What is the correct order of electrical events in the heart?

The sequence of electrical events in the heart begins with the firing of the SA (sinoatrial) node, which serves as the primary pacemaker of the heart. This impulse causes the atria to contract, leading to atrial systole. After the atria have contracted, the electrical signal travels to the AV (atrioventricular) node, where it is delayed briefly to allow the ventricles to fill with blood before they contract, resulting in ventricular systole.

This order is crucial for maintaining efficient blood flow through the heart and into the major arteries. The timing of these electrical events ensures that the heart chambers are not contracted simultaneously in a way that would compromise blood circulation.

In this context, the correct order illustrated in the chosen answer highlights the progression from the SA node firing, to atrial contraction, and then to ventricular contraction, reflecting the organized and coordinated activity of the heart's electrical conduction system.