In medical terms BUN stands for urea nitrogen, which is a metabolic waste produced by the body and excreted through the kidneys. There are more than one hundred kinds of metabolic wastes known to be excreted through the kidneys. These metabolic wastes are dissolved in urine and excreted out of the body to maintain the stability of the body’s internal environment, and these substances are maintained at relatively low levels in the body. Urea nitrogen, a representative of these metabolic wastes, is a nitrogen-containing metabolism that is produced by muscles. The accumulation of these metabolic wastes in the body means that the kidneys have a reduced ability to excrete them, which is commonly referred to as renal failure. Therefore, these metabolic wastes, creatinine or urea nitrogen, are clinically selected as representatives and blood tests are drawn to reflect the state of kidney function.