Excretion is the process of eliminating waste products of
metabolismand 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.
In humans, the two major excretory processes are the formation of urine in the
kidneysand 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 humans the main organs of excretion are the
kidneys and accessory urinary organs, through which urineis 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 fecesis 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.
* "Mucociliary excretion" is the excretion of
mucusin the respiratory system.
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 ammoniadirectly into the external environment, as this compound has high solubilityand 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.
mammals excrete nitrogenous wastes in the form of urea, an ancestral trait. Birds excrete their nitrogenous wastes as uric acidin the form of a paste. This is metabolically more expensive, but allows more efficient waterretention 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. Perspirationis another excretory process which removes salts and water from the body, although the primary purpose is cooling.
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.
* [http://www.biology.ualberta.ca/courses.hp/zool250/animations/Excretion.swf Animation of excretion]
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