This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Amino acid responsible for buffering action of Hemoglobin 
Select and explain. 
A. Histidine, B. Arginine, C. Valine, D. Lysine
A: Histidine
Answer: A: Histidine 
Explanation: The second buffer system is provided by the dissociation of the imidazole groups of the histidine residues in hemoglobin.Ref: Ganong&;s Review of medical physiology 25th edition. Page: 644