Urea is also called carbonyl diamine, carboxamide and urea. It is an organic compound composed of carbon, nitrogen, oxygen and hydrogen, also known as urea (the homonym of urine). Its chemical formula is CON2H4, CO(NH2)2 or CN2H4O, and its international drug name is urea. The appearance is white crystal or powder. It is the product of protein metabolism in animals and is usually used as nitrogen fertilizer for plants. Urea is synthesized in the liver and filtered out by the kidney. It is a nitrogen-containing metabolite secreted by mammals. This metabolic process is called urea cycle. Urea is the first organic compound obtained by artificial synthesis of inorganic substances.