Thirst

Thirst is the craving for potable fluids, resulting in the basic

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, the fluid inside the cell. This condition is called isotonic and occurs when the same levels 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 (or a lower concentration of water) 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. The animal will then become thirsty in response to the demand for water in the cell. After the animal drinks water, the interstitial fluid becomes less concentrated of solutes (more concentrated of water) than the intracellular fluid and 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.^[2]

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 hypovolemic shock. 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

Others

• Arterial baroreceptors sense a decreased arterial pressure, and signal to the
  nucleus tractus solitarii.^[2]
• nucleus tractus solitarii.^[2]

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.^[1]

Clusters of cells (

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.[3] This is also a result of the renin-angiotensin system activation.^{[medical citation needed]}

Elderly

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.^[6] 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.^[7][8][9] In 2009, the European Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time.^[10] 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.^[10][11]

Thirst quenching

According to preliminary research, quenching of thirst – the

Thirst quenching varies among animal species, with dogs, camels, sheep, goats, and deer replacing fluid deficits quickly when water is available, whereas humans and horses may need hours to restore fluid balance.[12]

Neurophysiology

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

See also

• Drought
• Hunger (motivational state)
• World Water Day
• Adipsia

References

1. ^
   PMID 26290293
   .
2. ^
   S2CID 58947441
   .
3. ^
   ISBN 978-0-205-23981-8
   .
4. ISBN 978-0-19-920314-7
   .
5. ISBN 978-1-4160-2328-9
   . Page 872
6. ^ Fish LC, Minaker, KL, Rowe JW. "Altered thirst threshold during hypertonic stress in aging man". Gerontologist;; 1985;25:A1189.
7. PMID 11753484
   .
8. PMID 10995083
   .
9. PMID 16222395
   .
10. ^
    ISSN 1831-4732
    .
11. ^ IoM (Institute of Medicine), 2004. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, Washington, D.C.
12. ^
    PMID 29932494
    .

Further reading

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

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