Every cell in your body needs a steady supply of oxygen for respiration and must get rid of carbon dioxide. Your breathing system pulls air deep into the lungs, where it meets the bloodstream across an enormous, ultra-thin surface — the alveoli. Spread flat, your alveoli would cover roughly half a tennis court.
Why this matters Gas exchange links directly to respiration, exercise and health. It also explains why smoking, asthma and pneumonia are so damaging, and why the alveoli are shaped the way they are — a favourite CAIE “explain the adaptations” question.
The route air takes
- Air enters through the nose/mouth and down the trachea (windpipe), held open by rings of cartilage.
- The trachea splits into two bronchi, one to each lung.
- Each bronchus branches into many smaller bronchioles.
- Each bronchiole ends in clusters of tiny air sacs, the alveoli, wrapped in capillaries.
Features of a good gas-exchange surface
Alveoli are superbly adapted, and CAIE expects you to link each feature to why it helps:
| Feature | Why it helps |
|---|---|
| Large surface area (millions of alveoli) | More area for diffusion, so faster exchange |
| Very thin walls (one cell thick) | Short diffusion distance for gases |
| Moist lining | Gases dissolve before diffusing across |
| Rich blood supply (many capillaries) | Maintains a steep concentration gradient |
Gases move by diffusion: oxygen from the alveolus (high O₂) into the blood (low O₂), and carbon dioxide the other way.
How we breathe in and out
- Inhalation — the diaphragm contracts and flattens, the intercostal muscles pull the ribs up and out. Chest volume increases, pressure inside falls below outside, and air is pushed in.
- Exhalation — the diaphragm relaxes and domes up, ribs move down and in. Volume decreases, pressure rises, and air is forced out.
Worked example. “Explain how the alveoli are adapted for efficient gas exchange (4 marks).” Score one mark per feature-plus-reason: (1) large surface area — more space for diffusion; (2) thin walls (one cell thick) — short diffusion distance; (3) moist lining — gases dissolve to diffuse; (4) good blood supply — keeps a steep concentration gradient. Always pair the feature with why.
Common exam mistake Muscles can only pull, not push. The diaphragm contracts to flatten on inhaling; it does not “push air out” — it relaxes when you breathe out. Also, air moves because of pressure differences, not because the lungs “suck”.
Memory hook Good exchange surfaces are “large, thin, moist and well-supplied” — the same four words work for alveoli, gills and leaves. For breathing in: diaphragm Down, air In.