Urea is a nitrogen containing compound which is formed in the body of mammals and gets excreted through their urine. It is synthesised in the liver as a byproduct of the oxidation of aminoacids or from ammonia, in the urea cycle.
Urea is made in the body as an excretory product which is a less toxic form of its precursor, ammonia. Urea formed in the liver is dissolved into the blood and carried to the kidneys to be expelled from the body along with the urine, thus being an efficient way to remove excess nitrogen from the body.
The urea levels in blood is determined by the BUN test (Blood Urea Nitrogen test). As the urea dissolved in the blood stream is continually filtered and removed by the kidneys, the urea levels are maintained at an average of 7 - 20 mg/100 ml.
A decreased BUN indicates liver failure or malnutrition. Other conditions like anabolic steriod intake, overhydration, or pregnancy may give a decreased urea level in blood. A severe liver failure affects the blood urea levels as liver is the site for urea synthesis. Decreased blood urea levels is known as hypouremia.