Terrestrial adaptation required the synthesis of less harmful nitrogenous wastes like urea and uric acid in order to conserve water. Since urea is 100,000 times less hazardous than ammonia, it may be tolerated at considerably higher concentrations. Ureotelism is the process through which urea is excreted. Ammonia generated by metabolism is converted to urea in these animals' livers and released into the bloodstream, where it is filtered and expelled by the kidneys. Ureotelic animals are mammals, numerous terrestrial amphibians, and marine fishes that mostly excrete urea.