The eye has an internal pressure created by production of a clear fluid called aqueous humor. This fluid is necessary to maintain the shape of the eye and it acts to maintain the function of the optically clear parts of the eye, including the lens and cornea. This fluid circulates through the eye and exits through the anterior chamber angle and ultimately drains into the bloodstream through blood vessels outside of the eye. In most forms of glaucoma, the aqueous humor outflow is obstructed, resulting in increased eye pressure and, eventually, optic nerve damage. However, glaucoma is not a disease of high pressure in the eye alone. Glaucoma is an optic nerve disease and high pressure within the eye is one of the major risk factors. The vision loss from glaucoma is the result of the death of the structures making up the optic nerve that convey the visual information gathered from the eye to the brain, which is interpreted as vision. The characteristic pattern of damage to the nerve fiber layer in glaucoma produces the patterns of visual field defects that are characteristic of the disease.