Hb is a good buffer because of
Histidine residues
Protein nature
Acidic nature
Iron molecules
Ref Robbins 9/e p650 Haemoglobin also functions in CO2 and proton transpo from tissues to lungs . Release of O2 from Oxy Hb at the tissue is accompanied by uptake of proton due to lowering of the pKa of histamine residues
Haematology