How does electrical activity of the heart cause the heart to pump?

The sinoatrial (SA) node is a group of specialized cells which sit in the wall of the right atrium. The SA node sets the pace of your heart and is therefore called your pacemaker. SA node sets this pace by generating an electrical current. The current the SA node generates travels in specialized fibres, similar to the wires in an electrical circuit. Where the current spreads, the muscle of the heart contracts.

In the heart's relaxed phase, diastole, the atria can fill with blood. The SA node fires, and the current spreads across the two atria, causing them to contract (atrial systole). This forces the blood into the ventricles. The electrical impulse then moves to an area known as the atrio-ventricular node, which sits just above the ventricles. Here, the electrical impulse is held up for a brief period. This is really important to allow all the blood from the atria to enter the ventricles before the ventricles themselves contract (ventricular systole). The electrical current then spreads down fibres known as the Bundle of His and then the right and left Purkinje fibres. This causes the ventricles to contract, forcing blood out of the heart and into the arteries. The heart relaxes and the cycle can begin again, triggered by the SA node!