The rhythm of heartbeat is maintained by the sinoatrial (SA) node. The impulse starts in a small bundle of specialized cells located in the right atrium. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles.

The heart beats at an average rate of 60 to 75 beats per minute and pumps about five litres of blood. This means that the total volume of a person's blood is moved in a period of just one minute. Under intense exercise, your heart rate can increase by more than two hundred beats.

Thus, the heart can beat up to 100,000 times each day. During this time, the heart will pump almost 7,000 litres of blood. The heart can do this because it can rest for a combined time of four hours between beats each day.

The image shows the electrical activity of the heart (ECG).