excretion

Excretion is the biological process of eliminating metabolic wastes and other substances that cannot be used by the body. It is distinct from secretion, which releases useful substances produced by cells. Excretion helps maintain internal homeostasis by regulating chemical composition, pH, and water balance.

In humans and many animals, major excretory organs include the kidneys, liver, lungs, and skin, with the digestive tract contributing waste elimination. The kidneys filter the blood, removing nitrogenous wastes such as urea, creatinine, and uric acid, while regulating water and electrolyte levels to form urine. The liver converts toxic ammonia to urea and produces bile pigments that are excreted in feces. The lungs eliminate carbon dioxide and water vapor, and the skin excretes salts and water through sweat.

Urine formation involves filtration at the glomerulus, reabsorption of needed substances in the tubules, and secretion of additional wastes. Urine travels via the ureters to the bladder and is excreted through the urethra during urination. These processes allow the body to control waste concentrations and fluid balance.

Across organisms, excretion can involve different waste products. For example, many aquatic animals excrete ammonia directly, while terrestrial animals convert it to urea or uric acid to conserve water. Plants also eliminate certain wastes and excess salts and gases, though excretion is often less focused on waste removal than on transport and storage of metabolites.

Clinical relevance: kidney dysfunction, dialysis, and liver disease illustrate the importance of excretory processes in health. Excretion also governs the elimination of drugs and their metabolites and is a key factor in managing hydration, pH, and toxin exposure.