN 0-205-23939-0 
  4. ^ a b c d e f g h i j k l MJ McKinley; AK Johnson 2004 "The Physiological Regulation of Thirst and Fluid Intake" News in Physiological Sciences 19 1: 1–6 doi:101152/nips014702003 PMID 14739394 Retrieved 2006-06-02 
  5. ^ Derek A Denton 8 June 2006 The primordial emotions: the dawning of consciousness Oxford University Press pp 118–19 ISBN 978-0-19-920314-7 
  6. ^ Walter F Boron 2005 Medical Physiology: A Cellular And Molecular Approach Elsevier/Saunders ISBN 1-4160-2328-3  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;83:1459
  12. ^ IoM Institute of Medicine, 2004 Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate National Academies Press, Washington DC

Further readingedit

  • "Scientists Identify Thirst-Controlling Neurons" National Institutes of Health NIH Retrieved 2016-02-11 

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