excretion

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excretion,

process of eliminating from an organism waste products of metabolism and other materials that are of no use. It is an essential process in all forms of life. In one-celled organisms wastes are discharged through the surface of the cell. The higher plants eliminate gases through the stomata, or pores, on the leaf surface. Multicellular animals have special excretory organs. In humans the main organs of excretion are the kidneys and accessory urinary organs, through which urine is eliminated (see urinary systemurinary system,
group of organs of the body concerned with excretion of urine, that is, water and the waste products of metabolism. In humans, the kidneys are two small organs situated near the vertebral column at the small of the back, the left lying somewhat higher than the
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), and the large intestinesintestine,
muscular hoselike portion of the gastrointestinal tract extending from the lower end of the stomach (pylorus) to the anal opening. In humans this fairly narrow (about 1 in./2.
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, from which solid wastes are expelled. The skin and lungs also have excretory functions: The skin eliminates water and salt in sweatsweat
or perspiration,
fluid secreted by the sweat glands of mammalian skin and containing water, salts, and waste products of body metabolism such as urea. The dissolved solid content of sweat is only one eighth that of an equal volume of urine, the body's main
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, and the lungs expel water vapor and carbon dioxide.

Excretion

 

the freeing of the organism from the end products of metabolism as well as from the foreign substances and excess water, salts, and organic compounds that have entered with food or were formed in the organism. As a result of metabolism, carbon dioxide, certain amino acids, urea, and other substances leave the cell and enter the inter-cellular fluid and then the blood. When excess salts or food substances are consumed or when metabolism is impaired, there is increased concentration of inorganic or organic substances in the blood (for example, glucose and amino acids). The organs of excretion have an important role in keeping constant the composition of the fluids of the internal environment (homeostasis). The process of excretion in vertebrates involves the kidneys, lungs or gills, glands of the gastrointestinal tract, skin, and sweat and salt glands (nasal, rectal). In invertebrates the process involves protonephridia, meta-nephridia, gills, and other organs; in protozoans the excretory organs are the contractile vacuoles. In some animals the products of metabolism and salts are deposited in accumulatory organs or integumentary tissues, which are discarded during molting. Excretion of carbon dioxide and other volatile substances occurs through the lungs or gills. Water, salts, and products of nitrogen metabolism (ammonia, urea, and uric acid) are mainly excreted by the kidneys. Sodium salts are excreted by seabirds and reptiles through the nasal glands and by fish through gills or the rectal gland. A human being weighing 70 kg and eating a normal diet discharges from 10,000 to 20,000 millimoles of carbon dioxide in a 24-hour period through the lungs, while nonvolatile mineral and organic acids and just 1-2 millimoles of bicarbonates are re-moved in the urine. The excretion of water is 1.2 liters in urine, 0.5 liter in sweat, and 0.1 liter in feces. The total quantity of nitrogen excreted in urine is 11 grams, in feces 1.7 grams, and in sweat 1 gram. In a 24-hour period the kidneys discharge 21 grams of urea, 0.63 grams of uric acid, 0.56 grams of hippuric acid, 1.05 grams of creatinine, and 0.78 grams of ammonia.

REFERENCES

Prosser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translation from English.)

IU. V. NATOCHIN

excretion

[ek′skrē·shən]
(physiology)
The removal of unusable or excess material from a cell or a living organism.
References in periodicals archive ?
The clear difference in relationship between size of excretion and time confined accords with the spiders excreting more regularly in the field than under confined laboratory conditions.
McNeill compares us to cyanobacteria, which refashioned the world's atmosphere two billion years ago by excreting oxygen and increasing its presence in the air from one part per trillion to the current one-fifth.
First spotted in the Los Angeles region in 1998, the lerp has spread rapidly to thousands of eucalyptus trees, damaging and even killing them while excreting a sticky goo that gums up people and cars.
And keep in mind that extra calcium could increase the risk of kidney stones in people who are already excreting calcium in their urine.