What happens when you breathe
Follow the air from your nose to your lungs, and see the muscle that pulls it in.
Meet your lungs
These are your lungs. They are two soft, spongy bags inside your chest. In a grown-up who is resting, air goes in and out of them about 15 times every minute, even during sleep; children breathe a bit faster. The right lung has three parts, the left has two, to leave room for your heart. Press play to watch them fill and empty.
The lungs sit either side of the heart inside the rib cage, each wrapped in a thin double membrane, the pleura. The right lung has three lobes (superior, middle, inferior); the left has two, with a notch where the heart sits. Adults at rest breathe 12 to 20 times a minute, moving roughly half a litre of air per breath. The swelling shown here is exaggerated a little so you can see it.
The road the air takes
Air comes in through your nose (or mouth), goes down your throat, and into your windpipe. The windpipe splits into two tubes, one for each lung. Those split into smaller and smaller tubes, like the branches of an upside-down tree. Watch the air travel in.
Air enters through the nasal cavity, where it is warmed, moistened and filtered, then passes through the pharynx and larynx into the trachea, a tube about 10 to 12 cm long held open by C-shaped rings of cartilage. The trachea divides into the right and left main bronchi, which divide into lobar bronchi (three on the right, two on the left), then segmental bronchi, then ever smaller bronchioles: about 23 generations of branching, ending in the alveoli. The epiglottis folds over the airway when you swallow so food goes down the oesophagus instead.
The muscle that does the work
Lungs cannot move by themselves. A big muscle under them, the diaphragm, does the pulling. It is shaped like a dome. When it tightens, it flattens and moves down, which makes the chest bigger, so air rushes in. When it relaxes, it moves back up and the air is pushed out. Watch it go down and up.
The diaphragm is a dome-shaped sheet of muscle separating the chest from the abdomen. When it contracts it flattens and descends by 1 to 2 cm in quiet breathing (up to 10 cm in deep breathing), and the external intercostal muscles lift the ribs. Chest volume rises, pressure inside the lungs falls below atmospheric pressure, and air flows in. Quiet exhalation is passive: the diaphragm relaxes and the elastic lungs recoil. Forced exhalation adds the abdominal and internal intercostal muscles.
The swap at the end of the road
At the very end of the tiniest tubes are millions of tiny air bubbles. Blood flows right past them in very thin pipes (the lit-up pipes bring blood to the lungs and carry it away again). Oxygen from the air hops into the blood, and a waste gas called carbon dioxide hops out of the blood into the air, ready to be breathed out.
The bronchioles end in about 300 to 500 million alveoli, thin-walled air sacs with a total surface area near 70 square metres. Each is wrapped in capillaries. Oxygen diffuses across the alveolar-capillary membrane into the blood and binds to haemoglobin; carbon dioxide diffuses the other way and is exhaled. The pulmonary arteries (from the right ventricle) bring oxygen-poor blood to the lungs, and the four pulmonary veins return oxygen-rich blood to the left atrium. Alveoli themselves are too small to model here; the lung lobes stand in for them.
In and out, all day long
Put it all together: the diaphragm pulls down, fresh air (blue) flows in, the lungs fill. Then the diaphragm relaxes, the lungs shrink, and used air (orange) flows out. About 20,000 times a day, and you never have to think about it. Your brain does it for you, even when you are asleep.
One breathing cycle: inspiration (diaphragm and external intercostals contract, air in) then expiration (passive recoil, air out). The rhythm is set by the respiratory centres in the brainstem (medulla and pons), which respond mainly to rising carbon dioxide in the blood, sensed by chemoreceptors. That is why you cannot hold your breath indefinitely. At 15 breaths a minute you take about 20,000 breaths a day and move roughly 10,000 litres of air. Exhaled air still holds about 16% oxygen, which is why rescue breathing works.