excretion

excretion

[ik-skree-shuhn]
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 system), and the large intestines, from which solid wastes are expelled. The skin and lungs also have excretory functions: The skin eliminates water and salt in sweat, and the lungs expel water vapor and carbon dioxide.

Bodily process for disposing of undigested food waste products and nitrogenous by-products of metabolism, regulating water content, maintaining acid-base balance, and controlling osmotic pressure to promote homeostasis. It refers to both urination and defecation and to the processes that take place in the digestive and urinary systems, as the kidney and liver filter wastes, toxins, and drugs from the blood and food reaches the last stage of digestion. Ammonia from protein digestion, the primary excretory product, is converted to urea to be excreted in urine.

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Excretion is the process of eliminating waste products of metabolism and other non-useful materials. 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.

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, 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, and the lungs expel carbon dioxide.

Other

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.

References

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