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{{Short description|Water content of an animal's body}}
{{Distinguish|Body of water}}

In [[physiology]], '''body water''' is the [[water]] content of an [[Animal#Structure|animal body]] that is contained in the tissues, the blood, the bones and elsewhere. The percentages of body water contained in various [[compartment (pharmacokinetics)|fluid compartments]] add up to '''total body water''' (TBW). This water makes up a significant fraction of the [[human body]], both by weight and by volume. Ensuring the right amount of body water is part of [[fluid balance]], an aspect of [[homeostasis]].

==Location==
[[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 |title=Fluid composition of the body 1.3 |url=https://openverse.org/image/877ec3c8-9c07-4f4c-9739-cc80434e45dd?q=body+fluid+compartments |vauthors=Sved A, Walsh D}}</ref>]]
By weight, the average adult human is approximately 60% water, and the average child is approximately 65% water.<ref>{{cite web| title = The water in you|url = https://water.usgs.gov/edu/propertyyou.html | publisher = [[ Howard Perlman]]| date = December 2016 }}</ref><ref name="lote">{{cite book |last=Lote |first=Christopher J. |title= Principles of Renal Physiology, 5th edition|publisher=Springer |page=2}}</ref> There can be considerable variation in body water percentage based on a number of factors like age, health, water intake, weight, and sex. In a large study of adults of all ages and both sexes, the adult human body averaged ~65% water. However, this varied substantially by age, sex, and adiposity (amount of fat in [[body composition]]). The figure for water fraction by weight in this sample was found to be 58 ±8% water for males and 48 ±6% for females.<ref>{{cite journal| title = Total body water volumes for adult males and females estimated from simple anthropometric measurements.|url = http://ajcn.nutrition.org/content/33/1/27.full.pdf| journal = [[The American Journal of Clinical Nutrition]]|url-access=subscription |volume = 33| date = January 1980| issue = 1 |pages = 27–39|doi = 10.1093/ajcn/33.1.27|last1 = Watson|first1 = P. E.|last2 = Watson|first2 = I. D.|last3 = Batt|first3 = R. D.|pmid = 6986753}}</ref> The body water constitutes as much as 75% of the body weight of a newborn infant, whereas some obese people are as little as 45% water by weight.<ref name="textbookofmedicalphysiology5thed">{{cite book| title=Textbook of Medical Physiology| last=Guyton| first=Arthur C.| year=1976| edition=5th| publisher=W.B. Saunders| location=Philadelphia| pages=[https://archive.org/details/textbookofmedica0005guyt/page/284 284, 424]| isbn=0-7216-4393-0| url=https://archive.org/details/textbookofmedica0005guyt/page/284}}</ref> This is due to how fat tissue does not retain water as well as lean tissue. These statistical averages will vary with factors such as type of population, age of people sampled, number of people sampled, and methodology. So there is not, and cannot be, a figure that is exactly the same for all people, for this or any other physiological measure.<!--Anchor for redirect of [Animal body water] article; caution. -->

Most animal body water is contained in various [[body fluid]]s. These include [[intracellular fluid]]; [[extracellular fluid]]; [[Blood plasma|plasma]]; [[interstitial fluid]]; and [[transcellular fluid]].<ref name="John T. Hansen, Bruce M. Koeppen, 2002">{{cite book |author1=John T. Hansen |author2=Bruce M. Koeppen |title=Netter's Atlas of Human Physiology |publisher=Icon Learning Systems |location=Teterboro, N.J |year=2002 |isbn=1-929007-01-9}}</ref>  Water is also contained inside organs, in [[Gastrointestinal physiology#Secretion|gastrointestinal]], [[cerebrospinal fluid|cerebrospinal]], [[peritoneal fluid|peritoneal]], and [[Aqueous humour|ocular]] fluids.  [[Adipose tissue]] contains about 10% of water, while [[muscle tissue]] contains about 75%.<ref name=aniprod/><ref>{{cite book|url=https://books.google.com/books?id=FcA6zIq0d48C|title=Nutrient Requirements of Nonhuman Primates|isbn=9780309172042|date=2003-02-01|publisher=National Academies Press }}</ref>

In ''Netter's Atlas of Human Physiology (2002)'', body water is broken down into the following ''compartments:''<ref name="John T. Hansen, Bruce M. Koeppen, 2002"/>
* [[Intracellular fluid]] (2/3 of body water) is fluid contained within cells. In a {{Convert|72|kg|lb|abbr=on|sp=us}} body containing 40 litres of fluid, about 25 litres is intracellular,<ref name="guyton2">{{cite book| title=Textbook of Medical Physiology| last=Guyton| first=Arthur C.| year=1976| edition=5th| publisher=W.B. Saunders| location=Philadelphia| pages=[https://archive.org/details/textbookofmedica0005guyt/page/275 275]| isbn=0-7216-4393-0| url=https://archive.org/details/textbookofmedica0005guyt/page/275}}</ref> which amounts to 62.5%. Jackson's texts states 70% of body fluid is intracellular.<ref name="Jackson1979">{{cite book | title=Anatomy & Physiology for Nurses |last=Jackson| first=Sheila| year=1985| edition=9th| series=Nurses' Aids Series| publisher=Bailliere Tindall| location=London| isbn=0-7020-0737-4}}</ref>
* [[Extracellular fluid]] (1/3 of body water) is fluid contained in areas outside of cells. For a 40-litre body, about 15 litres is extracellular,<ref name=guyton2/> which amounts to 37.5%.
** [[Blood plasma|Plasma]] (1/5 of extracellular fluid). Of this 15 litres of extracellular fluid, plasma volume averages 3 litres,<ref name=guyton2/> or 20%.
** [[Interstitial fluid]] (4/5 of extracellular fluid)
** [[Transcellular fluid]] (a.k.a. "third space," normally ignored in calculations) contained inside organs, such as the [[gastrointestinal]], [[cerebrospinal fluid|cerebrospinal]], peritoneal, and [[Human eye|ocular]] fluids.

==Measurement==
===Dilution and equilibration===
An individual’s total body water can be determined using [[flowing afterglow mass spectrometry|flowing-afterglow mass spectrometry]] (FA-MS) to measure the abundance of [[deuterium]] in breath samples.  A known dose of deuterated water ([[heavy water]], D<sub>2</sub>O) is ingested and allowed to equilibrate within the body water. Then, the FA-MS instrument measures the ratio D:H of deuterium to hydrogen in the water vapour in exhaled breath.  The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D<sub>2</sub>O ingested.

The water in individual compartments can be measured with different substances:<ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 7/7ch02/7ch02p13 |chapter-url=http://humanphysiology.tuars.com/program/section7/7ch02/7ch02p13.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section7/7ch02/7ch02p13.htm|archive-date=2016-03-24}}</ref>
* total body water: [[tritiated water]] or [[heavy water]].
* [[extracellular fluid]]: [[inulin]]
* [[blood plasma]]: [[Evans Blue (dye)|Evans blue]]

Intracellular fluid may then be estimated by subtracting extracellular fluid from total body water.

===Bioelectrical impedance analysis===
Another method of determining total body water percentage (TBW%) is via [[Bioelectrical Impedance Analysis|bioelectrical impedance analysis]] (BIA). In the traditional BIA method, a person lies on a cot and spot electrodes are placed on the hands and bare feet. Electrolyte gel is applied first, and then a weak current of frequency 50kHz is introduced. This AC waveform allows the creation of a current inside the body via the very capacitive skin without causing a DC flow or burns, and limited in the ~20mA range current for safety.<ref name="US Patent 4719922">{{cite web|url=http://www.patentstorm.us/patents/4719922/description.html|title=US Patent 4719922, Stimulator Apparatus - this website has ended|work=patentstorm.us|url-status=dead|archive-url=https://web.archive.org/web/20121013145453/http://www.patentstorm.us/patents/4719922/description.html|archive-date=2012-10-13}}</ref>

BIA has emerged as a promising technique because of its simplicity, low cost, high reproducibility, and noninvasiveness. BIA prediction equations can be either generalized or population-specific, allowing this method to be potentially very accurate. Selecting the appropriate equation is important to determining the quality of the results.{{Citation needed|date=November 2022}}

For clinical purposes, scientists are developing a multi-frequency BIA method that may further improve the method's ability to predict a person's hydration level. New segmental BIA equipment that uses more electrodes may lead to more precise measurements of specific parts of the body.

== Calculation ==
In humans, total body water can be estimated based on the [[Premorbidity|premorbid]] (or ideal) [[Human_body_weight#Ideal_body_weight|body weight]] and correction factor.

<math display="block">TBW = weight * C</math>

C is a coefficient for the expected percentage of weight made up of free water. For adult, non-elderly males, C = 0.6. For adult elderly males, malnourished males, or females, C = 0.5. For adult elderly or malnourished females, C = 0.45. A total body water deficit (TBWD) can then be approximated by the following formula:

<math display="block">TBWD=TBW*(1-\frac{[Na]t}{[Na]m})</math>

Where [Na]t = target sodium concentration (usually 140 mEq/L), and [Na]m = measured sodium concentration.

The resultant value is the approximate volume of free water required to correct a [[Hypernatremia|hypernatremic]] state. In practice, the value rarely approximates the actual amount of free water required to correct a deficit due to insensible losses, urinary output, and differences in water distribution among patients. <ref>{{Cite book|title=Goldman's Cecil medicine|last1=Lee.|first1=Goldman|last2=I.|first2=Schafer, Andrew|last3=Fayette.|first3=Cecil, Russell La|date=2012-01-01|publisher=Elsevier/Saunders|isbn=9781437716047|oclc=779501249}}</ref>

Total water may also be estimated by use of anthropometric equations:<ref name=Watson>{{cite journal |last1=Watson |first1=PE |last2=Watson |first2=ID |last3=Batt |first3=RD |title=Total body water volumes for adult males and females estimated from simple anthropometric measurements. |journal=The American Journal of Clinical Nutrition |date=January 1980 |volume=33 |issue=1 |pages=27–39 |doi=10.1093/ajcn/33.1.27 |pmid=6986753|doi-access=free }}</ref>
<math display="block">TBW_{\text{male}}\text{ (L)} = 2.447 - 0.09516 \times \text{age (years)} + 0.1074 \times \text{height (cm)} + 0.3362 \times \text{weight (kg)}</math>
<math display="block">TBW_{\text{female}}\text{ (L)} = -2.097 + 0.1069 \times \text{height (cm)} + 0.2466 \times \text{weight (kg)}</math>

The equations above (the Watson equations) have been found to give reasonable estimates in most cases, as a diverse set of subject data was used.<ref name=Watson/> Other equations have been given for specific populations, such as Americans,<ref>{{cite journal |last1=Chumlea |first1=William Cameron |last2=Guo |first2=Shumei S. |last3=Zeller |first3=Christine M. |last4=Reo |first4=Nicholas V. |last5=Baumgartner |first5=Richard N. |last6=Garry |first6=Philip J. |last7=Wang |first7=Jack |last8=Pierson |first8=Richard N. |last9=Heymsfield |first9=Steven B. |last10=Siervogel |first10=Roger M. |title=Total body water reference values and prediction equations for adults |journal=Kidney International |date=June 2001 |volume=59 |issue=6 |pages=2250–2258 |doi=10.1046/j.1523-1755.2001.00741.x|pmid=11380828 |doi-access=free }}</ref><ref>{{cite journal |last1=Chumlea |first1=William C. |last2=Schubert |first2=Christine M. |last3=Reo |first3=Nicholas V. |last4=Sun |first4=Shumei S. |last5=Siervogel |first5=Roger M. |title=Total body water volume for white children and adolescents and anthropometric prediction equations: The Fels Longitudinal Study |journal=Kidney International |date=November 2005 |volume=68 |issue=5 |pages=2317–2322 |doi=10.1111/j.1523-1755.2005.00692.x|pmid=16221235 |doi-access=free }}</ref> Tunisians,<ref>{{cite journal |last1=El Kouki |first1=Donia |last2=El Kari |first2=Khalid |last3=Draoui |first3=Jihéne |last4=Ben Jemaa |first4=Houda |last5=Ben Amor |first5=Nadia |last6=Monyeki |first6=Andries |last7=Jamoussi |first7=Henda |last8=Aguenaou |first8=Hassan |last9=Aouidet |first9=Abdallah |last10=Mankaï |first10=Amani |title=Development and validation of anthropometric predictive equations that estimate the total body water and fat-free mass in Tunisian adults |journal=European Journal of Clinical Nutrition |date=April 2023 |volume=77 |issue=4 |pages=447–453 |doi=10.1038/s41430-023-01262-2|pmid=36726031 |pmc=9891746 }}</ref> and Cameroonians.<ref>{{cite journal |last1=Medoua |first1=Gabriel Nama |last2=Essa'a |first2=Véronique J. |last3=Tankou |first3=Colman T. |last4=Ndzana |first4=Anne Christine A. |last5=Dimodi |first5=Henriette T. |last6=Ntsama |first6=Patricia M. |title=Validity of anthropometry- and impedance-based equations for the prediction of total body water as measured by deuterium dilution in Cameroonian haemodialysis patients |journal=Clinical Nutrition ESPEN |date=October 2015 |volume=10 |issue=5 |pages=e167–e173 |doi=10.1016/j.clnesp.2015.05.006|pmid=28531471 }}</ref> Anthropometric TBW equations do not generalize well beyond healthy adult subjects outside their measured population, and it has been recommended to use a more accurate method such as BIA for most clinical settings.<ref>{{cite journal |last1=Basile |first1=C. |last2=Vernaglione |first2=L. |last3=Bellizzi |first3=V. |last4=Lomonte |first4=C. |last5=Rubino |first5=A. |last6=Ambrosio |first6=N. D |last7=Iorio |first7=B. D. |title=Total body water in health and disease: Have anthropometric equations any meaning? |journal=Nephrology Dialysis Transplantation |date=7 March 2008 |volume=23 |issue=6 |pages=1997–2002 |doi=10.1093/ndt/gfm909|pmid=18208903 |doi-access=free }}</ref>

== Functions ==
Water in the animal body performs a number of functions: as a solvent for transportation of nutrients; as a medium for [[excretion]]; a means for heat control; as a lubricant for joints; and for shock absorption.<ref name="aniprod">{{cite book|url=https://books.google.com/books?id=0UPDQQOgr5YC&q=%22animal+body+water%22&pg=PA47|title=FCS Animal Production L2|isbn=9781868917297|year=2007|publisher=Pearson South Africa }}</ref>

==Changes==
The usual way of adding water to a body is by [[drinking]].  Water also enters the body with foods, especially those rich in water, such as plants, [[raw meat]], and fish. About 10% of human adult water intake comes as a by-product of metabolism.<ref>{{Cite journal|last1=Yamada|first1=Yosuke|last2=Zhang|first2=Xueying|last3=Henderson|first3=Mary E. T.|last4=Sagayama|first4=Hiroyuki|last5=Pontzer|first5=Herman|last6=Speakman|first6=John R. |date=2022|title= Variation in human water turnover associated with environmental and lifestyle factors|journal=Science|language=en|volume=378|issue=6622|pages=909–915|doi=10.1126/science.abm8668|pmid=36423296|pmc=9764345 |bibcode=2022Sci...378..909I }}</ref>

The amount of this water that is retained in animals is affected by several factors. Water amounts vary with the age of the animal. The older the [[vertebrate]] animal, the higher its relative [[Bone density|bone mass]] and the lower its body water content.

In diseased states, where body water is affected, the [[fluid compartments|fluid compartment or compartments]] that have changed can give clues to the nature of the problem, or problems. Body water is regulated by [[hormone]]s, including [[anti-diuretic hormone|antidiuretic hormone]], [[aldosterone]] and [[atrial natriuretic peptide]].

=== Loss of water ===
{{Main|Volume contraction|Dehydration}}

[[Volume contraction]] is a decrease in [[body fluid]] volume, with or without a concomitant loss of [[osmolyte]]s. The loss of the body water component of body fluid is specifically termed [[dehydration]].<ref>[http://www.medterms.com/script/main/art.asp?articlekey=2933 MedicineNet > Definition of Dehydration] {{Webarchive|url=https://web.archive.org/web/20131106025259/http://www.medterms.com/script/main/art.asp?articlekey=2933 |date=2013-11-06 }} Retrieved on July 2, 2009</ref>

Sodium loss approximately correlates with fluid loss from extracellular fluid, since sodium has a much higher concentration in extracellular fluid (ECF) than [[intracellular fluid]] (ICF). In contrast, K<sup>+</sup> has a much higher concentration in ICF than ECF, and therefore its loss rather correlates with fluid loss from ICF, since K<sup>+</sup> loss from ECF causes the K<sup>+</sup> in ICF to diffuse out of the cells, dragging water with it by [[osmosis]].{{Citation needed|date=February 2016}}

==References==
{{reflist}}

==Further reading==
*[https://books.google.com/books?id=en3Z1N8lF80C&pg=PT26 ''Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice'']
*[https://books.google.com/books?id=M0XvP6mgmnkC&pg=PA180 ''Essentials of Animal Physiology'']
*[https://books.google.com/books?id=JGBBhS-Wnz4C&pg=PA58 ''The Encyclopedia of Farm Animal Nutrition'']
*[https://books.google.com/books?id=jlZg1VF-sKIC&pg=PA120 ''Animal Osmoregulation'']
*[https://books.google.com/books?id=OG6qrtVKwTEC&pg=PA117 ''Animal Nutrition Science'']

==External links==
{{portal|Animals}}
* {{MeshName|Body+water}}
* {{MeshName|Body+fluid+compartments}}

{{Renal physiology}}

[[Category:Body water| ]]
[[Category:Human body]]

[[de:Körperwasser]]