Wastes
one of the excretory system's main functions is to remove cellular wastes. When proteins are broken down by cells, nitrogenous wastes form, and these wastes, and excess salt and water, are removed by the excretory system. The respiratory system gets rid of the carbon dioxide created by broken down carbohydrates, while the digestive system gets rid of feces, which is also created by broken down carbohydrates.
Urea
In the urine is a substance called urea, and urea is a nitrogenous waste. Urea's chemical structure is H2N- C(o)-NH2. Urea, a product of protein metabolism, is colorless solid that melts at 270.8°F. "Urea is formed by the liver primarily from the ammonia that results when excess amino acids are deaminated (or broken down). In this process--in which the amine group is removed from the amino acid--the resting ammonia is either used to make other nitrogen-containing compounds or transported to the liver, where it is converted to urea and excreted by way of the urine."