After being produced in the liver, urea merges into the blood (the concentration in the human body is between 2.5 and 7.5 micromoles per liter), and is finally excreted in the urine through the kidneys. A small amount of urea is excreted by sweat.
Biology uses carbon dioxide, water, aspartic acid and ammonia to synthesize urea. The metabolic pathway that promotes the synthesis of urea is an anabolism called the urea cycle. This process consumes energy, but it is necessary. Because ammonia is toxic and a common metabolite, it must be eliminated. When the liver synthesizes urea, it needs N-acetylglutamate as a regulation.
Nitrogen-containing wastes are toxic and are produced by the catabolism of proteins and amino acids
Amino action is a process in which amino acids are removed. The nitrogen-containing compounds produced by this process are converted into urea in the liver, and the nitrogen-free part is converted into sugars or fats, etc.). Most living things must be processed again. Marine organisms are usually discharged directly into seawater in the form of ammonia. Terrestrial organisms convert ammonia into urea or uric acid and then excrete it. Birds and reptiles usually excrete uric acid, while other animals (such as mammals) use urea. Exceptions are the aquatic tadpoles that excrete ammonia, but in the process of metamorphosis, they excrete urea; Dalmatian dogs mainly excrete uric acid, not urea, because the gene for a converting enzyme in the urea cycle is broken.
Mammals produce urea as a cyclic reaction in the liver. This cycle was first proposed in 1932, and the starting point of the reaction was the decomposition of ammonia. After clarifying the role of citrulline and argininosuccinic acid in the 1940s, it was fully understood. In this cycle, the amino groups from ammonia and L-aspartic acid are converted to urea, and the intermediaries are L-ornithine, citrulline, L-arginine-succinic acid and L-arginine .
The urea cycle is the main way for mammals and amphibians to excrete nitrogenous metabolic waste. But the same is true for other organisms, such as birds, invertebrates, insects, plants, yeasts, fungi and microorganisms.
Urea is basically a waste to biology, but it still has positive value. For example, the urea in the renal tubules is introduced into the renal cortex to increase its osmotic concentration, and promote the permeation of water from the renal tubules back to the body for reuse.
