They allow the osmolarity of their body fluids to match that of the environment.

It is the primary nitrogenous waste for aquatic invertibrates, teleosts and larval amphibians.

They keep the osmolarity of body fluids different from that of the environment.

It is produced by mammals, most amphibians, some reptiles, some marine fishes and some terrestrial invertebrates

It is excreted by birds, insects and terrestrials reptiles.