What is "blood urea nitrogen"

BUN \BUN\ n. [acronym] (Med.) same as

blood urea nitrogen; the concentration of nitrogen in blood present in the form of urea; -- used as a measure of kidney function.

Note: Blood usually contains 10 to 15 mg of nitrogen per 100 ml in the form of urea.
--Stedman.

The liver produces urea in the urea cycle as a waste product of the digestion of protein. Normal human adult blood should contain between 6 and 20 mg of urea nitrogen per 100 ml (6–20 mg/ dL) of blood. Individual laboratories will have different reference ranges as the assay used will vary between laboratories.