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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Efficient gas exchange surfaces share four key features: a large surface area to maximise diffusion, a thin barrier (short diffusion distance) so gases pass through quickly, a moist surface to dissolve gases before diffusion, and a good blood supply to maintain a steep concentration gradient.
Gas exchange surface features are tested across multiple contexts: alveoli, villi, root hairs, leaf mesophyll. Understanding the four general principles lets you apply them to any surface the examiner asks about, rather than memorising separate lists.
The four key features
Feature
Why it increases the rate of diffusion
Large surface area
More area for gas molecules to diffuse across simultaneously
Thin barrier (short diffusion distance)
Gas molecules travel a shorter distance, reaching the other side faster
Moist surface
Gases dissolve in the moisture before diffusing across the membrane
Diffusion occurs along a concentration gradient: from higher to lower concentration. For gas exchange to continue rapidly, this gradient must remain steep. Two mechanisms maintain it:
Blood flow: blood continuously carries oxygen away from the exchange surface and brings carbon dioxide to it (or vice versa in the lungs). Ventilation: breathing refreshes the air in the lungs, bringing in more oxygen and removing carbon dioxide. Together, blood flow and ventilation ensure the concentration on each side of the exchange surface stays different.
Applying the features to alveoli
The alveoli of the lungs are the primary gas exchange surface in humans. They demonstrate all four features: millions of alveoli create an enormous total surface area (approximately 70 m² in an adult); walls are one cell thick, giving a very short diffusion distance; the alveolar surface is kept moist by a thin film of fluid; and a dense capillary network surrounds each alveolus, maintaining the concentration gradient.
These same principles apply wherever efficient exchange is needed: from the gills of fish to the villi of the small intestine.
Try a focused question (4 marks)
Explain how the structure of alveoli is adapted for efficient gas exchange.
Show the answer and marking guidance
Large number of alveoli provides a very large total surface area for diffusion.
Alveolar walls are one cell thick, giving a short diffusion distance.
Moist lining allows gases to dissolve before diffusing.
Dense network of capillaries maintains a steep concentration gradient by carrying blood close to the alveolar surface.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Listing features without explaining how each one increases diffusion rate.
Saying thin walls rather than specifying one cell thick for alveoli.
Forgetting the role of ventilation in maintaining the concentration gradient.
Applying these features to contexts where they do not apply: for example, thick bark is not a gas exchange surface.