Excretion

Excretion is the "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." ^*

In mammals, for example, the two major excretory processes are the formation of urine in the kidneys and the formation of carbon dioxide (a human's most abundant metabolic waste) molecules as a result of respiration, which is then exhaled from the lungs.

The waste products are eliminated by urination and exhalation respectively. In urination, Hormone control over excretion occurs in the distal tubules of the kidneys as directed by the hypothalamus.

Perspiration is another excretory process which removes salts and water, 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 stuffs.

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