The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). Animals that use this cycle, mainly amphibians and mammals, are called ureotelic.
The urea cycle converts highly toxic ammonia to urea for excretion.[1] This cycle was the first metabolic cycle to be discovered by Hans Krebs and Kurt Henseleit in 1932,[2][3][4] five years before the discovery of the TCA cycle. The urea cycle was described in more detail later on by Ratner and Cohen. The urea cycle takes place primarily in the liver and, to a lesser extent, in the kidneys.