Urea is a product of protein metabolism in the body, mainly synthesized in the liver, produced and dissolved in the blood, and eventually excreted by urine through the kidneys and a small amount through sweat. The roles of urea are: first, to maintain the urea cycle and eliminate ammonia from the body, which is excreted from the body in the form of urea; second, urea in the renal tubules is introduced into the renal cortex to increase the osmotic concentration of the renal cortex and promote water infiltration from the renal tubules back to the body for utilization; third, clinical monitoring of blood urea can be a rough observation of glomerular filtration function; fourth, the degree of increase in blood urea can be staged for chronic renal failure, etc.