How does haemoglobin help in transport of oxygen from lung to tissue ?

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 Hconcentration 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.