How the Lungs Work: Breathing, Airways and Alveoli in 3D

The pump that breathes

The picture shows the chest from the front, and the lungs are shown inside the rib cage. Lungs have no muscles of their own. Breathing is done by the diaphragm, a dome-shaped sheet of muscle under the lungs, and by small muscles between the ribs. When you breathe in, the diaphragm contracts and flattens and the ribs swing up and out. The chest gets bigger, the pressure inside drops a little, and air flows in. When you breathe out, the muscles relax and the lungs spring back. At rest, adults take about 12 to 20 breaths a minute.

A tree made of tubes

The model fades the lungs and shows only the airways. Air enters through the nose or mouth and travels down the trachea, or windpipe, a tube about 10 to 12 centimetres long held open by rings of cartilage. At its lower end the trachea divides into two main bronchi, one for each lung, and these divide again and again, around 20 times or more, into ever finer bronchioles. The branching looks like an upside-down tree. Sticky mucus and tiny hairs called cilia line the tubes, catching dust and germs and sweeping them up and out. The right lung has three lobes and the left has two, leaving room for the heart.

Grapes at the end of every twig

The camera dives down to the end of the smallest airway. Here the tubes end in clusters of tiny air sacs, the alveoli, which look like bunches of grapes. An adult has an estimated 300 to 500 million of them, and the figure of about 480 million is often quoted. Each is a fraction of a millimetre across and wrapped in a net of capillaries. The wall between air and blood is so thin that gases pass straight through it. Together the alveoli give the lungs a gas-exchange surface of tens of square metres, many times the area of your skin.

Gas exchange by diffusion

Watch one alveolus. Oxygen is more plentiful in the air in the sac than in the blood arriving from the body, so it drifts across the thin wall into the blood. There it is grabbed by haemoglobin, the red protein inside red blood cells. Carbon dioxide, a waste product made by cells, is more plentiful in the blood, so it drifts the other way and leaves with your next breath. No pump is needed, only the difference in concentration, called diffusion. A soapy liquid called surfactant lines the sacs and stops them from collapsing.

What changes in the air

Counting numbers appear in the overlay. The air you breathe in is about 21 percent oxygen, about 78 percent nitrogen and only about 0.04 percent carbon dioxide. The air you breathe out is about 16 percent oxygen and roughly 4 percent carbon dioxide, with extra water vapour. So your body takes only about a quarter of the oxygen in each breath. That is why a rescuer's breath can still help someone who has stopped breathing. A quiet breath moves about half a litre of air. Multiply it by your rate and you get minute ventilation, which you can test in the sandbox.

Looking after your lungs

The last moment shows healthy lungs, and then lungs after years of smoking, calmly and without drama. Cigarette smoke contains tar and irritants. They paralyse the cilia, inflame the airways and wear away the thin walls of alveoli, which leads to chronic bronchitis, emphysema and lung cancer. The good news is that damage slows down when a person quits. Exercise helps the lungs and heart work together more efficiently, and clean air matters too. If you want to stop smoking, a doctor or a local health service can help.

The lungs and breathing