Urea comprises of nitrogen, carbon, hydrogen and oxygen. It is a waste product of protein catabolism and is eliminated from the blood through the kidneys. Nitrogen is removed from the body in the form of ammonia, in the case of aquatic animals and in the form of urea and uric acid in the land animals.

Blood urea

Liver malfunction results in decreased blood urea. It is a waste product, that is resorbed by the medulla of the kidneys. It is mixed in the blood and eliminated in the urine at an intensity of 2.5 to 7.5 mmol per litre. Birds and reptiles eliminate uric acid, while mammals and others excrete urea. It is utilized to decrease volume. Minute quantities of urea are eliminated by sweat. Urea, chemically is a fertilizer, and utilized in many plant enhancement preparations.

The quantity of nitrogen in the urea is estimated by blood urea nitrogen (BUN). Urea nitrogen in the blood is an indicator of kidney function, though changes in the blood urea nitrogen levels are seen in hepatic failure and catabolism of proteins. The adult BUN levels range from 7 to 20 mg/dl with the values increasing in senescence. The BUN values reduce by 25%, during pregnancy. The newborn infants have a lesser level than adults.

A reduction in blood urea nitrogen levels occur in the case of malnourishment, hepatic failure, nutrient malabsorption, pregnancy, consumption of anabolic steroids and over hydration. In the case of an individual suffering from "syndrome of inappropriate anti-diuretic secretion" (SIADH), the (ADH) or the anti-diuretic hormone which triggers the kidney to save water results in retention of excessive water in the blood. This results in low BUN levels and other vital substances, as the fluid capacity of blood raises. The normal range for BUN values are 7 to 20 mg/dl. This test is conducted to evaluate the kidney functioning capacity, treatment given and the presence of dehydration.

Blood urea nitrogen is evaluated with creatinine test. Serum creatinine proves helpful in diagnosing kidney failure and diseases. A directly proportional relationship exists between creatinine levels and kidney function. Both the blood urea nitrogen and creatinine levels are helpful in determining the BUN to creatinine ratio. This test is a diagnostic tool in identifying the causative factor for abnormal levels, such as dehydration, as it results in higher BUN to creatinine ratio. This is attributed to the fact, that BUN levels are greater than creatinine levels, during dehydration. Both BUN and creatinine levels are raised in the case of kidney malfunctioning or urinary flow damage.