Haemoglobin is a red coloured iron containing pigment present in the RBCs. O2 binds with haemoglobin in a reversible manner and forms oxyhaemoglobin.
Each haemoglobin molecule can carry a maximum of four molecules of O2. Binding of oxygen with haemoglobin is primarily related to partial pressure of O2.
The high pO2, low pCO2, lesser H+ concentration and lower temperature in the alveoli are favourable for the formation of
oxyhaemoglobin, whereas in the tissues,
low pO2, high pCO2, high H+ concentration and higher temperature exist which favours dissociation of the oxygen from the haemoglobin. Thus, O2 gets bound to haemoglobin in the lung surface and gets dissociated at the tissues.