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From Wikipedia, the free encyclopedia. The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions occurring in many animals that produces urea ((NH2)2CO) from ammonia (NH3). This cycle was the first metabolic cycle discovered (Hans Krebs and Kurt Henseleit, 1932), five years before the discovery of the TCA cycle. In mammals, the urea cycle takes place primarily in the liver, and to a lesser extent in the kidney.
The urea cycle. This cycle is active in the liver. The liver cleans the blood. It removes with this cycle ammonia (NH4 +) from the body in the form of urea. Urea gets excreted through the kidneys in the urine. The ammonia originates from the break down of amino acids. Amino acids are broken down when too much amino acids are present to be used directly.