Aqueous humour

The aqueous humour is a thick watery substance filling the space between the lens and the cornea. It is not to be confused with vitreous humour, which is contained within the larger cavity of the eye. It is transparent so as to allow light to pass through it.

Functions

 * Maintains the intraocular pressure and inflates the globe of the eye.
 * Provides nutrition (e.g. amino acids and glucose) for the avascular ocular tissues; posterior cornea, trabecular meshwork, lens, and anterior vitreous.
 * May serve to transport ascorbate in the anterior segment to act as an anti-oxidant agent.
 * Presence of immunoglobulins indicate a role in immune response to defend against pathogens.
 * Provides inflation for expansion of the cornea and thus increased protection against dust, wind, pollen grains and some pathogens.
 * for refractive index.

Composition

 * Amino acids: transported by ciliary epithelial cells.

Production and drainage
Aqueous humour is secreted into the posterior chamber by the ciliary body, specifically the non-pigmented epithelium of the ciliary body(pars plicata). It flows through the narrow cleft between the front of the lens and the back of the iris, to escape through the pupil into the anterior chamber, and then to drain out of the eye via the trabecular meshwork. From here, it drains into Schlemm's canal by one of two ways: directly, via aqueous vein to the episcleral vein, or indirectly, via collector channels to the episcleral vein by intrascleral plexus and eventually into the veins of the orbit.

Production

 * Filtration: As blood flows in the ciliary body's capillaries, it is coarsely filtered by the capillaries' endothelial cells. The resulting plasma is then refiltered by the pigmented and nonpigmented ciliary epithelial cells and is secreted into the posterior chamber before travelling between the lens and iris into the anterior chamber of the eye as aqueous humour.
 * Diamond-Bossert model: Active transport occurring in the nonpigmented cilary epithelial cells induces small osmotic pressure gradients in between the cells. A higher concentration of solutes in the proximal part of the intercellular space generates a flow of water. The concentration diminishes from the proximal part to the distal part, releasing the liquid into the posterior chamber.

Drainage
Aqueous humour is continually produced by the ciliary processes and this rate of production must be balanced by an equal rate of aqueous humour drainage. Small variations in the production or outflow of aqueous humour will have a large influence on the intraocular pressure.

The drainage route for aqueous humour flow is first through the posterior chamber, then the narrow space between the posterior iris and the anterior lens (contributes to small resistance), through the pupil to enter the anterior chamber. From there, the aqueous humour exits the eye through the trabecular meshwork into Schlemm's canal (a channel at the limbus, i.e., the joining point of the cornea and sclera, which encircles the cornea ) It flows through 25–30 collector canals into the episcleral veins. The greatest resistance to aqueous flow is provided by the trabecular meshwork, and this is where most of the aqueous outflow occurs. The internal wall of the canal is very delicate and allows the fluid to filter due to high pressure of the fluid within the eye. The secondary route is the uveoscleral drainage, and is independent of the intraocular pressure, the aqueous flows through here, but to a lesser extent than through the trabecular meshwork.

The fluid is normally 15 mm (0.6 inch) Hg above atmospheric pressure, so when a syringe is injected the fluid flows easily. If the fluid is leaking, due to collapse and wilting of cornea, the hardness of the normal eye is therefore corroborated.

Diseases and disorders
Glaucoma is a condition characterised by increased intraocular pressure (pressure within the eye) either through increased production or decreased outflow of aqueous humour. Increased resistance to outflow of aqueous humour may occur due to an abnormal trabecular meshwork or to obliteration of the meshwork due to injury or disease of the iris. However, increased intraocular pressure is neither sufficient nor necessary for development of primary open angle glaucoma, although it is a major risk factor. Uncontrolled glaucoma typically leads to visual field loss and ultimately blindness.