From a spark to a coordinated squeeze
A heartbeat is not a single twitch. The two upper chambers (atria) must contract first to top up the two lower chambers (ventricles), and only then should the ventricles squeeze to push blood out. For that to happen smoothly, the spark from the SA node has to spread in a precise sequence. The structures that carry it form the conduction system — think of it as the heart’s built-in electrical wiring.
- The SA node in the right atrium fires; the wave spreads across both atria, and they contract.
- The signal funnels into the AV node, the only normal electrical gateway between atria and ventricles.
- From there it runs down the bundle of His, splits into the left and right bundle branches along the septum.
- It spreads out through the Purkinje fibers into the ventricle walls, and the ventricles contract.
Why the AV node pauses on purpose
The AV node does something clever: it deliberately slows the signal down for a fraction of a second. This is the AV delay. Without it the ventricles would start squeezing while the atria are still pushing blood in, and the timing would be a mess. The pause gives the atria time to finish filling the ventricles before the big squeeze — a small wait that makes each beat efficient.
Notice the beautiful logic of the layout. The wave goes down to the bottom of the heart first (via the His–Purkinje cables) and then sweeps upward through the ventricle muscle. That bottom-up squeeze wrings blood toward the valves at the top — exactly the direction it needs to leave the heart.