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Aqueous is a clear fluid with a consistency similar to water that flows through the front part of the eye. Aqueous is continuously produced by the ciliary body, which is located behind the iris, the colored part of the eye. As the aqueous flows through the central opening in the iris (the pupil) to enter the eye’s front chamber (the space between the iris and the cornea), it bathes and nourishes the eye’s lens and cornea. The aqueous then exits the eye through a meshwork tissue called the trabeculum, located at the “angle” where the iris connects to the inside wall of the eye (see Diagram 1), to enter drainage canals where it is absorbed into the veins of the general circulatory system.

As fluid enters the eye, it must exit at the same rate to maintain a stable fluid pressure inside the eye. That pressure is called the intraocular pressure (sometimes called IOP), and it typically ranges from 12 to 21 millimeters of mercury (mm Hg). The primary concern about intraocular pressure is that if it is too high, it can damage the optic nerve, which can result in lose of peripheral vision and blindness if not treated.

The idea of “normal” intraocular pressure simply means that the pressure in a particular eye may fall within the 12 to 21 mm Hg range. However, in some people “normal” intraocular pressures can damage the optic nerve, a condition called normal-tension glaucoma. Conversely, in certain other individuals, an intraocular pressure above 21 mm Hg might not damage the optic nerve.

Diagram 1

A nourishing fluid called aqueous humor enters the eye, flows through the front portion of it, and exits through a meshwork of tissue (trabecular meshwork), located at the “angle,” and into drainage canals. A balance of inflow and outflow must be maintained to keep fluid pressure in the eye at acceptable levels.( Illustration courtesy National Eye Institute)

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