As protein is metabolized in the liver, toxic nitrogenous wastes such as ammonia, urea, and uric acid are produced. To avoid a buildup of these toxic wastes, mammalian vertebrates have evolved a complex excretory system.

During protein metabolism, the amino group (NH₂) from amino acids is removed by enzymes found in the liver. This process, called deamination, results in the formation of a toxic by-product called ammonia (NH₃).

Ammonia is so toxic it cannot be stored for any time in the body, so it must be converted into a less harmful form. This form is usually urea, which becomes a compound in urine. The formation of urea occurs when ammonia combines with carbon dioxide, also in the liver. Another nitrogenous waste that is formed primarily from the breakdown of certain nucleic acids is uric acid. 

Watch

Watch the video to the right. This is a good introduction to the function of the kidney and its role in excreting waste.

Read

Read "The Problem of Wastes" and "The Solution to Wastes: Excretion" on page 306 of your textbook.