excretory

Excretory refers to the biological process of eliminating waste products of metabolism from an organism and maintaining internal fluid and electrolyte balance. Excretion is distinct from secretion, which involves the release of substances produced by cells for use elsewhere.

In animals, the excretory system typically includes the kidneys, along with supporting organs such as the liver, lungs, and skin. The kidney contains nephrons that filter blood, reabsorb essential solutes, and secrete wastes to form urine. Through filtration at the glomerulus and processing along the renal tubules, it maintains water and electrolyte balance and removes nitrogenous wastes such as urea.

Beyond urine, excretion occurs via other routes: the lungs expel carbon dioxide and water vapor; the skin releases salts in sweat; and the digestive system eliminates undigested material as feces.

Invertebrate excretory systems vary: flatworms use protonephridia, while annelids employ metanephridia, and many insects rely on Malpighian tubules. Nitrogenous wastes range from ammonia in aquatic species to urea or uric acid in terrestrial forms.

Excretion maintains homeostasis and health. Impaired excretory function, such as kidney failure, disrupts fluid balance and waste removal, with potential systemic effects.