AV node

The atrioventricular node is the heart's gatekeeper and traffic-calming junction. It sits at the floor of the right atrium, near the boundary between the atria and ventricles, and it is the one electrical route that normally lets the atrial impulse pass downward into the ventricles. Everywhere else, a sheet of fibrous tissue insulates the atria from the ventricles.

The node's signature trick is that it conducts slowly. As the impulse crawls through it, the descent into the ventricles is deliberately held back by a fraction of a second. This pause — the AV delay — gives the atria time to finish squeezing blood into the ventricles before the ventricles contract, so the chambers fill properly. The node's cells reset slowly too, which limits how many impulses can get through; in atrial fibrillation, this gatekeeping is what stops the chaotic atrial firing from driving the ventricles dangerously fast.

The atrioventricular node also serves as a backup pacemaker. If the sinoatrial node fails, junctional tissue around the node can generate its own beats at roughly 40 to 60 per minute — slower than normal but enough to keep blood moving. Disease or excessive vagal tone in the node produces the various degrees of heart block.