Haemoglobin is a red coloured iron containing pigment present in the RBCs. Oxygen binds with Haemoglobin in a reversible manner and forms oxyhaemoglobin.

Each Haemoglobin molecule can carry a maximum of four molecules of oxygen. Binding of oxygen with Haemoglobin is related to partial pressure of oxygen.

The high pO2 , low pCO2 , lesser H+ concentration and lower temperature in the alveoli are favourable for formation of oxyhaemoglobin, where as in the tissues, low pO2, high CO2 , high H+ concentration and higher temperature exists which favour dissociation of oxygen from haemoglobin. Thus O2 binds to heamoglobin in lung surface and gets dissociated at the tissues in reversible conditions.