The S.A node serves as the "pacemaker"
Because it starts each heartbeat and sets the pace for the whole heart.
The SA node normally functions as the pacemaker for the entire heart because its intrinsic rate is the fastest in the system. From the SA node, impulses spread out over the atria, causing them to...
This is because the SA node starts contracting on its own and propagates that contraction to the rest of the heart, causing the heartbeat.
A pacemaker is a relacement for the heart when you have hearts problems or something like that. One of my Primary school friends had a pacemaker.