Útskili

Útskili is the biological process by which an organism eliminates waste products produced by metabolism. In humans and many other animals, it is a key component of homeostasis, helping to regulate the chemical composition of body fluids, pH, and fluid balance.

The main routes of excretion are the urinary system (kidneys producing urine), the digestive system (feces from the gut), the integument (sweat), and the respiratory system (exhaled air). Nitrogenous wastes arise from protein breakdown and include ammonia, which is often converted to urea or uric acid for safe elimination. In aquatic animals ammonia is excreted directly; in mammals, urea is predominant; birds and reptiles mainly excrete uric acid.

Mechanisms: in the kidneys, filtration in the glomeruli, along with reabsorption and secretion in tubules, concentrates wastes into urine. In the liver, toxins may be transformed and excreted in bile. The gut eliminates waste material and bilirubin via feces. The lungs expel carbon dioxide and water vapor, and the skin releases salts and other substances in sweat.

Clinical relevance: measurement of excretory function (for example, urine composition, creatinine clearance, and glomerular filtration rate) is central in diagnosing kidney or liver disorders. Decreased excretion can lead to toxin buildup; excessive excretion can indicate other problems. Excretion varies across species and environments, reflecting adaptations to conserve water or manage toxins.