Explain how the structure of the human lung's capillary network surrounding the alveoli facilitates efficient gas exchange.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Capillaries around the alveoli are only one endothelial cell thick, so the diffusion distance for O₂ and CO₂ is minimal.
The capillaries form a dense network, giving a huge surface area in contact with the alveolar wall and therefore a high rate of diffusion.
Blood flows continuously through these capillaries, keeping a steep concentration gradient for O₂ and CO₂ and driving rapid exchange.
Because the capillaries are about the same diameter as a red blood cell, the cells are slowed as they pass through, giving them more time to load O₂ and unload CO₂.
The capillaries form a dense network, giving a huge surface area in contact with the alveolar wall and therefore a high rate of diffusion.
Blood flows continuously through these capillaries, keeping a steep concentration gradient for O₂ and CO₂ and driving rapid exchange.
Because the capillaries are about the same diameter as a red blood cell, the cells are slowed as they pass through, giving them more time to load O₂ and unload CO₂.
Examiner tips
- Use the exact phrases from the mark scheme – e.g. "one cell thick", "dense network", "steep concentration gradient", "slowed as they pass through".
- Show the logical link: thin walls → short distance; large surface area → high rate; continuous flow → gradient; slow flow → more time.
Common mistakes
- Omitting the fact that capillaries are only one cell thick.
- Confusing the direction of gas flow or the role of blood flow with the concentration gradient.
Mark scheme (4 marks)
- Capillaries have walls that are only one cell (endothelium) thick, reducing the diffusion distance for gases.
- The dense network of capillaries creates a large surface area in contact with the alveolar surface, increasing the rate of diffusion.
- Continuous blood flow through the capillaries maintains a steep concentration gradient for both oxygen and carbon dioxide.
- Red blood cells are slowed as they pass through the narrow capillaries (which are approximately the same diameter as a red blood cell), allowing more time for gas exchange to occur.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Gas exchange questions
- Explain how countercurrent flow in the gills of bony fish increases the efficien…
- Explain how the ventilation of the lungs maintains the concentration gradients n…
- Explain how the tracheal system of insects allows gas exchange to occur without …
- Explain how the structure of a gill lamella is adapted to facilitate efficient g…
- Explain how the structure of the human trachea and bronchioles enables efficient…
- Explain how the structure of an alveolus is adapted to facilitate efficient gas …