# Explain why oxygen moves from the air sacs (alveoli) in the lungs into the blood in the surrounding capillaries.

> Edexcel GCSE Biology (1BI0) — 1.5 Transport in cells · Explain · 4 marks

> During gas exchange in the lungs, oxygen must cross from the alveoli into the blood to be transported around the body. Oxygen concentration in the alveoli is higher than in the blood arriving at the lungs.

## Mark scheme (4 marks)

1. Names the correct process as diffusion
2. States that there is a concentration gradient / higher concentration of oxygen in the alveoli than in the blood
3. States that particles / molecules move from a region of higher concentration to a region of lower concentration
4. States that this continues until equilibrium is reached / until the concentration gradient no longer exists, OR links the continuous movement of blood to maintaining the concentration gradient

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](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-oxygen-moves-from-the-8554e37c) · Published by Druglandscape Ltd.