# Explain why the concentration of carbon dioxide is higher in the air inside the alveoli than in the blood arriving at the alveoli from the pulmonary artery.

> Pearson Edexcel International GCSE Biology (4BI1) — 2.9 Gas exchange: humans · Explain · 4 marks

> During gas exchange in the lungs, carbon dioxide moves from the blood into the air inside the alveoli. The direction of this movement depends on concentration gradients.

## Mark scheme (4 marks)

1. Carbon dioxide is produced during (aerobic) respiration in body cells / tissues
2. Carbon dioxide diffuses from body cells into the blood / blood picks up / carries carbon dioxide from respiring tissues
3. So the blood arriving at the alveoli has a high concentration of carbon dioxide / the concentration of carbon dioxide in the blood is higher than in alveolar air
4. Therefore carbon dioxide diffuses down the concentration gradient from the blood into the alveolar air / from higher to lower concentration

## Key terms

- [alveoli](https://www.gradenine.co.uk/glossary/alveoli)

## Related

- [Revision notes for Pearson Edexcel International GCSE Biology (4BI1)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-concentration-of-carbon-d3b11e64) · Published by Druglandscape Ltd.