excretion

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Excretion is the transfer of substances out of a living organism into its environment. At its simplest, for single-cell forms of life, this involves extrusion across the cell membrane of the unwanted or potentially toxic by-products of respiration and metabolism. This is also what is happening continually in the individual cells of the animal body, but from their immediate environment substances must move into the blood to be carried away to the site of their ultimate disposal. In the animal body there is also another type of excretion: expulsion of the residue of substances which have not been absorbed into the body proper from the gut (which can be considered a tunnel through the body of the external world).

In human terms ‘excreta’ normally refers only to urine and faeces, whereas the definition of excretion would also include both carbon dioxide and heat, and these will be considered first.

Carbon dioxide

carbon dioxide (CO₂), along with water, is the end-product in cells which use oxygen to release their energy supply from food sources, and those cells are in the vast majority. If this CO₂ were to accumulate the cells would become too acidic for their internal chemistry to proceed. Continual generation of CO₂ maintains a concentration gradient from inside to outside so that it moves by diffusion out of the cells into the surrounding fluid, and thence into the blood in the nearby capillaries. So the blood picks up CO₂ as it circulates, until it converges from the whole body into the right side of the heart, carrying an amount of CO₂ which varies with the total rate of energy release by body cells. This venous blood, low in oxygen and high in CO₂, is pumped through the lungs, where CO₂ is excreted by the reverse process to that of its uptake from cells — it diffuses out down a gradient, because breathing keeps the concentration lower in the gas in the lungs than it is in the incoming blood.

Heat

Heat is continually generated by resting metabolic activity, and to a much greater extent by working muscles. Unless conservation of body heat is required in cold conditions to maintain body temperature, it is ‘excreted’ from the surface of the body when there is a temperature gradient from the skin to the environment. This gradient, and therefore heat loss, is regulated by the mechanisms for temperature regulation: dilation of skin blood vessels brings heat to the surface and increases the gradient; when this mechanism is inadequate, sweating comes into play as well.

Excretion in the urine

The kidneys are responsible for filtering off a continual sample of the watery component of the blood plasma, with its solutes, at a rate equivalent to the whole of the plasma volume about every twenty minutes. The further processes within the kidneys could be likened to ‘quality control’ and correction. Not only the filtered water, but also many dissolved substances, are largely reabsorbed, but the reabsorption is fine-tuned according to any need for correction of the blood composition; nitrogenous waste (mainly urea) from protein breakdown is allowed to escape, and waste acid (H⁺) and other substances present in excess are actively secreted into the urine. The end result is production of urine at a variable rate depending on fluid intake, but on average less than one-hundredth the rate of filtration of fluid from the blood, and containing all that needs to be excreted minute by minute.

Excretion from the bowel

That which is voided consists of the residue that remains after digestion and absorption of food breakdown products in the stomach and small intestine, and after absorption of most of the remaining water in the large intestine. This is also the route for voiding of cholesterol, excreted by the liver into the bile. The colour of the faeces is derived from bile pigments: although these are recycled to a large extent, the remainder becomes stercobilin and leaves by this route.

— Sheila Jennett

See also faeces; urine.

noun

The act or process of discharging bodily wastes or foreign substances: elimination, evacuation, purgation. Medicine catharsis. See keep/release.

The elimination of metabolic wastes (including carbon dioxide and urea) from the body.

1. the act, process or function of excreting.
2. material that is excreted.
Ordinarily, what is meant by excretion is the evacuation of feces. Technically, excretion can refer to the expulsion of any matter, whether from a single cell or from the entire body, or to the matter excreted.

• virus e. — see shedding.

• Digestive and Excretory System - excretion: discharge of waste products from body
• Physiology - excretion: loss of substance through urine, feces, or other material expelled from body

Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin.^[1] This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. Excretion is an essential process in all forms of life.

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.

Contents 1 Non-mammalian 2 See also 3 References 4 External links

Non-mammalian

Chemical structure of uric acid.

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, and exits through stomata, root cell walls, and other routes. Plants can get rid of excess water by transpiration and guttation. 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. These processes do not need added energy, they act passively.^[2] Plants also excrete some waste substances into the soil around them.^[3]

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.

White cast of uric acid defecated with the dark feces from a lizard. Insects, birds and some other reptiles also undergo a similar mechanism.

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][free/online citation requested]}

See also

• Countercurrent exchange
• Eructation
• Homeostasis
• Osmoregulation
• Respiration (physiology)

References

Wikimedia Commons has media related to: Excretion

1. ^ ^{a b} Beckett, B. S. (1987). Biology: A Modern Introduction. Oxford University Press. pp. 110. ISBN 0-19-914260-2. 
2. ^ "excretion." Encyclopædia Britannica. Encyclopaedia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010
3. ^ Tutorvista.com

External links

Look up excretion in Wiktionary, the free dictionary.

• UAlberta.ca, Animation of excretion

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