8.3.4—Cardiac conduction system

The heart is myogenic: its own conduction system starts and distributes the excitation wave. The sequence is SAN → atria → AVN delay → bundle of His → Purkyne tissue → ventricular muscle, so atrial contraction precedes coordinated ventricular contraction.

1. SAN starts the wave: the sinoatrial node in the right atrium generates a wave of excitation, causing the atria to contract.
2. AVN delays the wave: non-conducting tissue between atria and ventricles prevents the wave travelling straight through; the atrioventricular node receives it and delays it, allowing the ventricles time to fill after atrial contraction.
3. His bundle carries it through the septum: after the delay, the wave passes down the bundle of His.
4. Purkyne tissue spreads it from the apex: the bundle divides into Purkyne fibres that spread through the ventricular walls, initiating excitation from the apex upward.
5. Ventricles contract together: this coordinated wave makes the ventricles contract and forces blood into the pulmonary artery and aorta.

The key control relationship is electrical excitation → muscle contraction → coordinated pumping. The AVN delay is essential for atrial-then-ventricular timing. This card covers the conduction pathway, not ECG interpretation, drug effects or other unsupported clinical detail; the mechanical valve-pressure sequence is covered in 4613. No image generated or bound.