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
Gatekeeper of the hea 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: The atrioventricular (AV) node is the "gatekeeper" between the atria and the ventricles and is located at the AV junction on the right side of the hea (Figure 1A). The primary role of the node is to conduct the action potential from the atria to the ventricles. However, the AV node also acts as a backup pacemaker in the case of failure of the sinus node and stops arrhythmias in the atria, such as atrial fibrillation, from being conducted into the ventricles at dangerously high rates. Atrial conductive system is organised so that the cardiac impulse do not travel frim atria to ventricle too rapidly. This delay allows time for the atria to empty their blood into ventricle before ventricular contravtion begins .