waste must be eliminated from the body & something must be eliminated from the parts of something else before it can be eliminated from the entirety something else ->  waste must be eliminated from the parts of the body before it can be eliminated from the entirety of the body  waste must be eliminated from the parts of the body before it can be eliminated from the entirety of the body & blood is a part of the body ->  waste must be eliminated from the blood before it can be eliminated from the body  waste must be eliminated from the blood before it can be eliminated from the body & if an action must be done before a process can be done, that action is a step in that process ->  removing waste from blood is a step in removing waste from the body  removing waste from blood is a step in removing waste from the body & kidney is a vehicle for removing bodily waste / cell waste / byproducts from the blood ->  the kidneys perform a step in removing waste from the body  the kidneys perform a step in removing waste from the body & if an action is a step in a process, then that action helps in the performance of that process -> the kidneys help remove waste from the body