# Explain how the structure of the alveolar wall in the human lung is adapted to maximise the rate of gas exchange by diffusion.

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

## Mark scheme (4 marks)

1. The alveolar wall is only one cell thick (a single layer of squamous epithelial cells), which minimises the diffusion distance for oxygen and carbon dioxide.
2. The alveolar epithelial cells are squamous (flattened), which further reduces the diffusion distance across the gas-exchange surface.
3. The alveolar surface is covered by a thin film of moisture / surfactant, allowing gases (oxygen and carbon dioxide) to dissolve and diffuse across the membrane in solution.
4. The large number of alveoli provides a very large total surface area, increasing the overall rate of diffusion according to Fick's law (rate ∝ surface area).

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