# Explain how the pupil of the human eye changes size in bright light and describe the role of the antagonistic muscles involved in this response.

> Pearson Edexcel International GCSE Biology (4BI1) — 2.15 Coordination and response: humans · Explain · 4 marks

## Mark scheme (4 marks)

1. In bright light, the pupil constricts / becomes smaller / decreases in diameter
2. The circular (constrictor) muscles of the iris contract
3. The radial (dilator) muscles of the iris relax
4. This reduces the amount of light entering the eye / reaching the retina, preventing damage to the retina / photoreceptors

## Key terms

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

## 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-how-the-pupil-of-the-c6287613) · Published by Druglandscape Ltd.