These are organs which bring the blood into contact with clean fresh air – because nothing is better than a good bath of oxygen to purify the blood.

At each breath, air is drawn into more than 1,500,000 little air sacs in the lungs, which if spread out would cover an area of some 200 square metres – the size of a little vegetable plot. These little balloon-like sacs are made of a thin elastic tissue which allows air to pass through but prevents blood from oozing in.

The blood is carried to the lungs through 50,000,000,000 tiny hair thin tubes which form a close network all along the outside of the little sacs of the lungs. Each day they bring in some 10,000 litres of blood. Oxygen is sucked in by the red blood cells, while waste products of the body like carbon dioxide and water are given up by the blood, pass into the little air sacs, and are breathed out.

If the capillaries of the lungs are laid end to end, they would stretch for 2400 km.

As long as a child is in the womb of its mother, its lungs do not function, and the flow of blood is turned away from the lungs by means of a special little door in the heart. As soon as it is born, the baby, who is on the verge of suffocation, utters a loud cry. The cry produces a whole series of wonderful changes. The great bags of the lungs open and air rushes in to fill them. A great flow of blood is 37 drawn into the lungs which, like a violent draught of air slams shut the little door inside the heart which had hitherto turned the blood away.

At the end of the smallest passages are bunches of air sacs called alveoli. These are like tiny balloons and fill with air when you breathe in. All together there are about 300 million alveoli in your lungs. Their total surface area is about 70 square metres, which is about 40 times the area of your skin.

THE KIDNEYS,

WATER AND WASTE

About two thirds (65%) of your body consists of water. The body of the average adult contains 45 litres of water. This has to be kept at a consistent level and evenly distributed throughout your cells. You take in water every time you eat and drink, and water is produced inside your cells during respiration. You lose water mainly in your urine, which is produced in varying amounts by your kidneys. In general, the more you drink the more urine your kidneys produce. If you are losing a lot of water, by sweating for example, they produce less. Your kidneys also help to control the level of substances such as salt in your body and they get rid of waste products such as urea.

INSIDE YOUR KIDNEYS

Each of your kidneys contains over a million microscopic blood filtering units called nephrons. In all, the nephrons filter about 150- 180 litres of fluid a day from your blood though only about 1.5 litres