# Explain why oxygen moves from the air in the alveoli into the blood in the surrounding capillaries during gas exchange in the lungs.

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

> During gas exchange in the human lungs, gases move between the alveoli and the blood in the capillaries that surround them. The blood arriving at the lungs has returned from the body's tissues, where it has delivered oxygen to respiring cells.

## Mark scheme (4 marks)

1. Oxygen moves by diffusion (down a concentration gradient)
2. The concentration of oxygen is higher in the alveoli than in the blood arriving at the lungs
3. This is because oxygen has been used up / removed by respiration in the body's tissues, so the returning blood has a low concentration of oxygen
4. The thin walls of the alveoli / capillaries (one cell thick) allow rapid diffusion / maintain the concentration gradient by continuously supplying deoxygenated blood

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-oxygen-moves-from-the-0700a3ac) · Published by Druglandscape Ltd.