ABSTRACT
The kidneys are the centerpiece of the executory system in most vertebrates, even though the skin, respiratory, and digestive systems may also have excretory functions. In vertebrates other than fish, nitrogenous wastes are excreted through the kidneys. In mammals and amphibians, they are first converted to urea, a soluble compound. In birds and many reptiles, they are converted to uric acid, which can be concentrated almost into a solid. In fish, the kidneys have a smaller role. Most nitrogenous wastes are excreted as ammonia through the gills, which also regulate salt concentration.
