Animals which excrete urea as the major excretory product are said to be ureotelic. Urea is a useful product for a semi-terrestrial animal that is not exposed to scarcity of water but is not necessarily surrounded by large quantities of it. It is, therefore, the main product of some earthworms, shore gastropods, adult amphibians, and semi-terrestrial tortoises, turtles, and mammals. In humans, blood contains about 0.04 g/100 ml of urea, whereas the level of urea in the bladder is 2 g/100 ml of urine.