We use a tonopen to measure the pressure in eyes. What is intraocular pressure and how is it maintained?
The size and shape of the normal eye is maintained by the amount of fluid within the eyeball. The pressure of the fluid inside the front or anterior chamber of the eye is known as the intra-ocular pressure (IOP). Fluid inside the eye is constantly produced by a structure called the ciliary body. In addition to producing this fluid (aqueous humor), the ciliary body contains the suspensory ligaments which hold the lens in place. Muscles in the ciliary body pull on the suspensory ligaments, controlling the shape and focusing ability of the lens.

Aqueous humor contains nutrients and oxygen that are used by the structures within the eye. The excess fluid is constantly drained from the eye between the cornea and the iris. This area is called the iridocorneal angle or the filtration and drainage angle.

The intra-ocular pressure remains constant as long as the production and absorption or drainage of aqueous is equal.