How do the lungs work?

Every breath takes the same journey. Air comes in through your nose (or mouth), where it is warmed, wetted and filtered. It passes the pharynx (your throat) and the larynx, your voice box, where a flap called the epiglottis shuts when you swallow, so food goes down the food pipe instead.

Then comes the trachea, or windpipe: about 11 cm long, held open by C-shaped rings of cartilage. At the carina it splits into two bronchi, and these split again and again, about 23 times, into thinner and thinner bronchioles. At the very ends sit bunches of tiny air sacs, the alveoli. You have about 480 million of them.

Your two lungs are not twins. The right lung has three lobes. The left lung has two, and a dent called the cardiac notch makes room for your heart (see HeartClear). Each lung sits in a slippery two-layer bag, the pleura. Underneath is a dome of muscle, the diaphragm, and all around are the ribs with the intercostal muscles between them.

Your lungs have no muscles of their own. They are stretchy bags that follow the chest wall, because the thin film of fluid in the pleura sticks them to it, like two wet sheets of glass.

To breathe in, the diaphragm contracts: its dome flattens and moves down. The intercostal muscles lift the ribs up and out, like a bucket handle. The chest gets bigger, so the air inside the lungs has more room. By Boyle’s law (see the gas laws), a gas given more room has a lower pressure. The pressure in your air sacs drops about 1 cmH₂O below the air outside, a change of only 0.1%, and air flows in until the pressures match again.

Breathing out at rest takes no effort at all. The muscles relax and the stretched lungs recoil like a balloon, squeezing the air back out. The bell jar on the left shows the same idea: pull the rubber sheet down and the balloons inside fill up.

A quiet breath is about 500 mL, the tidal volume. Adults at rest breathe 12 to 20 times a minute, so about 6 to 8 litres of air flow in and out every minute.

About 150 mL of each breath only fills the airways, the dead space, and never reaches the air sacs. So slow, deep breaths refresh the air sacs better than fast, shallow ones.

At the end of every airway is a bunch of tiny bubbles, the alveoli. Each is about a fifth of a millimetre across, and you have around 480 million. Spread flat, their walls would cover about 70 m², most of a badminton court, all folded into your chest.

Each alveolus is wrapped in a net of capillaries, blood vessels so narrow that red blood cells pass in single file. Between air and blood lies a wall only about 0.5 µm thick, a hundred times thinner than a sheet of paper.

No pump pushes the gases across. They diffuse, spreading from where there is more to where there is less (see diffusion and osmosis). Air in the alveoli has an oxygen partial pressure of about 100 mmHg; blood arriving from the body has only 40. So oxygen moves into the blood. Carbon dioxide goes the other way, from 46 in the blood to 40 in the air. Fick’s law says the flow grows with the area and the pressure difference, and shrinks as the wall gets thicker.

The inside of each alveolus is wet, and a wet bubble wants to collapse. Your lungs make surfactant, a soapy film that lowers the surface tension. By Laplace’s law, P = 2T/r, small bubbles need more pressure to stay open, so without surfactant small alveoli would empty into big ones.

Very little oxygen dissolves in blood plasma. Almost all of it rides on haemoglobin, the protein that makes red blood cells red. Each haemoglobin molecule has four parts, and each part holds an iron atom that can grab one O₂ molecule, so four in all. One red cell carries about 270 million haemoglobin molecules.

Haemoglobin is a team player. Once one oxygen sticks, the others stick more easily. That gives the oxygen–haemoglobin dissociation curve its S shape: at the 100 mmHg in your lungs the blood fills to about 97–98%, and half full needs only about 27 mmHg, the P50. On the steep middle part, small drops in oxygen let lots of it go, right where your tissues need it.

Busy muscles are warmer, more acidic and full of CO₂. All three push the curve to the right, so haemoglobin lets go of more oxygen. That is the Bohr effect.

High up, the air is thinner. At Leh (about 3,500 m) the air pressure is two-thirds of sea level, and on the summit of Everest it is about a third: 253 mmHg. Climbers breathe hard to blow off CO₂, which makes room for more oxygen in the air sacs.

A pulse oximeter shines red and infrared light through your finger. Oxygen-rich blood absorbs less red light and more infrared, so the ratio of the two pulsing signals tells the saturation, SpO₂. Most healthy people read 95% or more at sea level. Readings can be less accurate on darker skin, cold fingers or nail polish, and they are only part of the picture: if you are worried about your breathing or a reading, see a doctor.

A spirometer measures how much air you breathe in and out. The old kind is an upturned bell floating in water: breathe out into it and it rises. A pen draws your lung volume on a turning drum, breathing in going up.

A quiet breath, the tidal volume, is only about 500 mL. Breathe in as far as you can and you add the inspiratory reserve, about 3 litres more. Push out as hard as you can and you empty the expiratory reserve, about 1 litre. All of that together is the vital capacity, around 4.5 to 5 litres for a young adult man. Even then about 1.2 litres stays behind, the residual volume, so your lungs never collapse. The total lung capacity is about 6 litres.

Doctors often ask you to blow out as hard and fast as you can. The litres out in the first second are the FEV1; the total is the FVC. Healthy lungs push out about 70 to 85% of the total in that first second. If the airways are narrowed, as in asthma or COPD, the air comes out more slowly and the ratio drops.

Not all the air you breathe is useful. About 150 mL of every breath just fills the nose, throat and airways, the dead space, and never reaches the alveoli.

“Normal” depends on your age, height, sex and ancestry, so a single number means little on its own. Lung tests are read by doctors against the right reference for each person.

Every day you breathe in dust, pollen, germs and soot. Your airways clean themselves. They are lined with sticky mucus that traps particles, and millions of tiny hairs called cilia beat 10 to 15 times a second, sweeping the mucus up to your throat, where you swallow it. This is the mucociliary escalator. A cough blasts out anything stuck, with air rushing out at around 8 m/s; a sneeze can throw droplets 7 to 8 metres, so cover it with your elbow.

Smoking coats the airways in tar and slows and damages the cilia, so dirt and germs stay put. In India about 29% of adults use tobacco in some form (GATS-2, 2016–17). Smoke is the main cause of lung cancer and of COPD, a long-term illness where airways stay narrowed and alveoli are destroyed. Smoke from cooking fires can cause COPD too.

Air pollution matters just as much. The tiniest specks, PM2.5, are 30 times thinner than a hair and reach deep into the lungs. The WHO guideline is a yearly average of 5 µg/m³. India’s average is about 50, and Delhi’s is about 100, some 20 times the guideline (IQAir).

In asthma, airways swell and their muscles squeeze. Because resistance grows with 1/r⁴ (see how fluids flow), halving an airway’s radius makes it 16 times harder to breathe through. In pneumonia, an infection fills alveoli with fluid, so oxygen can’t reach the blood. Tuberculosis (TB) is caused by a germ that spreads through coughs; India has about a quarter of the world’s cases (WHO, 2025). TB is curable, and tests and treatment are free at government health centres in India.

These are general facts, not medical advice. A cough that lasts more than two weeks, breathlessness, chest pain or coughing blood are reasons to see a doctor. Moving your body helps your lungs and heart work well together (see where energy goes).