How does the electrical system of the heart work?

The electrical activity of the heart begins in the sinoatrial node which is a collection of pacemaker cells located in the right atrium. These cells spread electrical activity across the atrium causing both atrium to contract to pump blood from the atrium into the ventricles. The electrical signals now spread to the atrioventricular node located between the right atrium and right venticle. From here the signals run down the bundle of His in the heart's septum to the purkinje fibres at the base of the ventricles. This then causes both ventricles to contract so blood can be pumped out of the heart to the body and lungs.