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Excretion is the process of eliminating waste products of metabolism and other non-useful materials.[1] 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.
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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,[2] 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 skin and lungs also have excretory functions: the skin eliminates metabolic wastes like urea and lactic acid through sweating,[3] 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.[1]
References
- ^ a b Beckett, B. S. (1986). Biology: A Modern Introduction. Oxford University Press, 110. ISBN 0199142602.
- ^ Tigerstedt, Dr. Robert (1906). A Text-book of Human Physiology. D. Appleton and Co., 384-390.
- ^ (Tigerstedt, pg. 395)
See also
External links
Wikipedia content modification information:
- This page was last modified on 12 August 2008, at 13:40.
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