Explain how the structure of alveoli makes them well adapted as an exchange surface for gaseous exchange in the lungs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The lungs contain millions of tiny air sacs called alveoli. Oxygen must move from the air inside the alveoli into the blood, while carbon dioxide moves in the opposite direction.
Model answer (5 marks)
1. The alveoli have a very large total surface area, which increases the rate of diffusion and allows a large amount of gas to be exchanged at once.
2. The walls of the alveoli are extremely thin, reducing the diffusion distance and allowing gases to move across more quickly.
3. The alveolar walls are lined with a moist membrane, which lets gases dissolve and diffuse across the membrane.
4. The alveoli are supplied by a rich capillary network, maintaining a steep concentration gradient for oxygen and carbon dioxide.
5. These structural features together maximise the rate of diffusion of oxygen into the blood and carbon dioxide out, meeting the demands of aerobic respiration.
2. The walls of the alveoli are extremely thin, reducing the diffusion distance and allowing gases to move across more quickly.
3. The alveolar walls are lined with a moist membrane, which lets gases dissolve and diffuse across the membrane.
4. The alveoli are supplied by a rich capillary network, maintaining a steep concentration gradient for oxygen and carbon dioxide.
5. These structural features together maximise the rate of diffusion of oxygen into the blood and carbon dioxide out, meeting the demands of aerobic respiration.
Examiner tips
- Use the exact terms: surface area, thin walls, moist lining, rich blood supply, diffusion gradient.
- Show the causal link: each adaptation leads to faster diffusion.
- Keep each point concise and separate.
- Mention the final outcome: meeting aerobic respiration demands.
Common mistakes
- Confusing alveoli with bronchioles or forgetting the moist lining.
- Using vague terms like "big" instead of "large surface area".
- Failing to link the adaptations to the rate of diffusion or to aerobic respiration.
Mark scheme (5 marks)
- Large surface area (of alveoli) increases the rate of diffusion / allows more gas to be exchanged at once
- Thin walls reduce the diffusion distance so gases can diffuse across more quickly
- Good/rich blood supply maintains a steep concentration gradient (for oxygen / carbon dioxide)
- Moist lining allows gases to dissolve so they can diffuse across the membrane
- Overall, these adaptations maximise the rate of diffusion of oxygen into the blood (and carbon dioxide out), meeting the demands of aerobic respiration
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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