Elimination is the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. Through excretion organisms control osmotic pressure--the balance between inorganic ions and water--and maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism's internal environment.

Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination, which may be considered to encompass all of the various mechanisms and processes by which life forms dispose of or throw off waste products, toxic substances, and dead portions of the organism. The nature of the process and of the specialized structures developed for waste disposal vary greatly with the size and complexity of the organism.

In human beings, the primary organs of elimination are the skin, which releases water, nitrogenous wastes and salts, the large intestine, which releases water and undigested food particles, and the kidneys which balance pH by controlling the release of hydronium ions, balance salt and water levels in the blood, and dispose of metabolic wastes like creatinine and urea.