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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.
Urea is a waste product made when protein is broken down in your body. Urea is made in the liver and passed out of your body in the urine. The levels of blood creatinine and blood urea nitrogen (BUN) can be used to find the BUN-to-creatinine ratio.