1) The sinoatrial node (SAN) is a group of cells found in right atrium of the heart, that acts as a pacemaker.
2) A wave of electrical activity spreads out from the SAN, across the atria causing them to contract simultaneously.
3) The atriventricular septum prevents the wave crossing to the ventricles (this allows the atria to complete their contraction and completely empy of blood).
4) The wave passes to the atrioventricular node (AVN), which lies between the atria.
5) After a short delay, the AVN passes on the wave along specialised fibres called the Bundle of His.
6) When the wave of electrical activity is released from the fibres, the ventricles can contract.
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