The heart is situated on the left side of the chest, between the lungs and above the diaphragm, and is protected by the ribs. Its strong muscular pumping action works in two directions. It pushes dark-red, de-oxygenated blood to the lungs to pick up oxygen and get rid of carbon dioxide, and then pumps this bright-red, newly oxygenated blood round the body.

The right side of the heart is the pump which sends blood to the lungs to be oxygenated and the left side is the pump which supplies the whole body with oxygenated blood. Each separate pump has two chambers, an atrium above which receives blood and a ventricle below which sends it out of the heart again. Between each atrium and ventricle, and between each ventricle and outflow blood vessel, is a valve which prevents the blood flowing in the wrong direction.

In a healthy person, the heart beats regularly and spon­taneously throughout life. The heart rate can be altered by such factors as emotion, food, drugs, exercise, altitude and certain illnesses. Normally the beating of the heart is unnoticeable, but sometimes it seems to thump in the chest. With each beat, the whole heart contracts because of a shortening and tightening of its muscle fibres. The two atria contract before the two ventricles, so the atria push blood into the ventricles in time for them to squeeze it out, as they contract, into the outflow vessels. Any turbulence in the flow of blood creates a noise called a murmur.

The blood vessels carrying blood away from the heart are called arteries. All the arteries except the pulmonary artery carry oxygenated blood. The nearer an artery is to the tissue it supplies, the smaller it becomes. These tiny branches are called arterioles and they eventually become minute capillaries which form a network around the cells and allow oxygen, nutrients and various other substances carried by the blood to diffuse out into the tissue fluid and into the cells.

The blood vessels carrying blood to the heart are called veins. All the veins except the pulmonary vein carry de-oxygenated blood. The nearer the heart, the larger the blood vessels: blood flows from capillaries to venules to veins and back to the right atrium. Veins tend to be larger than arteries and have thinner walls with less muscle in them.

The pulmonary artery carries de-oxygenated blood from the heart to the lungs, where carbon dioxide is removed and oxygen added. The oxygenated blood is then returned to the heart by the pulmonary vein, to be circulated round the body.

Nutrients from the food we eat are taken up from the digestive system into the blood and circulated to the tissues. Waste products are carried to the kidneys, which filter and remove many of them. Drugs and other chemicals entering the bloodstream after being eaten, breathed in. or taken in through the skin are also carried round and used or eventually removed. Hormones from the endocrine glands are taken by the blood to their various sites of action.

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