The eye is filled with aqueous humor and vitreous humor.

Aqueous humor is a clear fluid in the front part of the eye.

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• Vitreous humor is a clear, jelly-like substance that fills the eye behind the lens and helps the eyeball keep its shape.

In a normal eye, aqueous humor is produced, circulates through the eye, and then drains out through the trabecular meshwork, which is the eye's filtration system. This is a series of tiny channels near the angle formed by the cornea (the clear portion of the eye), the iris (the colored portion of the eye), and the sclera (the white of the eye).

If there is any sort of blockage in these channels, pressure builds up inside the eyeball.

Untreated pressure in the eye can damage and eventually destroy the optic nerve, leading to blindness. But, surprisingly, there are some people who suffer from glaucoma even though they have normal pressure in their eyes. There also are people who have pressure in their eyes and yet do not suffer from glaucoma. Researchers are still trying to figure out why.

• Somewhere between 20 and 25 percent of people with glaucoma have normal (and maybe even low) eye pressure.
   
• Some people with high eye pressure, which is also called ocular hypertension, do not have glaucoma, and never will.