Blood urea nitrogen (BUN) measures the amount of urea nitrogen, a waste product of protein metabolism, in the blood. Urea is formed by the liver and carried by the blood to the kidneys for excretion.The amino acid deamination produces NH3 and C02, and which synthesis to urea in the liver.Per gram of protein metabolism of urea is 0.3g. The nitrogen have almost half content of 28/26 in the urea. Diseased or damaged kidneys cause an elevated BUN because the kidneys are less able to clear urea from the bloodstream. In conditions in which renal perfusion is decreased, such as hypovolemic shock or congestive heart failure, BUN levels rise.