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

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

## Mark scheme (4 marks)

1. Ventilation brings fresh air (high oxygen / low carbon dioxide) into the alveoli, maintaining a higher partial pressure of oxygen in the alveoli than in the blood of the pulmonary capillaries.
2. Expired air removes carbon dioxide from the alveoli, keeping the partial pressure of carbon dioxide in the alveoli lower than in the blood arriving from the body tissues.
3. Because gases diffuse down their concentration gradients, oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses from the blood into the alveolar air.
4. Without continuous ventilation the gradients would diminish as oxygen is consumed and carbon dioxide accumulates, reducing the rate of gas exchange; ventilation therefore sustains efficient exchange.

## 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 Standard 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-the-ventilation-of-the-2a5855cf) · Published by Druglandscape Ltd.