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
Gatekeeper of the heart is 
Select and explain. 
A. SA node, B. AV node, C. Purkinje fibers, D. Bundle of His
B: AV node
Answer: B: AV node 
Explanation: 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