This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
[{'content': 'Gatekeeper of the heart is \nSelect and explain. \nA. SA node, B. AV node, C. Purkinje fibers, D. Bundle of His', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. (b) AV node(Ref: Ganong, 25th ed/p.521)AV node conduction is slow. There is a delay of about 0.1 s in spread of impulseThis delay is mainly responsible for the sequential contraction of atrium followed by ventricles. It is for this reason, AV node is also called "Gatekeeper" to the ventricles', 'role': 'assistant'}]