Urea nitrogen is the terminal product of protein metabolism. Amino acid gives birth to NH3 and CO2 through deamination and these two substances synthetize into urea within the Liver. Generally, one protein can produce .03g urea and the content of nitrogen accounts for nearly half of the urea. Kidneys are the main organ for urea excretion. urea is filtrated through glomerulus into renal tubules, and then renal tubules reabsorb a portion of the urea. The more quickly the urine flows in renal tubules, the less urea will be absorbed by renal tubules, and the more effectively will urea be excreted. The same as Serum Creatinine(High Creatinine Levels), Blood Urea Nitrogen increase only occurs when glomerular filtration rate decrease to less than half of the normal standard, in another word, the level of Blood Urea Nitrogen can stay within the normal standard during the early stages of renal diseases. Generally, the quantity ratio between Blood Urea Nitrogen and Serum Creatinine is 10 to 1. And protein-rich diet, high catabolism, Hydropenia, blood insufficiency of the kidneys, Hypovolemia and some kind of nephropathies can increase the ration to 20～30 to 1. This is called Hypernitremia(High Blood Pressure and Kidney Disease).
For adults, the empty stomach Blood Urea Nitrogen level should between 3.2—7.1mmol/L. various pathological changes of renal tissues, such as Glomerularnephritis, Interstitial nephritis, acute renal failure and other Renal occupying and damaging pathological changes. Generally speaking when indexes show that the Glomerular Filtration Rate is less than 50ml/min and the Blood Urea Nitrogen is within 9mmol/L, the disease has progressed into Renal Compensatory period, with Blood Urea Nitrogen being more than 9mmol/L but less than 20mmol/L in Renal Discompensation stage. And when Blood Urea Nitrogen increases to more than 20mmol/L, the disease has already progressed into Uremia.
As I have mentioned above, Blood Urea Nitrogen can be adopted as an index to measure the damage level of glomerulus and renal tubules, especially during the later stages of renal diseases. However, the increase of Blood Urea Nitrogen generally wont occur during the early stages of renal problems, so other symptoms(Kidney Disease Symptoms and Treatment) like protein urine, occult blood or hematuresis must be closely watched in order to detect renal problems promptly.
