Urea is a waste product formed in the liver as a result of protein metabolism. When animals consume proteins, these proteins are broken down into amino acids during digestion. The amino acids are then used by the body for various functions, such as building new proteins.
However, not all amino acids are needed, and the excess ones are broken down further in the liver. This process, known as deamination, removes the amino group (NH2) from the amino acids. The amino group is then converted into ammonia (NH3), which is highly toxic to the body.
To neutralize this toxicity, the liver converts ammonia into urea through a series of reactions known as the urea cycle. Urea is much less toxic than ammonia and can be safely transported in the blood to the kidneys. The kidneys then filter the urea out of the blood and excrete it in the urine.
In summary, the source of urea in an animal’s body is the liver, where it is produced as a byproduct of protein metabolism and the detoxification of ammonia.