Editing Excretion

{{short description|Elimination by an organism of metabolic waste products}}
{{about|a biological process|a pharmacokinetics phase|Elimination (pharmacology)}}
[[File:Urinary System Large Unlabeled.jpg|thumb|Mammals excrete urine through the [[urinary system]].]]
'''Excretion''' is elimination of [[metabolic waste]], which is an essential process in all [[organism]]s. In [[vertebrate]]s, this is primarily carried out by the [[lungs]], [[Kidney (vertebrates)|kidneys]], and [[skin]].<ref name='beckett'>{{cite book | vauthors = Beckett BS | title = Biology: A Modern Introduction | publisher = [[Oxford University Press]] | year = 1987 | pages = 110 | isbn = 0-19-914260-2 }}</ref> This is in contrast with [[secretion]], where the substance may have specific tasks after leaving the [[Cell (biology)|cell]]. For example, [[placental mammals]] expel [[urine]] from the [[bladder]] through the [[urethra]],<ref name="Wake1992">{{cite book|author=Marvalee H. Wake|title=Hyman's Comparative Vertebrate Anatomy|url=https://books.google.com/books?id=VKlWjdOkiMwC&q=placental+mammal+urethra&pg=PA583|access-date=6 May 2013|date=15 September 1992|publisher=University of Chicago Press|isbn=978-0-226-87013-7|pages=583–}}</ref> which is part of the [[excretory system]]. [[Unicellular organism]]s discharge waste products directly through the surface of the cell.

During life activities such as [[cellular respiration]], several chemical reactions take place in the body. These are known as [[metabolism]]. These chemical reactions produce waste products such as [[carbon dioxide]], water, [[Salt (chemistry)|salts]], [[urea]] and [[uric acid]]. Accumulation of these wastes beyond a level inside the body is harmful to the body. The excretory organs remove these wastes. This process of removal of metabolic waste from the body is known as excretion.

== Processes across various types of life ==
{{Expand section|date=January 2025}}

=== Plants ===
Green [[plants]] excrete carbon dioxide and water as respiratory products. In green plants, the [[carbon dioxide]] released during respiration gets used during photosynthesis. [[Oxygen]] is a byproduct generated during [[photosynthesis]], and exits through [[stomata]], root cell walls, and other routes. Plants can get rid of excess water by transpiration and [[guttation]]. It has been shown that the leaf acts as an 'excretophore' and, in addition to being a primary organ of photosynthesis, is also used as a method of excreting toxic wastes via diffusion. Other waste materials that are exuded by some plants&nbsp;— [[resin]], saps, [[latex]], etc. are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells. These latter processes do not need added energy, they act passively. However, during the pre-abscission phase, the metabolic levels of a leaf are high.<ref>{{cite journal | vauthors = Ford BJ | title = Even plants excrete | journal = Nature | date = October 1986 | volume = 323 | issue = 6091 | page = 763 | doi = 10.1038/323763a0 | bibcode = 1986Natur.323..763F | s2cid = 4344886 |url=http://www.brianjford.com/wleaf03.htm | doi-access = free }}</ref><ref>{{cite book | chapter = Excretion | title = Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite.  | location = Chicago | publisher = Encyclopædia Britannica | date = 2010 }}</ref> Plants also excrete some waste substances into the soil around them.<ref>http://www.tutorvista.com/content/science/science-ii/excretion/excretion-plants {{dead link|date=November 2016}}</ref>

=== Animals ===
[[File:Uric Acid.svg|left|thumb|Chemical structure of [[uric acid]].]]

In animals, the main excretory products are [[carbon dioxide]], [[ammonia]] (in [[ammoniotelic]]s), [[urea]] (in [[ureotelic]]s), [[uric acid]] (in [[uricotelic]]s), [[guanine]] (in [[Arachnida]]), and [[creatine]]. The liver and kidneys [[clearance (medicine)|clear]] many substances from the [[blood]] (for example, in [[Renal physiology#Filtration|renal excretion]]), and the cleared substances are then excreted from the body in the [[urine]] and [[feces]].<ref>{{cite journal | vauthors = Weiner ID, Mitch WE, Sands JM | title = Urea and Ammonia Metabolism and the Control of Renal Nitrogen Excretion | journal = Clinical Journal of the American Society of Nephrology | volume = 10 | issue = 8 | pages = 1444–58 | date = August 2015 | pmid = 25078422 | pmc = 4527031 | doi = 10.2215/CJN.10311013 }}</ref>

[[Aquatic animal]]s usually excrete [[ammonia]] directly into the external environment, as this compound has high [[solubility]] and there is ample water available for dilution. In [[terrestrial animal]]s, ammonia-like compounds are converted into other nitrogenous materials, i.e. [[urea]], that are less harmful as there is less water in the environment and ammonia itself is [[toxic]]. This process is called detoxification.<ref>{{Cite web|title=Excretion - General features of excretory structures and functions|url=https://www.britannica.com/science/excretion|access-date=2021-02-05|website=Encyclopedia Britannica|language=en}}</ref>

==== Birds ====
[[File:Feces and uric-acid.jpg|thumb|250px|White cast of uric acid defecated along with the dark feces by a [[lizard]]. Insects, birds and some other reptiles also use a similar mechanism.]]

[[Bird]]s excrete their nitrogenous wastes as [[uric acid]] in the form of a paste. Although this process is metabolically more expensive, it allows more efficient [[water]] retention and it can be stored more easily in the [[egg (bird)|egg]]. Many avian species, especially [[seabird]]s, can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the [[beak]].{{Citation needed|date=January 2025}}

==== Insects ====
In [[insect]]s, a system involving [[Malpighian tubule]]s is used to excrete [[metabolic waste]]. Metabolic waste diffuses or is actively transported into the tubule, which transports the wastes to the intestines. The metabolic waste is then released from the body along with fecal matter.{{Citation needed|date=January 2025}}

The excreted material may be called ''ejecta''.<ref name="EOS1887">{{cite journal | vauthors = Carmichael J | title = Gastro-Intestinal Disorder in Sucklings | journal = The Transactions of the Edinburgh Obstetrical Society | volume = 12 | issue = | pages = 164–173, 169 | date = 1887 | pmid = 29613104 | pmc = 5487197 | doi = | url = | location = Edinburgh  | publisher = Oliver and Boyd  }}</ref> In [[pathology]] the word ejecta is more commonly used.<ref>{{cite book | chapter-url = https://www.oxfordreference.com/view/10.1093/acref/9780199891573.001.0001/acref-9780199891573-e-2074 | chapter = Ejecta | title = Oxford English Dictionary | publisher = Oxford University Press | edition = 2nd | date = 1989 }}</ref>

== See also ==
{{cmn|colwidth=30em|* [[Defecation]]
* [[Homeostasis]]
* [[Human excreta]]
* [[Osmoregulation]]
* [[Respiration (physiology)]]
* [[Sanitation]]
* [[Urination]]
}}

== References ==
{{Commons category}}
{{Reflist}}

== External links ==
{{Wiktionary|excretion}}
* [http://www.biology.ualberta.ca/courses.hp/zool250/animations/Excretion.swf UAlberta.ca], Animation of excretion
* [http://www.brianjford.com/wleaf03.htm Brian J Ford on leaf fall in Nature]

{{Authority control}}
{{Pharmacology}}

[[Category:Excretion| ]]
[[Category:Pharmacology]]
[[Category:Digestive system]]
[[Category:Metabolism]]
[[Category:Physiology]]
[[Category:Health]]
[[Category:Biology]]
[[Category:Biological processes]]