The eye contains a fluid known as aqueous humor that provides nourishment to its internal structures. This fluid is produced behind the iris (colored portion of the eye) and then is drained out through a sieve-like structure called the trabecular meshwork at the front part of the eye. In some eyes, abnormalities in the drainage system lead to impairment of the normal aqueous humor outflow, and IOP increases. This high IOP may damage the optic nerve head located in the back part of the eye and impair the communication between the eye and the part of the brain responsible for the vision. In other cases, IOP may be relatively normal, but glaucoma occurs anyway because of the inability of the eye to handle mechanical stress where the nerve fibers leave the eye, or because of poor blood supply to these same nerve fibers.