The elimination by an organism of the waste products that arise as a result of metabolic activity. These products include water, carbon dioxide, and nitrogenous compounds. Excretion plays an important role in maintaining the constancy of an organism’s internal environment (see homeostasis). The simplest method of excretion, which occurs, for example, in plants, involves diffusion of waste products from the body, but many animals have specialized organs and organ systems devoted to this function (see malpighian tubule; nephridium). Examples of excretory organs in vertebrates are the lungs (for carbon dioxide and water), and the kidneys (for nitrogenous waste and water). In addition, mammals excrete small amounts of urea, salts, and water from the skin in sweat. Removal of excess salt from the blood is accomplished by rectal glands in elasmobranch fishes, and by salt glands in marine birds and reptiles.