This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Haematology, Subject name: Anatomy
[{'content': 'Hb is a good buffer because of \nSelect and explain. \nA. Histidine residues, B. Protein nature, C. Acidic nature, D. Iron molecules', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]