The blood urea nitrogen (BUN) test measures the amount of nitrogen content in the blood derived from urea. Urea is an organic compound, which is secreted by the liver. Urea is not a toxic substance. The liver produces urea as a waste product during protein digestion in the body. Kidneys remove urea from the blood, which is excreted as urine.
Normal human blood of an adult contains 7 to 25 mg of urea nitrogen per 100 ml of blood.
  The increased level of BUN or Azotemia is caused by poor functioning of kidneys. The BUN level greater then 60 mg/decilitre indicates moderate to severe degree of renal failure. Renal urea excretion is also affected if a person is under shock, suffering from dehydration, and acute or chronic kidney diseases.

 Increased level of BUN, when creatinine level is normal, can be caused due to reduced blood flow to the kidney. This generally occurs in case of heart failure or dehydration. However, increase of BUN level in isolation may indicate excessive formation of urea without any compromise to the kidneys. The production of urea also increases due to moderate or heavy bleeding in the upper gastrointestinal tract caused by ulcers. Enhanced protein metabolism is caused by high protein diets, use of steroids, burns, and fevers, which enhances urea production.

 Accumulation of urea and other nitrogenous wastes due to renal failure causes neurological disturbances such as altered cognitive function, impaired taste, loss of appetite, and nausea, vomiting, or bleeding from dysfunctional platelets.

 A low BUN level does not cause significant problems. However, low BUN level is caused by malnutrition, liver problems, or excessive intake of alcohol. BUN content is also lowered during pregnancy due to changes in renal blood flow and excessive hydration from intravenous fluids.