excretions

Excretion is the biological process of eliminating waste products of metabolism and other non-useful substances from an organism. It is distinct from secretion, which releases substances produced by cells for use elsewhere; excretion removes wastes from the body.

In animals, primary excretory organs include the kidneys, skin, lungs, and the digestive tract. The kidney’s nephrons filter blood, form urine, and regulate water and solute balance through reabsorption and secretion. The skin excretes salts and some wastes in sweat; the lungs expel carbon dioxide and water vapor; the digestive tract eliminates undigested material and certain waste products via feces.

Nitrogenous waste varies by lineage: ammonia is soluble but highly toxic and is typically excreted by aquatic animals; urea is less toxic and conserves water, as in mammals and many marine species; and uric acid is the least water-dependent, used by birds, reptiles, and many insects. Some organisms rely on specialized excretory systems such as Malpighian tubules in insects.

Excretion also involves other routes, such as the liver’s processing of waste products that are then excreted into bile and eliminated with feces, and the role of the digestive system in removing waste and toxins. Plants manage waste and excess minerals through various pathways, including root and leaf processes, though their excretory strategies differ from those of animals.

Excretion supports homeostasis by maintaining fluid and electrolyte balance, acid–base balance, and detoxification. Impairment of excretory function can lead to waste accumulation and disease, such as kidney failure. Overall, excretion is a fundamental biological process for removing metabolic byproducts and environmental toxins.