Excretion
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[[Image:Gray1120.png|right|thumb|The kidneys are important excretory organs in vertebrates.]]
'''Excretion''' is the process of eliminating waste products of [[metabolism]] and other non-useful materials.<ref name='beckett'>{{cite book | last = Beckett | first = B. S. | authorlink = | coauthors = | title = Biology: A Modern Introduction | publisher = Oxford University Press | date = 1986 | location = | pages = 110 | url = | doi = | id = | isbn = 0199142602 }}</ref> It is an essential process in all forms of life. It contrasts [[secretion]], where the substance may have specific tasks after leaving the cell.
In [[Microorganism|single-celled organisms]], waste products are discharged directly through the surface of the [[Cell (biology)|cell]]. [[Multicellular organism]]s utilize more complex excretory methods. Higher [[plant]]s eliminate gases through the [[stomata]], or pores, on the surface of leaves. Animals have special excretory organs.
==Human excretion==
In humans, the two major excretory processes are the formation of urine in the [[kidneys]] and the formation of [[carbon dioxide]] (a human's abundant [[metabolism|metabolic]] waste) molecules as a result of respiration, which is then exhaled from the lungs. These waste products are eliminated by urination and exhalation respectively. In urination, hormonal control over excretion occurs in the distal tubules of the kidneys as directed by the hypothalamus.
===In kidney===
{{Main|Clearance (medicine)}}
In humans the main organs of excretion are the [[kidney]]s and [[urinary system|accessory urinary organs]], through which [[urine]] is eliminated,<ref name='tigerstedt'>{{cite book | last = Tigerstedt | first = Dr. Robert | authorlink = | coauthors = | title = A Text-book of Human Physiology | publisher = D. Appleton and Co. | date = 1906 | location = | pages = 384-390 | url = | doi = | id = | isbn = }}</ref> and the [[large intestine]]s, from which [[feces|solid wastes]] are expelled. In strict biological terminology, the expulsion of [[faeces|feces]] is not considered to be excretion, since faeces is indigestible food, and not metabolic waste. The [[skin]] and [[lung]]s also have excretory functions: the skin eliminates water and [[salt]]s in [[sweat]],<ref>(Tigerstedt, pg. 395)</ref> and the lungs expel [[water vapor]] and [[carbon dioxide]].
===Other===
*''Mucociliary excretion'' is the excretion of [[mucus]] in the [[respiratory system]].
==Non-human==
[[Image:Uric acid.png|left|thumb|Chemical structure of uric acid.]]
Plants have been shown (by British biologist [[Brian J. Ford]]) to translocate wastes into leaves which are then shed. In this fashion, the leaf, in addition to acting as an [[Photosynthesis|energy-trapping structure]], is also a plant's organ of excretion.
[[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 as there is less water in the environment and ammonia itself is [[toxic]].
Most [[mammal]]s excrete [[nitrogen]]ous wastes in the form of [[urea]], an ancestral trait.
[[Bird]]s excrete their nitrogenous wastes as [[uric acid]] in the form of a paste. This is metabolically more expensive, but 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]].
[[Perspiration]] is another excretory process which removes salts and water from the body, although the primary purpose is cooling.
In [[insect]]s, a system involving [[Malpighian tubule]]s is utilized 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.
==Etymology==
Many people misuse the term excretion as a euphemism for [[defecation]], and use excrement for [[feces]], but this is medically inexact.<ref name='beckett'/>
== References ==
{{reflist}}
==See also==
{{Commonscat}}
{{wiktionary|excretion}}
*[[Countercurrent exchange]]
*[[Eructation]]
*[[Flatulence]]
*[[Homeostasis]]
*[[Osmoregulation]]
*[[Respiration (physiology)]]
==External links==
*[http://www.biology.ualberta.ca/courses.hp/zool250/animations/Excretion.swf Animation of excretion]
[[Category:Digestive system]]
[[Category:Metabolism]]
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