Normally, under natural light, the diameter of a normal adult pupil is about 2.5 to 4mm, but the exact size varies from person to person, with older people and young children having slightly smaller pupils.
The pupil is the channel through which light enters the eye. The contraction of the pupil sphincter and the pupil opening muscle on the iris can cause the pupil to dilate and enlarge, thus controlling the amount of light entering the eye, in order to protect the eye and provide clear vision. Pupil dilation and narrowing can be seen in both physiologic and pathologic conditions.
In high light, the pupil will narrow to prevent too much light from entering the eye, while in low light, the pupil will widen to allow more light to enter the eye to ensure clear vision. Pathologic pupil narrowing can be seen in organophosphorus pesticide poisoning, while pupil dilation can be seen in atropine poisoning, glaucoma, brain hernia and other diseases.
The size of the pupil is an important indicator of vital signs, and it is recommended that patients with abnormal pupil size consult a doctor promptly to assess their condition and follow medical advice.