The heart beats when the cardiac muscles around the outside of the organ contract. The heart contracts in two phases, first squeasing the blood that is in the two atria into the ventricles, and then squeasing the blood from the ventricles out of the heart, either to the lungs or to the rest of the body. When the cardiac muscles are contracting the heart is described as being in systole, and when it is relaxed it is described as being at diastole.

The heart muscle is made up of many cells, bound to each other my "intercolated discs". These intercolated discs spread electrical signals over the whole muscle very quickly, so when a message is sent to the muscle, it all acts as once, which is very important when trying to move large volumes of blood around. When the atria contract, the "sino-atrial node" sends a signal out, which spreads across all of the muscle. This signal is stopped by a non-conductive layer between the muscle surrounding the atria and the muscle surrounding the ventricles, so the whole of the heart doesn't contract at once. The atrial muscles contract, squeasing blood into the ventricles of the heart.

The signal from the sino-atrial node activates another node, the "atrioventricular node". Once the atrial muscles have contracted, the atrioventricular node sends another electrical signal down the "bundle of Hes", a clump of nerve cells bridging the non-conductive material between the atrial and the ventricular muscles. The signal travels down this, which half way down splits into two. At the bottom, or "apex" of the heart, the electrical signal is released from the nerve cells out into the muscles, and then spreads throughout the ventricular muscles making the ventricles contract.

The reason for the electrical signal travelling to the bottom of the heart before the ventricular muscles contract is two fold:
  • To provide a short delay between atrial and ventricular contractions
  • To make the ventricles contract from the bottom upwards if there is any delay at all in the signal spreading, ensuring that all the blood is squeased out of the heart

Systole lasts around 0.6 seconds when at rest (this will be shorter when the heart is beating fast). The heart then goes through diastole, a period of resting, which lasts around 0.3 seconds when at rest. During this time all the changes which have occured during systole are reset, and the heart prepares to contract again. Although the two sides of the heart pump to different places in the body (the right side pumps blood to the lungs for oxygenation, and the left side pumps oxygenated blood to the rest of the body), both sides contract at the same time as it is simply easier to make all the muscle contract at once.

All this makes the cardiac muscles surrounding the heart very active, so they use up a lot of oxygen and glucose respiring in order to keep up enough energy. As a result, 5% of the oxygenated blood pumped out by the heart is fed straight back in to the heart muscles to power them.

An interesting note is that the heart muscles are "Myogenic". This means that even when taken out of the body, the signals will still be send by the sino-atrial node for the heart to beat, and, as long as it has sufficient levels of glucose and oxygen, the heart will continue to beat. This is what makes heart transplants possible, as by cooling them the usage of oxygen and glucose can be kept to a minimum, and so the heart can be kept beating for up to a day (although it is usually no longer than 4 hours).