Urea is a product of the degradation of amino acids. In the process amino acids lose their amine group which is then transformed into ammonia. In the liver ammonia reacts with carbon dioxide to form urea and water, a process called ureogenesis.
In the intermediary reactions of the ureogenesis a molecule of ornithine is consumed and another is produced. For this reason ureogenesis is also known as the ornithine cycle.
