Alveoli adaptations for gas exchange

Alveoli are the tiny air sacs at the ends of the airways in your lungs, where oxygen passes into the blood and carbon dioxide passes out. Their adaptations all speed up diffusion: a huge surface area (about 75 m²), walls only one cell thick for a short diffusion path, a dense network of capillaries, and ventilation by breathing. The last two keep the concentration gradients steep.

What are alveoli?

Air enters through the trachea, which splits into two bronchi, one to each lung. These branch into narrower and narrower tubes that end in clusters of alveoli, like bunches of grapes. OpenStax notes that gas exchange happens only in the alveoli: the airways leading to them just carry the air. AQA's GCSE specification limits the lungs to the trachea, bronchi, alveoli and the capillary network around the alveoli.

How alveoli are adapted for gas exchange

AQA's GCSE specification lists what makes any exchange surface effective: a large surface area, a thin membrane for a short diffusion path, an efficient blood supply, and, for gas exchange, ventilation. Alveoli have all four:

The numbers in the table come from OpenStax. The three things that set the rate of diffusion are the same ones AQA lists: the concentration difference, the temperature and the surface area, and a thinner barrier helps too. How they combine is covered in should an exchange surface be thick or thin, which explains Fick's law.

Why the concentration gradient matters

Particles spread out by diffusion, so overall they move from where they are more concentrated to where they are less concentrated. Air in the alveoli has more oxygen than the blood arriving from the body, so oxygen diffuses in. That blood has more carbon dioxide than the alveolar air, so carbon dioxide diffuses out. If the blood stood still, or the air was never replaced, the two sides would even out and diffusion would slow to nothing. Blood flow and breathing stop that happening.

Worked example: where 75 m² comes from

OpenStax gives a diameter of 200 to 300 micrometres for each alveolus and about 300 million alveoli per lung. Model each alveolus as a sphere of diameter 200 µm and check the total surface area.

1. Radius = 100 µm = 1.0 × 10⁻⁴ m.
2. Surface area of one sphere = 4πr² = 4 × π × (1.0 × 10⁻⁴)² = 1.26 × 10⁻⁷ m².
3. Two lungs hold about 2 × 300 million = 6.0 × 10⁸ alveoli.
4. Total = 6.0 × 10⁸ × 1.26 × 10⁻⁷ = 75 m², matching the figure OpenStax quotes.

Splitting the lungs into many tiny sacs is what gives the huge area: small spheres have a far bigger surface area to volume ratio than one large one. For a sphere the ratio is 3 ÷ r, so halving the radius doubles it. The same surface area to volume idea explains why a single-celled organism doesn't need lungs at all, which AQA also asks about.

Alveoli at A-level

AQA's A-level specification adds the essential features of the alveolar epithelium as a gas exchange surface, the mechanism of breathing (the diaphragm and the antagonistic external and internal intercostal muscles changing the pressure in the thorax), and interpreting data on how lung disease, pollution and smoking affect gas exchange. Any disease that thickens the barrier or destroys alveolar walls works against one of the adaptations above.

Common mistakes

• Saying the alveoli have "thin walls" and stopping there. Say why: a short diffusion path, so faster diffusion.
• Saying oxygen is "pumped" or "breathed" into the blood. It diffuses, down its concentration gradient.
• Forgetting the capillary wall. The diffusion path crosses two thin walls: the alveolus and the capillary.
• Calling ventilation and gas exchange the same thing. Ventilation moves air in and out; gas exchange is diffusion across the alveolar surface.
• Saying "large surface area" for one alveolus. It is the huge number of tiny alveoli that gives the large total area.

Gas exchange is in GCSE Biology and Combined Science. Practise it on today's GCSE Biology Higher short timed paper. The human physiology course has an interactive alveolus in its gas exchange section, and the heart guide follows the blood on from the lungs.

Read next

Biology · Organisation and transportShould an ideal exchange surface be thick or thin?An ideal exchange surface is thin, for a short diffusion path. Why it matters, the other features, Fick's law, and examples from lungs to leaves.