Nitrogen urea blood testing or eBay .Yu.(BUN) in medicine, the amount of urea present in the blood, defined by the liver in the urea cycle as a waste product digestive system or digestion of proteins can be produced. To determine some diseases have the the amount tested. In human blood, adult, and healthy, about 6 to 20 mg of nitrogen of urea per 100 ml (6-20 mg / dL) there.