Why does your body need to exhale oxygen?

Well your body doesn't need to exhale oxygen, it is just a natural thing that happens since all of the red blood cells cant pick up all the oxygen we inhale so some, probably lots of it, is exhaled. There is such a thing as oxygen toxicity, but that is not really due to a need to exhale oxygen, and you would need to be hooked up to 100% Oxygen and breath quickly for that to occur.