Explain how the structure of the alveoli in the human lung is adapted to maximise the rate of gas exchange.

IB DP Biology Higher Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Alveoli have a very large total surface area, increasing the area available for diffusion of oxygen and carbon dioxide.
Alveolar walls consist of a single layer of squamous (type I) epithelial cells, minimising the diffusion distance for gases.
Alveoli are surrounded by a dense network of pulmonary capillaries, maintaining a steep concentration gradient by continuously removing oxygen and delivering carbon dioxide.
The surface of alveoli is coated with a thin film of moisture/surfactant, allowing gases to dissolve and diffuse across the epithelium.

Examiner tips

Use the word ‘adapted’ to link structure to function. Mention the large surface area, single‑cell wall, capillary network and surfactant film. Keep each point concise and use correct terminology (e.g. ‘squamous epithelial cells’, ‘diffusion distance’).

Common mistakes

Failing to mention the single‑cell wall or the surfactant film. Using vague terms like ‘big’ instead of ‘large surface area’. Mixing up the direction of gas movement (e.g. saying oxygen is removed from the alveoli).

Mark scheme (4 marks)

Alveoli have a very large total surface area, increasing the area available for diffusion of oxygen and carbon dioxide.
Alveolar walls consist of a single layer of squamous (type I) epithelial cells, minimising the diffusion distance for gases.
Alveoli are surrounded by a dense network of pulmonary capillaries, maintaining a steep concentration gradient by continuously removing oxygen and delivering carbon dioxide.
The surface of alveoli is coated with a thin film of moisture / surfactant, allowing gases to dissolve and diffuse across the epithelium.

Key terms in this question

alveoli