A pump that starts itself
Most machines need someone to switch them on. The heart is different: it switches itself on, again and again, about once every second, for your whole life. Each heartbeat begins not with a message from outside but with a tiny electrical spark generated inside the heart itself. That ability — to fire on its own without being told to — is called automaticity.
The spark comes from a small cluster of cells in the upper right of the heart called the sinoatrial (SA) node. These are not ordinary muscle cells. They are pacemaker cells — specialists whose job is not to squeeze, but to set the tempo. They leak charge in a slow, steady way until they reach a tipping point and fire, and that firing is the signal for the whole heart to beat.
What sets the pace
When the SA node is in charge and everything runs in its normal order, doctors call the resulting rhythm sinus rhythm — “sinus” because it comes from the sinoatrial node. A healthy adult at rest usually has a heart rate somewhere between about 60 and 100 beats per minute, though fit athletes are often slower and that is perfectly fine.
Although the SA node sets its own base tempo, it does listen to the body. Nerves and hormones can nudge it faster when you climb stairs or feel afraid, and slower when you rest or sleep. So the SA node is like a metronome that runs by itself but can be sped up or slowed down by the conductor. The whole network it feeds into is the heart’s conduction system, which the next guides will walk through.