# Explain how ventilation of the lungs maintains the concentration gradients necessary for gas exchange in the alveoli.

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

## Mark scheme (4 marks)

1. Oxygen is continuously removed from alveolar air by diffusion into the blood, so fresh air must be delivered to maintain a high alveolar oxygen partial pressure / concentration.
2. Carbon dioxide diffuses from the blood into the alveoli, and expiration removes this CO₂-rich air, maintaining a low partial pressure / concentration of carbon dioxide in the alveoli.
3. The steeper / maintained concentration gradient increases the net rate of diffusion of gases across the alveolar membrane, according to Fick's law / principle.
4. Continuous blood flow (perfusion) through the alveolar capillaries works in concert with ventilation: blood constantly carries oxygen away and delivers CO₂, preventing equilibration of gases on the blood side of the membrane.

## Key terms

- [ventilation](https://www.gradenine.co.uk/glossary/ventilation)
- [concentration gradient](https://www.gradenine.co.uk/glossary/concentration-gradient)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-ventilation-of-the-lungs-168aebe3) · Published by Druglandscape Ltd.