# Explain how oxygen moves from the air sacs (alveoli) in the lungs into the blood, and why the concentration of oxygen in the alveoli remains higher than in the blood.

> Pearson Edexcel A-Level Biology A (9BI0) — 1.6 Membranes and transport · Explain · 5 marks

> The alveoli in the lungs are surrounded by a dense network of capillaries. Oxygen must move from the air inside the alveoli into the blood in these capillaries so it can be transported around the body.

## Mark scheme (5 marks)

1. Oxygen moves by diffusion
2. Diffusion occurs down a concentration gradient, from high concentration in the alveoli to low concentration in the blood
3. Blood arriving at the alveoli has a low concentration of oxygen (because it has been used up by respiring cells / returned from the body)
4. Breathing (ventilation) continuously replaces air in the alveoli, maintaining a high oxygen concentration there
5. The continuous flow of blood through the capillaries removes oxygen, keeping blood oxygen concentration low and maintaining the concentration gradient

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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-oxygen-moves-from-the-96665e6e) · Published by Druglandscape Ltd.