excrétion

Excrétion, in biology, is the process by which organisms eliminate waste products produced by metabolism. It encompasses the removal of nitrogenous wastes such as ammonia, urea, and uric acid, as well as other metabolic by-products, excess water, and salts. Excrétion is distinct from secretion, which involves the production and release of substances that serve a physiological function rather than waste disposal.

In animals, excretory systems remove wastes from circulation. Vertebrates typically rely on kidneys to filter blood and form urine, concentrating wastes while regulating water and electrolyte balance. The liver contributes to waste processing by converting toxic ammonia to urea in mammals and by producing bile acids. The lungs excrete carbon dioxide and water vapor, and the skin releases small amounts of salts and organic compounds in sweat. Some invertebrates use simpler organs such as nephridia (annelids), Malpighian tubules (insects), or contractile vacuoles (protozoa) to expel wastes and maintain osmotic balance.

In plants and microorganisms, excrétion is less centralized. Plants primarily manage gas exchange and excretion of certain metabolites through stomata and other surfaces, while root exudates release compounds into the soil. Plants also release volatile secondary metabolites as part of defense or communication. Microorganisms excrete enzymes and metabolic by-products into their surroundings, influencing their environment and nutrient cycles. The study of excrétion highlights homeostasis and the adaptation of different lineages to manage internal waste.