What structure serves as the backup pacemaker for the heart?
a. Atrioventricular (AV) node
b. Sinoatrial (SA) node
c. Bundle of His
d. Purkinje fibers
ANS: A
After leaving the atria, the impulse moves to the AV node, located in the intraventricular septum in the inferior aspect of the right atrium (Figure 18-2). The AV node is the "backup" pacemaker because it has the second greatest degree of automaticity in the healthy heart.
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