The heart's "master pacemaker" is the sinoatrial (SA) node (Upper R-atrium). Here electrical impulses quickly spread through the R L atria. The atrioventricular (AV) node (Lower R-atrium) is stimulated. The heart's lower chamber (the ventricles) fill with blood contract. The R Bundle is His are stimulated next followed by the Purkinje fibres. The ventricles contract the blood is ejected. Any abnormalities in the heart's electrical pacing system results in a cardiac arrhythmia.