The Heart

Every cell in your body needs a steady supply of oxygen and food to stay alive and do its work. The same cells also make carbon dioxide and other waste, which must be carried away quickly. Blood does this carrying. Blood cannot move by itself, though. It needs a pump to push it and a set of tubes to flow through. The heart is the pump, and the blood vessels are the tubes.

The first person to study this system was William Harvey. Its main job is to carry oxygen from the lungs to every cell of the body, using the red blood cells in the blood. It also carries the nutrients absorbed by the digestive organs to the cells that need them, and it takes away things the body cannot use, such as carbon dioxide, urea and other chemical wastes.

The tubes that carry blood out to every part of the body and bring it back again are called blood vessels. Going by the direction the blood flows and the size of the tube, there are three kinds:

• Arteries: carry blood away from the heart
• Veins: bring blood back to the heart
• Capillaries: very fine tubes that join arteries to veins

Nearly every artery in the body carries pure blood, meaning blood full of oxygen. The farther an artery runs from the heart, the narrower and finer it becomes, and the more branches it gives off. Think of a tree: a thick trunk splits into big branches, the big branches into small ones, and the small ones into thin twigs. A large artery divides in the same way into countless smaller arteries. The main artery that leaves the heart is the largest artery in the body.

Almost all veins carry impure blood. Here too there is one exception: the pulmonary veins bring pure, oxygen-rich blood from the lungs to the heart. The largest veins of the body are called the venae cavae (one of them is a vena cava), and the smallest veins are called venules. Arteries split and get smaller as they leave the heart, but veins work the other way round. Tiny venules join together into bigger and bigger veins as they travel back to the heart.

The two largest veins, the great veins, are formed like this:

• Superior vena cava: formed where the veins from the head, the neck and the arms join together.
• Inferior vena cava: formed where the veins from the lower parts of the body, such as the legs, kidneys, intestines, stomach and liver, join together.

Blood enters an artery with the full force of the heart's push, so an artery needs a thick, strong wall to take that pressure. By the time blood is flowing back through a vein, most of that push has been used up, so a vein manages with a thin wall.

As an artery travels farther from the heart, it keeps dividing into smaller branches. These small arteries are called arterioles. When arterioles divide into even finer parts, the result is capillaries. A capillary is five millimetres long and 8 to 10 microns thick. One micron is one thousandth of a millimetre, so capillaries are far too fine to see with the naked eye.

Capillaries join the arteries to the veins: blood coming from an artery passes through the capillaries and then into a vein. Because a capillary's wall is so thin, this is where oxygen and food pass out of the blood into the body's cells, and carbon dioxide passes from the cells into the blood.

The heart is an important organ made of muscle. It sits inside the chest, between the two lungs. It is shaped roughly like a leaf, broad at the top and narrowing to a point, and it is about the size of your own closed fist. Its lower tip is slightly pointed and leans a little towards the left, which is why a heartbeat is easiest to feel just left of the middle of the chest. A heart normally weighs 225 to 290 grams.

The wall of the heart is built from three separate layers:

Inside, the heart is divided into four chambers: two above and two below. A thick wall down the middle, the septum, separates the right side of the heart from the left. Because of this wall, the impure blood on the right side never mixes with the pure blood on the left side.

The ventricles are bigger than the atria and have thicker walls, because they do the hard work of pushing blood out of the heart. Of the two, the left ventricle is thicker and stronger still. The right ventricle only has to send blood to the lungs close by, but the left ventricle must pump blood to the whole body, from the head down to the tips of the toes.

The heart has four valves: the tricuspid valve, the bicuspid valve, the pulmonary valve and the aortic valve. A valve works like a one-way door. It opens to let blood go forward and shuts so that the blood cannot come back.

The valve marked Mitral valve in the diagram is the bicuspid valve. It has two flaps, which is where the name bicuspid comes from. The tricuspid valve has three flaps.

Day and night, without stopping, the heart does three main jobs:

• It sends blood to every organ of the body.
• It sends the impure, carbon dioxide-rich blood that returns from the organs to the lungs.
• It collects the pure, oxygen-rich blood that comes back from the lungs and sends it out to every organ of the body.

The heart is the most important organ of circulation. It works like a pump: when it squeezes, or contracts, it pushes blood out, and when it relaxes it fills with blood again. One full journey of the blood, from the body to the heart, on to the lungs to be cleaned, back to the heart and out to the body again, happens in this order:

• Impure, carbon dioxide-rich blood from the cells, tissues and organs of the body flows through the superior vena cava and the inferior vena cava and collects in the right atrium.
• When the right atrium is full, it contracts. The tricuspid valve opens and the blood passes into the right ventricle.
• When the right ventricle is full, it contracts. The pulmonary valve opens and the impure blood travels along the pulmonary artery to the lungs.
• In the tiny air sacs of the lungs, called alveoli, carbon dioxide and oxygen are exchanged. The blood gives up its carbon dioxide there and takes in oxygen, and so it becomes pure.
• Four pulmonary veins bring this pure blood from both lungs to the left atrium.
• When the left atrium is full of pure blood, it contracts. The bicuspid valve opens and the pure blood passes into the left ventricle.
• When the left ventricle is full, it contracts. The aortic valve opens and the pure blood enters the aorta.
• The aorta carries the pure blood to every part of the body. Once the body has used its oxygen, the blood is impure again and the whole journey starts over.

In systemic circulation, blood carrying oxygen and nutrients leaves the left ventricle through the aorta. It flows along the branches and smaller branches of the arteries until it reaches the capillaries in every part of the body. Inside the cells, oxygen is used to release energy from food. This is called oxidation. The carbon dioxide made by oxidation mixes into the blood. The blood, now impure, flows from the capillaries into the small branches of the veins, then into larger branches, and finally into the two great veins, the superior vena cava and the inferior vena cava, which empty it into the right atrium.

In pulmonary circulation, blood leaves the right ventricle through the pulmonary valve and travels along the right and left pulmonary arteries to the air sacs of the right and left lungs. There it gives up its carbon dioxide. It picks up oxygen from the air in the air sacs and returns through the pulmonary veins to the left atrium. From there, systemic circulation takes over again. In this way blood keeps circulating through our body steadily, without a break.