Topic 11.1 · Core
The Human Breathing System
Cambridge doesn’t just want you to know the names of the breathing system’s parts — it wants you to recognise them in a diagram you haven’t seen before, in an unfamiliar orientation or artistic style, and still trace the correct route air takes to reach the alveoli.
The structures you need to identify
You must be able to identify the lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli, and the capillaries associated with the alveoli. That identification skill is itself part of what’s being examined — a diagram that draws these structures at an unfamiliar scale, or isolates just one alveolus with its capillary net, should still be readable once you know what each part does.
The airway, in order
A useful route to hold in your head is: trachea → bronchi → bronchioles → alveoli. The larynx sits at the top of this route, at the upper end of the trachea. The trachea itself is the main airway leading down from the larynx, and it divides into two bronchi — one entering each lung. Inside the lungs, the bronchi branch repeatedly into progressively smaller bronchioles, which eventually lead to clusters of alveoli, the tiny air spaces where gas exchange actually happens. Wrapped closely around each alveolus is a network of capillaries — blood vessels, not air passages — bringing the alveolar air into close contact with the blood supply. The full mechanics of what alveoli and their capillaries achieve is covered in gas exchange surfaces and alveoli; here, the job is simply to identify the structure correctly.
A common slip is treating bronchioles as just “tiny bronchi” or alveoli as just “tiny bronchioles”. Keep the roles distinct: bronchi and bronchioles are airway branches of decreasing size, while alveoli are the exchange surfaces themselves, not simply the smallest airway in the sequence.
Ribs, intercostal muscles and the diaphragm
The ribs form part of the wall of the thorax, and intercostal muscles lie between them. The diaphragm is a sheet of muscle below the lungs. At Core level, you only need to identify these structures in a diagram; what they actually do to move air in and out of the lungs — including the separate roles of the internal and external intercostal muscle layers — is Supplement content covered on its own page, mechanism of breathing.
Cartilage in the trachea
The trachea is supported by rings of cartilage. As pressure inside the breathing system changes during each breath, this cartilage stops the trachea from collapsing, so air can keep moving through it freely. It’s worth being precise about what cartilage is actually doing here: it supports the airway and keeps it open, rather than causing air to move — that job belongs entirely to the pressure changes described in mechanism of breathing.
Protecting the breathing system
Air entering the breathing system carries particles and microorganisms, some of which are pathogens, and the airway lining has a coordinated defence built specifically to stop them reaching the delicate gas-exchange surfaces deeper in the lungs. Cambridge requires three roles here, kept strictly separate:
| Structure | Role |
|---|---|
| Goblet cells | Produce mucus — they don’t move it |
| Mucus | Traps pathogens and particles — it doesn’t move itself |
| Ciliated cells | Beat their cilia to move the mucus, and everything trapped in it, towards the throat and away from the gas-exchange surfaces |
The protective value comes from the sequence running in that exact order — goblet cells produce mucus, mucus traps pathogens and particles, cilia move the contaminated mucus away from the lungs — and stating only “there is mucus” doesn’t explain how the trapped material actually gets removed. It’s also worth keeping the mechanism in proportion: this mucus-and-cilia defence belongs to the conducting airways such as the trachea and bronchi, not to the alveoli themselves. A thick mucus layer over the thin, moist alveolar wall described in gas exchange surfaces and alveoli would block diffusion rather than protect it, which is exactly why this defence is stationed further up the airway, before contamination ever reaches the exchange surface. For more on the pathogens this system is built to intercept, see pathogens and disease transmission.
Don’t confuse
Keep each structure attached to its own role rather than treating the airway as one interchangeable tube. Bronchi and bronchioles are airway branches, not exchange surfaces; capillaries are blood vessels, not air passages; cartilage supports the airway rather than moving air through it; and goblet cells, mucus and cilia each do one distinct job in a sequence, not three versions of the same job.