Oxygen enters our body through our lungs in the air we breathe. O₂ makes up about 21% of the total volume of air on Earth. From the lungs, O₂ diffuses into the bloodstream, where it binds to a molecule called hemoglobin in our red blood cells. The binding affinity of O₂ to hemoglobin is pH-dependent such that oxygen can readily be picked up by red blood cells near the lungs and then released into tissues and other areas of the body that need it.