Although Earth's atmosphere at sea level contains 21% oxygen by volume the human body only uses about 5% out of the available 21% oxygen per breath. Oxygen is used to facilitate the conversion of glucose into energy (ADP - adenosine triphosphate) by acting as a proton trasnfer agent in the electron transfer chain after the Kerbs Cycle. It is because we only require about 5% out of the available 21% that we can save other peoples' lives via artificial respiration during CPR; ie: we have 16% that we breath out so the other person has more than they need.