In ureotelic animals, urea is formed by:

Explanation

Urea is formed of two molecules of ammonia and one molecule of CO 2 . During catabolism, proteins break up into amino acids. The amino acids are converted into ketoacids (such as aketoglutaric acid) with the removal of ammonia (deamination). The ketoacid enters the Krebs cycle to yield energy. Carbon dioxide is formed during decarboxylation. Ornithine (amino acid) combines with one molecule of NH 3 and CO 2 producing citrulline (amino acid) and water. Citrulline combines with another molecule of ammonia and forms arginine (essential amino acid) and water. Arginine is broken into urea and ornithine in the presence of an enzyme arginase and water. Thus the cycle is repeated. This is called urea or ornithine or KrebsHenseleit cycle after the names of its discoverers. Most of the urea is produced in the liver. The liver cells continuously release urea into the blood and kidneys withdraw it from the blood to excrete it in urine.