Course information from the University of Cincinnati states that the excretory system is important to the human body because without it, nitrogenous wastes would build up in blood and tissues to toxic levels. The kidneys play a major role, filtering the nitrogenous wastes from the blood and mixing them with water to make urine. The kidneys filter large volumes of blood every day for this purpose.
According to course information from the University of Cincinnati, the first nitrogenous compound that usually forms during an animal's metabolic processes is ammonia. Ammonia is a base, so any build-up in the blood begins to raise its pH. Each organism has an optimal pH for its biological processes, so too much of a change causes damage or death. Thus, animals require some way of getting rid of ammonia. Ammonia is water soluble, so animals that live in the water can just release it from their cells and let it diffuse around them. Animals that live on land, however, must eliminate it in other ways. Mammals, such as humans, bind ammonia into another water-soluble compound, urea. Birds and bats instead create a non-water-soluble compound, uric acid. Humans also create some uric acid, but it is hard to eliminate, and an excessive build-up can lead to diseases such as gout.