The principle of Superposition states that, in an undisturbed sequence, the oldest rocks are at the bottom of the sequence and successive layers are younger than those below them. This is different from relative dating because, relative dating examines rock position in aq sequence to determine the order of events the rock's relative age.
I would say that the kidneys purpose is to filter waste material from the blood primarily to remove waste fluid from the blood and leave just the right of salt and minerals in the blood and remove the waste fluid which is known as urine.
