Likewise, if we have a fully saturated oxymoglobin molecule and one of the hein groups unloads or releases an oxygen molecule then that will create a conformational change in the structure of the four polypeptides and that will make it more likely for the other three occupied hein groups to actually release an oxygen molecule. So this is what we mean by Cooperativity. Now, this leads to the sigmoidal shape the S shape of the oxygen hemoglobin dissociation curve that can be seen in the following diagram. So the blue curve is the oxygen hemoglobin dissociation curve. So this S shaped sigmoidal curve. Now, the Y axis is, as always, the percent saturation of hemoglobin for this blue curve.