Final answer:
Urea is formed when amino acids are catabolized to remove nitrogen from the body. The liver synthesizes urea from ammonia and carbon dioxide, and it is excreted in urine.
Step-by-step explanation:
When amino acids are catabolized, urea is formed to remove nitrogen from the body. The amino group of amino acids is converted into ammonia, which is toxic and needs to be eliminated. The liver synthesizes urea from ammonia and carbon dioxide and it is excreted in urine.