blood urea nitrogen. Urea nitrogen is a waste product of protein metabolism. When the kidneys are working well they filter urea nitrogen from the blood stream and excrete it in the urine. When the kidneys are acutely injured, or in people with chronic kidney disease, urea nitrogen isn't effectively excreted and BUN levels rise. Some non-renal illnesses can also cause BUN levels to rise. These include (but aren't limited to): aging; eating a high-protein diet; having an upper gastrointestinal hemorrhage; pregnancy; and the use of some medications, such as steroids.