Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. For example, placental mammals expel urine from the bladder through the urethra, which is part of the excretory system. Unicellular organisms 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, 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
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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 — 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. Plants also excrete some waste substances into the soil around them.
Animals
In animals, the main excretory products are carbon dioxide, ammonia (in ammoniotelics), urea (in ureotelics), uric acid (in uricotelics), guanine (in Arachnida), and creatine. The liver and kidneys clear many substances from the blood (for example, in renal excretion), and the cleared substances are then excreted from the body in the urine and feces.
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, 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.
Birds
Birds 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. Many avian species, especially seabirds, can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the beak.[citation needed]
Insects
In insects, a system involving Malpighian tubules 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]
The excreted material may be called ejecta. In pathology the word ejecta is more commonly used.
See also
- Defecation
- Homeostasis
- Human excreta
- Osmoregulation
- Respiration (physiology)
- Sanitation
- Urination
References
- Beckett BS (1987). Biology: A Modern Introduction. Oxford University Press. p. 110. ISBN 0-19-914260-2.
- Marvalee H. Wake (15 September 1992). Hyman's Comparative Vertebrate Anatomy. University of Chicago Press. pp. 583–. ISBN 978-0-226-87013-7. Retrieved 6 May 2013.
- Ford BJ (October 1986). "Even plants excrete". Nature. 323 (6091): 763. Bibcode:1986Natur.323..763F. doi:10.1038/323763a0. S2CID 4344886.
- "Excretion". Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica. 2010.
- http://www.tutorvista.com/content/science/science-ii/excretion/excretion-plants [dead link]
- Weiner ID, Mitch WE, Sands JM (August 2015). "Urea and Ammonia Metabolism and the Control of Renal Nitrogen Excretion". Clinical Journal of the American Society of Nephrology. 10 (8): 1444–58. doi:10.2215/CJN.10311013. PMC 4527031. PMID 25078422.
- "Excretion - General features of excretory structures and functions". Encyclopedia Britannica. Retrieved 2021-02-05.
- Carmichael J (1887). "Gastro-Intestinal Disorder in Sucklings". The Transactions of the Edinburgh Obstetrical Society. 12. Edinburgh: Oliver and Boyd: 164–173, 169. PMC 5487197. PMID 29613104.
- "Ejecta". Oxford English Dictionary (2nd ed.). Oxford University Press. 1989.
External links
- UAlberta.ca, Animation of excretion
- Brian J Ford on leaf fall in Nature
Excretion is elimination of metabolic waste which is an essential process in all organisms In vertebrates this is primarily carried out by the lungs kidneys and skin This is in contrast with secretion where the substance may have specific tasks after leaving the cell For example placental mammals expel urine from the bladder through the urethra which is part of the excretory system Unicellular organisms discharge waste products directly through the surface of the cell Mammals excrete urine through the urinary system 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 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 lifeThis section needs expansion You can help by adding to it 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 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 Plants also excrete some waste substances into the soil around them Animals Chemical structure of uric acid In animals the main excretory products are carbon dioxide ammonia in ammoniotelics urea in ureotelics uric acid in uricotelics guanine in Arachnida and creatine The liver and kidneys clear many substances from the blood for example in renal excretion and the cleared substances are then excreted from the body in the urine and feces 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 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 Birds 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 Birds 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 Many avian species especially seabirds can also excrete salt via specialized nasal salt glands the saline solution leaving through nostrils in the beak citation needed Insects In insects a system involving Malpighian tubules 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 The excreted material may be called ejecta In pathology the word ejecta is more commonly used See alsoDefecation Homeostasis Human excreta Osmoregulation Respiration physiology Sanitation UrinationReferencesWikimedia Commons has media related to Excretion Beckett BS 1987 Biology A Modern Introduction Oxford University Press p 110 ISBN 0 19 914260 2 Marvalee H Wake 15 September 1992 Hyman s Comparative Vertebrate Anatomy University of Chicago Press pp 583 ISBN 978 0 226 87013 7 Retrieved 6 May 2013 Ford BJ October 1986 Even plants excrete Nature 323 6091 763 Bibcode 1986Natur 323 763F doi 10 1038 323763a0 S2CID 4344886 Excretion Encyclopaedia Britannica Encyclopaedia Britannica Ultimate Reference Suite Chicago Encyclopaedia Britannica 2010 http www tutorvista com content science science ii excretion excretion plants dead link Weiner ID Mitch WE Sands JM August 2015 Urea and Ammonia Metabolism and the Control of Renal Nitrogen Excretion Clinical Journal of the American Society of Nephrology 10 8 1444 58 doi 10 2215 CJN 10311013 PMC 4527031 PMID 25078422 Excretion General features of excretory structures and functions Encyclopedia Britannica Retrieved 2021 02 05 Carmichael J 1887 Gastro Intestinal Disorder in Sucklings The Transactions of the Edinburgh Obstetrical Society 12 Edinburgh Oliver and Boyd 164 173 169 PMC 5487197 PMID 29613104 Ejecta Oxford English Dictionary 2nd ed Oxford University Press 1989 External linksLook up excretion in Wiktionary the free dictionary UAlberta ca Animation of excretion Brian J Ford on leaf fall in Nature
