excretion

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.

excretion Elimination of materials from the body that have been involved in metabolism. Such waste materials, particularly nitrogenous wastes, would be toxic if allowed to accumulate. In mammals, these wastes excrete mainly as urine, and to some extent by sweating. Carbon dioxide excretes through the lungs during breathing. Defaecation is not excretion, as faeces consist mostly of material that has never been part of the body.