Here's why:
* SA Node: The SA node is known as the "pacemaker" of the heart. It normally generates electrical impulses at a rate of 60-100 beats per minute. This intrinsic rate is the fastest among all parts of the conduction system.
* Atrioventricular (AV) Node: The AV node has a slower intrinsic rate of 40-60 beats per minute.
* Bundle of His, Right & Left Bundle Branches, Purkinje Fibers: These structures have even slower intrinsic rates, typically around 20-40 beats per minute.
If the SA node malfunctions, the AV node can take over as the pacemaker, but at a slower rate. The other parts of the conduction system can also act as pacemakers, but at even slower rates. This is why a heart with a damaged SA node will have a slower heart rate.
