Describe how the alveoli are adapted to act as an efficient exchange surface for gaseous exchange.

Cambridge International IGCSE Biology (0610) — 11.1 Gas exchange in humans · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Large surface area of the alveoli increases the area available for diffusion.
Thin alveolar walls and capillary walls minimise the diffusion distance.
A dense network of capillaries supplies blood, keeping a steep concentration gradient.
Ventilation of air into the alveoli maintains a steep gradient for oxygen and carbon dioxide.

Examiner tips

Use the four key points in the order given by the mark scheme. Show how each feature reduces diffusion distance or increases area. Mention the concentration gradient explicitly.
Keep sentences concise and use exam terminology such as "diffusion", "capillaries" and "ventilation".

Common mistakes

Confusing alveoli with bronchioles. Omitting the role of ventilation or blood supply. Using vague terms like "big" instead of "large surface area".

Mark scheme (4 marks)

Large surface area to allow more efficient/rapid diffusion
Thin walls / short diffusion distance to allow rapid diffusion
Good blood supply / network of capillaries to maintain a steep/concentration gradient
Good ventilation with air to maintain a steep/concentration gradient

Key terms in this question

alveoli · exchange surface · gaseous exchange