You have two lungs, one on either side of your heart. They fill the inside of your chest. In an adult, each lung weighs about 1lb (0.45kg). However, the right lung is a little larger than the left because there is more room for it. Your left lung has to share space with your heart. Two thin layers of tissue, called the pleura, cover each lung. These layers – or membranes – slide over each other as you breathe so your lungs can expand and contract. The inside of your lungs look 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 around 300 million alveoli and if they were spread out they would cover an area roughly the size of a tennis court. Alveoli have very thin walls. They are criss-crossed with the finest of blood vessels called capillaries. Your lungs are protected by your rib cage. Between your ribs are muscles that are essential for breathing. Below your lungs is a dome-shaped muscle called the diaphragm. The diaphragm is an important muscle for breathing.

Diagram of lungs


Every part of your body needs oxygen to survive. It is carried around the body by red blood cells in your bloodstream. Oxygen cannot get into your blood directly through the skin, so a complicated system in your lungs absorbs it from the air and transfers it into your bloodstream.

Before birth a baby relies on its mother’s blood for oxygen and its lungs are filled with fluid. From the moment of birth, a child must draw air into its lungs and get its own oxygen. Your brain is constantly receiving signals from your body about the amount of oxygen needed. This depends on how active you are. For example, when you are asleep you need far less oxygen than if you are running to catch a bus. So you will breathe more slowly. Once your brain knows how much oxygen is needed, it sends messages along nerves to your breathing muscles, so that the right amount of air is breathed into your lungs. When the nerves to your breathing muscles tell you to breathe in, your diaphragm is pulled flat. At the same time, the muscles between your ribs pull your rib cage upwards and outwards. This means your lungs have the largest possible space to expand and pull in air.

Each time you breathe, air is drawn into your nose or mouth, down through your throat and into your windpipe, also called your 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 (diagram below). Breathing out is usually just a matter of relaxing your diaphragm and the muscles between the ribs. This pushes the air out and your lungs return to their resting size.

How does oxygen get into the bloodstream?

45219675721Inside the alveoli, oxygen moves across paper-thin walls to tiny blood cells called capillaries and into your blood. It is then picked up by a chemical called haemoglobin in the red blood cells that carry it round your body. At the same time, waste products from your body, in the form of carbon dioxide, come out of the capillaries back into the alveoli, ready to be breathed out. Blood with fresh oxygen is carried from your lungs to the left side of your heart, which pumps blood around your body through the arteries. Once the oxygen has been used up, the blood returns through the veins, to the right side of your heart. From there it is pumped to your lungs to remove carbon dioxide and breathe in more oxygen.

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