The eye is filled with clear fluid, called aqueous humor, which is produced by special cells inside the eye. Aqueous humor drains from the eye and into the blood stream. Normally, the rate of aqueous humor production is equal to the rate of drainage. The constant flow of aqueous humor maintains a constant pressure that supports the structures inside the eye; it also provides oxygen and nutrients to the inside of the eye.

In those who have glaucoma, the drainage system does not function normally. As a result, aqueous humor does not drain from the eye. This increases the pressure inside the eye, which damages the optic nerve. It is not known why the drainage system malfunctions.