The circulatory system is the network of elastic tubes that allows blood to flow throughout the body. It includes the heart, lungs, arteries , arterioles (small arteries), and capillaries (minute blood vessels). These blood vessels carry oxygen- and nutrient-rich blood to all parts of the body. The circulatory system also includes venules (small veins) and veins, the blood vessels that carry oxygen- and nutrient-depleted blood back to the heart and lungs. If all these vessels were laid end-to-end, they'd extend for about 60,000 miles - far enough to encircle the earth more than twice.

The circulating blood brings oxygen and nutrients to all the organs and tissues of the body, including the heart itself. It also picks up waste products from the body's cells. These waste products are removed as they're filtered through the kidneys, liver and lungs.

What is the structure of the heart?

The heart has four chambers through which blood is pumped. The upper two are the right and left atria ; the lower two, the right and left ventricles . Four valves open and close to let blood flow in only one direction when the heart contracts (beats).

The four heart valves are…

• the tricuspid valve, located between the right atrium and right ventricle;
• the pulmonary or pulmonic valve, between the right ventricle and the pulmonary artery;
• the mitral valve, between the left atrium and left ventricle;
• the aortic valve, between the left ventricle and the aorta.

Each valve has a set of flaps (also called leaflets or cusps).

The mitral valve has two flaps; the others have three. Under normal conditions, the valves permit blood to flow in just one direction. Blood flow occurs only when there's a difference in pressure across the valves that causes them to open.

How does the heart pump blood?

The heart pumps blood to the lungs and to all the body's tissues by a highly organised sequence of contractions of its four chambers.

For the heart to work properly, the four chambers must beat in an organised manner. This is governed by an electrical impulse. A chamber of the heart contracts when an electrical impulse moves across it. Such a signal starts in a small bundle of highly specialised cells located in the right atrium - the sinoatrial node (SA node), also called the sinus node. A discharge from this natural "pacemaker" causes the heart to beat. This pacemaker generates electrical impulses at a given rate, but emotional reactions and hormonal factors can affect its rate of discharge. This lets the heart rate respond to varying demands.