Urea is the chief nitrogenous waste of mammals. Most of nitrogenous waste comes from the breakdown of amino acids. This occurs by deamination. Deamination of amino acids results in the production of ammonia (NH3). Ammonia is an extremely toxic base and its accumulation in the body would quickly be fatal. However, the liver contains a system o f carrier molecules and enzymes which quickly converts the ammonia (and carbon dioxide) into urea. This is called the urea cycle.