Editing Dehydration (section)

== Mechanism ==
[[File:Fluid composition of the body 1.3.png|alt="Fluid composition of the body 1.3" by Alan Sved and David Walsh is licensed under CC BY-SA 4.0.|thumb|384x384px|Diagram depicting the distribution of total body water into percentages of intracellular and extracellular fluid.<ref>{{Cite web | vauthors = Sved A, Walsh D |title=Fluid composition of the body 1.3 |url=https://openverse.org/image/877ec3c8-9c07-4f4c-9739-cc80434e45dd?q=body+fluid+compartments}}</ref>]]Water content of a human body varies from 70-75% in newborns to 40% and less in obese adults,{{sfn|Schoeller|2005|p=35}} an average value of 60% is suggested.<ref>{{Cite book | vauthors = Kamel KS, Halperin ML |title=Fluid, electrolyte, and acid-base physiology: a problem-based approach|date=2017 |publisher=Elsevier |isbn=978-0-323-35515-5 |edition=Fifth |location=Philadelphia, PA}}</ref> Within the body, water is classified as intracellular fluid or extracellular fluid. Intracellular fluid refers to water that is contained within the cells. This consists of approximately 57% of the total body water weight.{{sfn|Schoeller|2005|p=35}} Fluid inside the cells has high concentrations of potassium, magnesium, phosphate, and proteins.<ref name="Garden_2023">{{Cite book | vauthors = Garden J, Parks R, Wigmore S |title=Principles and Practice of Surgery  |publisher=Elsevier Limited |year=2023 |isbn=978-0-7020-8251-1 |edition=8th |pages=32–55}}</ref> Extracellular fluid consists of all fluid outside of the cells, and it includes blood and interstitial fluid. This makes up approximately 43% of the total body water weight. The most common ions in extracellular fluid include sodium, chloride, and bicarbonate.
The concentration of dissolved molecules and ions in the fluid is described as [[Osmotic concentration|Osmolarity]] and is measured in osmoles per liter (Osm/L).<ref name="Garden_2023" /> When the body experiences a free water deficit, the concentration of solutes is increased. This leads to a higher serum osmolarity. When serum osmolarity is elevated, this is detected by osmoreceptors in the hypothalamus. These receptors trigger the release of [[Vasopressin|antidiuretic hormone]] (ADH).<ref>{{Cite book | vauthors = White BA, Harrison JR, Mehlmann LM |title=Endocrine and reproductive physiology |date=2019 |publisher=Elsevier |isbn=978-0-323-59573-5 |edition=5th |series=Mosby physiology series |location=St. Louis, MI}}</ref> ADH resists dehydration by increasing water absorption in the kidneys and constricting blood vessels. It acts on the [[Vasopressin receptor 2|V2 receptors]] in the cells of the collecting tubule of the [[nephron]] to increase expression of aquaporin. In more extreme cases of low blood pressure, the hypothalamus releases higher amounts of ADH which also acts on V1 receptors.<ref>{{cite journal | vauthors = Webb AJ, Seisa MO, Nayfeh T, Wieruszewski PM, Nei SD, Smischney NJ | title = Vasopressin in vasoplegic shock: A systematic review | journal = World Journal of Critical Care Medicine | volume = 9 | issue = 5 | pages = 88–98 | date = December 2020 | pmid = 33384951 | pmc = 7754532 | doi = 10.5492/wjccm.v9.i5.88 | doi-access = free }}</ref> These receptors cause contractions in the peripheral vascular smooth muscle. This increases systemic vascular resistance and raises blood pressure.