The ability of Haemoglobin to take up oxygen in places of high oxygen tension and high pH and release it in areas of low pH, such as respiring tissues is known as the Bohr effect. Hemoglobin's oxygen binding affinity is inversely related to the concentration of carbon dioxide in the blood, it is represented by a sigmoid shape graph.
The shape of this graph can be explained by the quarternary structure of haemoglobin. The heme groups to which the oxygen bind can be found on the inside of the molecule, hence the initial difficulty in the binding of the first haemoglobin molecule, which will take place only in areas of high oxygen concentration. Once this first molecule takes place, there is a conformational change in which the salt bridges between polypeptide chains are broken and the two beta chains are moved towards one another, exposing the remaining binding sites on the haemoglobin molecule. This explains the increasing facility with which each successive haemoglobin molecule binds and the S shape of the curve. It is difficult to achieve 100% saturation even is areas of high O2 concentration.