Also called carbamide, urea is a colorless, crystalline compound. It is the chief nitrogenous end product resulting from the metabolic breakdown of proteins in all mammals and some fishes. Commercially, urea is used as a fertilizer and feed supplement and is a base material for the manufacture of plastics and drugs. Urea functions as a stabilizer in nitrocellulose explosives and is a constituent of synthetically prepared resins.
Urea occurs not only in the urine of all mammals but also in their blood, bile, milk, and perspiration. During protein metabolism, amino groups (NH2) are removed from the amino acids and converted to ammonia (NH3). Ammonia is toxic to the body and is converted to urea by the liver. The urea then passes to the kidneys and is eventually excreted in the urine. The compound was first isolated from urine in 1773 and synthesized in 1828.
Excess nitrogen in the body is excreted in one of three forms: ammonia (as the ammonium ion), urea, and uric acid. Animals, such as fish, that live in the water excrete nitrogen as ammonia, which is quickly diluted by the aqueous environment. In terrestrial animals, the primary waste product of nitrogen metabolism is urea, a water-soluble compound. Birds excrete excess nitrogen in the form of uric acid, an insoluble chemical that allows birds to remove nitrogen without the use of water.