The urea cycle converts ammonia into urea for excretion, measured as BUN.

“the nitrogen group here combines with a carbon dioxide and enters the urea cycle this is how the body gets rid of nh4 plus which is ammonia not a good thing to have hanging out in the human body it's uh fairly toxic to the human physiology [...] at the end of this product of this cycle is urea so what you'll see here is that a number of amino acids come in here citrulline aspartate comes in there are a number of enzymes in the cycle but urea is what's produced and a urea is a water-soluble form of this nitrogen which we excrete in the blood and the urine so if you get a blood work panel and you see bun that is your blood urea nitrogen”