Every part of your body needs oxygen from the air to survive and oxygen is carried around the body by the red blood cells in the bloodstream. Since oxygen cannot get into the blood directly, through the skin, a complicated system is present in the lungs to absorb it from the air and transfer it into the bloodstream.

Your lungs lie on either side of your heart and fill the inside of your chest. In an adult, each lung weighs about 1lb, although the right lung is a little larger than the left one because there is more room for it - the left lung has to share its space with the heart. Both lungs are made up of lobes - three on the right and two on the left. The inside of your lungs looks like a giant sponge. It is a mass of fine tubes, the smallest of which end in tiny air sacs called the alveoli. There are an incredible 300 million or so of these alveoli and if they were spread out they would cover a piece of ground roughly the size of a tennis court. These alveoli have very thin walls which are crisscrossed with the finest of blood vessels called capillaries.

The lungs are protected by the rib cage. Between the ribs are muscles that are essential for breathing and below the lungs is a dome-shaped muscle called the diaphragm, which separates the chest from the abdomen and is also involved in breathing. Two thin layers of tissue, called the pleura, cover each lung. These layers - or membranes - slide back and forth over each other as we breathe so that the lungs can expand and contract.

Before birth a baby relies on its mother's blood for oxygen and its lungs are filled with fluid. But from the moment of birth we must draw air into our lungs and get our own oxygen.

The breathing centre in the brain is constantly receiving signals from the body about the amount of oxygen that is needed and this will depend on how active you are. When you are asleep you will need far less oxygen, for example, than if you are running to catch a bus, so you will breathe more slowly.

Once it knows how much oxygen is needed, the brain sends messages along nerves to the breathing muscles so that the right amount of air is breathed into the lungs. When the nerves to your breathing muscles tell you to breathe in, your diaphragm is pulled flat and, at the same time, the muscles between your ribs shorten and pull your rib cage upwards and outwards. This ensures that the lungs have the largest possible amount of space to expand into and pulls air into them.

Each time you breathe, air is drawn into your nose or mouth down through your throat and into your windpipe, or trachea. This windpipe is a tube about four or five inches long in adults and splits into two smaller air tubes called the bronchi, one of which goes to the left lung and the other to the right lung. The air passes down the bronchi which divide another 15 to 25 times into thousands of smaller airways, called bronchioles, until the air reaches the alveoli. Breathing out is usually just a matter of relaxing the diaphragm and the muscles between the ribs so that the air is pushed out and the lungs return to their resting size.

Inside the alveoli, oxygen moves across the paper-thin walls to the capillaries (tiny blood vessels) and into the blood, where it is picked up by chemicals (haemoglobin) in the red blood cells ready to be carried around the body. At the same time, waste products from the body, in the form of carbon dioxide, come out of the capillaries back into the alveoli, ready to be breathed out. Freshly oxygenated blood is carried from the lungs to the left side of the heart which pumps blood around the body through the arteries. Once the oxygen has been used up, the blood returns, through the veins, to the right side of the heart. From there it is pumped to the lungs so that the carbon dioxide can be removed and more oxygen taken on board.

10,000 litres of air move in and out of the lungs every day, carrying germs and other foreign bodies as well as oxygen. So the lungs provide a complex physical and chemical defence system to try and prevent unwanted material from getting into the body. Tiny hairs line the bronchi and help waft unwanted materials up to the mouth. Mucus produced in the walls of the airways helps to keep them clean and well lubricated. Cells in the lungs contain enzymes that produce chemical changes in the blood.

The delicate structure of the lungs is beautifully adapted to carry out the complex business of breathing and, at the same time, helps protect the body from outside attack. However, the lungs can be damaged - by cigarette smoke, air pollution (such as car exhaust and dust) as well as occupational gases and dusts. If the lungs are damaged, it can lead to breathlessness and disease.

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