Urea nitrogen is mainly excreted in urine through glomerular filtration of the kidney, and some secretion is also excreted from the renal tubules. When the renal parenchyma is damaged, the decrease of glomerular filtration rate will lead to the increase of urea nitrogen in the blood. Increased urea nitrogen is mostly seen in chronic renal failure caused by glomerulonephritis, pyelonephritis, renal tumor and polycystic kidney. However, due to the compensatory function of the kidneys, blood urea nitrogen cannot be used as an early indicator of renal impairment. For chronic renal failure, especially in uremia, the degree of increase in urea nitrogen is generally positively correlated with the severity of the disease. If there is severe dehydration, cardiac failure, severe ascites, hepatorenal syndrome and other causes of blood volume insufficiency, reduced renal blood flow to appear urine, then there will also be blood urea nitrogen increased, most of them can be reduced after dilatation. If the body protein decomposition or excessive intake of high fever, hemorrhage, massive burns, major surgery, high protein diet, etc. blood urea nitrogen can also be mildly elevated, to be alleviated by the primary condition of urea nitrogen value will also decline. High urea nitrogen can affect many situations, may be a reference indicator of renal impairment, may also indicate undercapacity, in addition, it is often used as a rough observation of the glomerular filtration function of the kidneys.