Every breath travels from the larynx down the trachea, bronchi and bronchioles to the alveoli, where oxygen passes into the blood and carbon dioxide passes out. This lesson names every part, explains why the alveoli are such a good gas exchange surface and (Extended) shows how cartilage, mucus and cilia keep the airways open and clean.
Air passes through the larynx, trachea, bronchi and bronchioles to the alveoli, tiny air sacs surrounded by capillaries. Gas exchange surfaces in humans have four features: a large surface area, a thin surface, a good blood supply and good ventilation with air. Together they keep oxygen diffusing into the blood and carbon dioxide diffusing out as fast as possible.
Rotate the 3D model and click each part. Air enters through the nose or mouth, passes the larynx (voice box) and goes down the trachea. The trachea divides into two bronchi, one to each lung, and these branch again and again into bronchioles that end in clusters of alveoli.
| Part | What it is |
|---|---|
| larynx | the voice box at the top of the trachea |
| trachea | the windpipe, carrying air from the larynx to the bronchi |
| bronchi | the two branches of the trachea, one to each lung |
| bronchioles | smaller and smaller tubes branching from the bronchi |
| alveoli | tiny air sacs at the ends of the bronchioles, where gas exchange happens (about 700 million) |
| capillaries | a dense network around every alveolus, bringing blood very close to the air |
| lungs | the two spongy organs of gas exchange, inside the thorax |
| ribs | protect the lungs and change the volume of the thorax when they move |
| intercostal muscles | muscles between the ribs |
| diaphragm | a sheet of muscle below the lungs that separates the thorax from the abdomen |
The order matters: trachea, then bronchi, then bronchioles, then alveoli. A common slip is to put bronchioles before bronchi. The singular of bronchi is bronchus, and of alveoli is alveolus.
Oxygen diffuses from the air in an alveolus into the blood in the capillary, and carbon dioxide diffuses the other way. All four features start switched on. Switch one off at a time and watch the oxygen rate fall from 10 per second.
| Feature | How the lungs provide it | Why it speeds up diffusion |
|---|---|---|
| large surface area | millions of tiny alveoli instead of one big sac | more gas can cross at the same time |
| thin surface | alveolus wall and capillary wall are each one cell thick | short diffusion distance |
| good blood supply | dense capillary network with blood always flowing | oxygen is carried away and carbon dioxide brought, keeping the concentration gradients steep |
| good ventilation with air | breathing replaces the air in the alveoli | keeps oxygen high and carbon dioxide low in the alveoli, keeping the gradients steep |
"Thin" refers to the surface (one cell thick), not to the lungs being thin. And a good blood supply does not push oxygen in: it carries oxygen away so the concentration gradient stays steep.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the breathing system model, which part carries air from the larynx to the bronchi?
About how many alveoli does the sim say there are in the lungs?
In the alveoli tab, switching off which single feature lowers the oxygen rate the most?
Why does the oxygen rate fall when the blood supply is turned off in the sim?
Extended. The air we breathe carries dust, pollen, bacteria and viruses. In the breathing system tab, set the View to "lining of the trachea (Extended)" to see how the lining of the airways traps and removes them before they reach the alveoli.
Cilia do not trap anything: the mucus traps, the cilia move the mucus. Goblet cells make mucus; they are not ciliated. Keep the three jobs separate.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which list shows the correct order in which air passes through the breathing system?
Which feature of the alveoli gives a short diffusion distance?
Extended: what is the function of the cartilage in the wall of the trachea?
Extended: how do ciliated cells protect the breathing system?
This skill is now recorded on your profile. Keep the chain going.