How does the heart work?

The heart is a hollow muscle. A wall through the middle (known as the septum) divides it into two halves. Each half has two chambers called the atrium and ventricle. The left ventricle pumps oxygen-rich blood out of the heart and into the body (systemic circulation) through the main artery, called the aorta. The first blood vessels that branch off from the aorta are the coronary arteries. They go straight to the muscle of the heart itself, supplying it with oxygen and nutrients.

The blood that has been "used" by the body – and is then low in oxygen – flows back to the heart. More specifically, it enters the right atrium and from there it flows into the right ventricle. The right ventricle pumps the low-oxygen blood into the pulmonary artery, which leads to the lungs (pulmonary circulation). In the lungs, the blood releases carbon dioxide and picks up oxygen. This oxygen-rich blood then flows back to the heart through the pulmonary veins – this time to the left side of the heart. From there, the blood is pumped back into the body.

The heart valves are found between the atria and the ventricles, as well as where the blood leaves the heart through the arteries. There are four of them in total:

• the tricuspid valve between the right atrium and the right ventricle
• the pulmonary valve, at the point where the blood flows from the heart into the pulmonary artery
• the mitral valve between the left atrium and the left ventricle
• the aortic valve, at the point where the blood flows from the heart into the aorta

These heart valves ensure that the blood flows in the right direction and doesn't flow back.

The valves of the heart function like one-way gates. Each valve is made up of a ring to which two or three flaps of tissue (called cusps or leaflets) are attached. The flaps are somewhat curved – a bit like sails billowing in the wind. When the blood pushes against these flaps in the direction of the “wind,” they close the valve. When the blood pushes in the other direction, it is able to flow through the valve.

The heart muscle is made up of special cells – the heart muscle cells. An electrical "wiring system" ensures that these muscle cells contract rhythmically, causing the heart to beat. Each heartbeat is triggered by an electrical impulse that starts in the sinus node, which is a group of cells in the wall of the right atrium. Special pathways of nerves carry the electrical signals to the muscle cells in the atria and ventricles, setting the pace of the heartbeat.