International A Level Biology / The nervous system and neurones
How the pupil dilates and contracts
Edexcel International A Level Biology (YBI11), chapter 23: The nervous system and neurones, specification point 8.6. Part of the full course.
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The pupil changes size when the circular and radial muscles in the iris contract or relax. These reflex responses control how much light reaches the retina, and you can compare them by calculating the change in pupil area.