Anatomy
Aqeous Humor and Vitreous Humor
Aqueous and Vitreous
The eye's anterior and posterior segments are filled with fluids, or humors, that play a vital role in maintaining ocular function. These fluids are called the aqueous humor and vitreous, respectively.
Aqueous Humor
The anterior segment, which lies between the cornea and lens, is filled with a clear, watery fluid called the Aqueous humor, or simply aqueous. After passing through the cornea, light enters the aqueous humor where it is refracted again towards the centre of the eye. This refraction is due to the difference in density between the cornea and aqueous humor.
Aqueous flow from the posterior chamber to the anterior chamber.
The aqueous humor supplies nutrients and oxygen to, and carries metabolic waste and carbon dioxide away from, the lens and the cornea. The aqueous humor is the sole source of nutrients and oxygen for the lens, as well as the endothelium layer of the cornea. Therefore, proper flow is critical for the health of these structures.
The aqueous humor is continually being produced by the ciliary body in the posterior chamber and drained out of the eye through the trabecular meshwork and canal of Schlemm in the anterior chamber. About 1 to 2.5 microliters of aqueous humor are formed and drained each minute.
For ocular pressure to remain constant, the rate of drainage must also be 1 to 2.5 microliters per minute. This balanced formation and drainage system normally maintains a constant intraocular pressure.
Read MoreIntraocular Pressure
Intraocular pressure (IOP) is defined as the pressure exerted on the eye by the fluids it contains. Like blood pressure, IOP is measured in millimetres of mercury (mm Hg).
Distribution of intraocular pressure in the general population
Normal IOP is about 12-22 mm Hg. The use of the term "normal" does not signify "correct" but instead indicates the "statistically" normal range. In most people, IOP varies over a diurnal cycle; the highest level occurs in mid-morning and the lowest level occurs at night, during sleep.
The most important contributor to IOP is the aqueous humor. The eye maintains normal IOP by balancing the amount of aqueous fluid produced by the ciliary process and the amount drained out through the trabecular meshwork and canal of Schlemm’s.
Although both production and drainage are important to regulating IOP, the primary method of physiologic regulation is by increasing or decreasing the amount of aqueous fluid that drains out of the canal of Schlemm. IOP is regulated to a lesser extent by altering the amount of aqueous humor produced by the ciliary processes.
Together, the conventional and uveoscleral drainage routes for aqueous humor provide balanced IOP. The majority of resistance to outflow of aqueous humor is provided by the trabecular meshwork of the conventional drainage route.
Read MoreVitreous
After passing through the cornea, aqueous humor and lens (anterior segment of the eye), light finally enters the vitreous (also called vitreous humor). Vitreous humor is a transparent, jelly-like substance that fills the vitreous body. The vitreous body is the space created by the lens, retina and optic disc. This is also referred to as the posterior segment of the eye.
The main functions of the vitreous are to transmit light to the retina, and to exert enough pressure to keep the retinal layers tightly pressed together. This pressure helps maintain the round shape of the eye so the lens can focus sharp images on the retina.
The vitreous is very different from the aqueous humor. This clear gel is made up of small fibres and water and is permanently formed at birth. Although substances can diffuse slowly through the vitreous, there is very little fluid flow.
The vitreous does not undergo a regular formation and drainage process like the aqueous humor. Instead, it stays permanently in the vitreous body of the eye.
Because the vitreous does not undergo continuous replacement, its contribution to the total fluid pressure of the eye remains relatively constant. As a result, changes in the internal pressure of the eye are due almost entirely to changes in the rate of formation or drainage of aqueous humor.