Excretion

Excretion is the process of eliminating waste products of metabolism and other non-useful materials.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 ] 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 single-celled organisms, waste products are discharged directly through the surface of the cell. Multicellular organisms utilize more complex excretory methods. Higher plants 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 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

In humans the main organs of excretion are the kidneys and accessory urinary organs, through which urine is eliminated,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 = ] and the large intestines, from which solid wastes are expelled. In strict biological terminology, the expulsion of feces is not considered to be excretion, since feces is indigestible food, and not metabolic waste. The skinand lungs also have excretory functions: the skin eliminates metabolic wastes like urea and lactic acid through sweating, [(Tigerstedt, pg. 395)] and the lungs expel carbon dioxide.

Other

* "Mucociliary excretion" is the excretion of mucus in the respiratory system.

Non-human

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 energy-trapping structure, is also a plant's organ of excretion.

Aquatic animals usually excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution. In terrestrial animals ammonia-like compounds are converted into other nitrogenous materials as there is less water in the environment and ammonia itself is toxic.

Most mammals excrete nitrogenous wastes in the form of urea, an ancestral trait.
Birds 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. Many avian species, especially seabirds, 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 insects, a system involving Malpighian tubules 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.

References

See also

* Countercurrent exchange
* Eructation
* Flatulence
* Homeostasis
* Osmoregulation
* Respiration (physiology)

External links

* [http://www.biology.ualberta.ca/courses.hp/zool250/animations/Excretion.swf Animation of excretion]


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  • excrétion — [ ɛkskresjɔ̃ ] n. f. • 1534; bas lat. excretio « action de séparer » ♦ Physiol. 1 ♦ Action par laquelle les déchets de l organisme sont rejetés au dehors. Excrétion des matières fécales, de l urine. ⇒ élimination, évacuation, expulsion. « Dans… …   Encyclopédie Universelle

  • Excretion — Excrétion Cette vache urine pour vider sa vessie. L excrétion (du verbe excréter) désigne l action par laquelle des substances sécrétées (des déchets d un métabolisme pour la plupart) d un organism …   Wikipédia en Français

  • Excretion — Ex*cre tion, n. [Cf. F. excr[ e]tion.] 1. The act of excreting. [1913 Webster] To promote secretion and excretion. Pereira. [1913 Webster] 2. That which is excreted; excrement. Bacon. [1913 Webster] …   The Collaborative International Dictionary of English

  • excretion — c.1600, from Fr. excrétion (16c.), from L. excretionem, noun of action from pp. stem of excernere to discharge (see EXCREMENT (Cf. excrement)) …   Etymology dictionary

  • Excretion — (v. lat.), so v.w. Aussonderung, Ausleerung (s.d.); daher Excretoria organa, Aussonderungsorgane; Excretorius ductus, Ausführungsgang …   Pierer's Universal-Lexikon

  • excretion — [eks krē′shən, ikskrē′shən] n. [VL excretio] 1. the act or process of excreting 2. waste matter excreted; sweat, urine, etc …   English World dictionary

  • excretion — excretion1 /ik skree sheuhn/, n. 1. the act of excreting. 2. the substance excreted, as urine or sweat, or certain plant products. [1595 1605; < LL excretion (s. of excretio) that which is sifted out. See EXCRETE, ION] excretion2 /ik skree… …   Universalium

  • Excrétion — Cette vache urine pour vider sa vessie. L excrétion (du verbe excréter) désigne l action par laquelle des substances sécrétées (des déchets d un métabolisme pour la plupart) d un organisme vivant sont rejetés au dehors sous forme …   Wikipédia en Français

  • excrétion — (èk skré sion) s. f. Terme de physiologie. Action par laquelle certains organes creux rejettent au dehors les matières liquides ou solides qu ils contiennent. L excrétion des matières fécales L excrétion de l urine. L excrétion de la salive, du… …   Dictionnaire de la Langue Française d'Émile Littré

  • Excretion — (Roget s Thesaurus) < N PARAG:Excretion >N GRP: N 1 Sgm: N 1 excretion excretion discharge emanation Sgm: N 1 exhalation exhalation exudation extrusion secretion effusion extravasation ecchymosis Sgm: N 1 evacuation …   English dictionary for students


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