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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Mammalian excretion
In mammals, the excretory processes are the formation of urine in the kidneys and the formation of carbon dioxide (a mammal'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. If excretion does not occur in an organism, waste products will accumulate, which will sicken and eventually kill the organism.
In kidney
In mammals 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, undigested food expelled in the feces is not considered to be excretion, since it is not metabolic waste. Substances secreted into the bile and then eliminated in the feces are considered to be excreted, however. 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.
- Biliary excretion occurs via the bile which is delivered to the duodenum and removed in the feces.
- Perspiration is the excretion of salts and water from the body, although the primary purpose is to cool the body.
Non-mammalian
In plants, breakdown of substances is much slower than in animals. Hence accumulation of waste is much slower and there are no special organs of excretion. Green plants in darkness or plants that do not contain chlorophyll produce carbon dioxide and water as respiratory waste products. Carbon dioxide released during respiration gets utilized during photosynthesis.Oxygen itself can be thought of as a waste product generated during photosynthesis. Plants can get rid of excess water by transpiration. Waste products may be stored in leaves that fall off. Other waste materials that are exuded by some plants — resins, saps, latexes, etc. are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells. Plants also excrete some waste substances into the soil around them.[4] Excreta is brown in colour.
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.
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.
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.
Many people misuse the term excretion as a euphemism for defecation, and use excrement for feces, but this is biologically incorrect.[1]
See also
References
- ^ a b Beckett, B. S. (1986). Biology: A Modern Introduction. Oxford University Press. pp. 110. ISBN 0199142602 [Amazon-US | Amazon-UK].
- ^ Tigerstedt, Dr. Robert (1906). A Text-book of Human Physiology. D. Appleton and Co.. pp. 384–390.
- ^ (Tigerstedt, pg. 395)
- ^ Tutorvista.com
External links
- UAlberta.ca, Animation of excretion
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