It has something to do with the partial pressure of O2 vs CO2 on either side of the lung's aveoli. Better off left to a physiologist. Hemoglobin normally binds oxygen at high partial pressures of oxygen.
Pressure
https://www.khanacademy.org/science/...rtial-pressure
Last edited by wanabee777; 12th Mar 2016 at 12:17.