aqueous humor

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2008)

Aqueous humour
Schematic diagram of the human eye.
Latin	humor aquosus

The aqueous humour is a thick watery substance filling the space between the lens and the cornea.

Locations

The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens.^[1] Within the anterior segment are two fluid-filled spaces divided by the iris plane:

• 1) the anterior chamber between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris.
• 2) the posterior chamber between the iris and the front face of the vitreous.^[1]

Aqueous humour fills these spaces within the anterior segment to provide nutrients to the lens and corneal endothelium, and its pressure maintains the convex shape of the cornea.^[2][3]

In a healthy eye, the aqueous humour does not mix with the firm, gel-like vitreous humour because of the lens and its Zonule of Zinn between the two.

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.
• Carries away waste products from metabolism of the above avascular ocular tissues.
• 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.
• Maintains proper brain energy consumption.

Its main function is to provide diopteric power to the cornea.

Composition

The fluid is essentially the same as blood plasma although with less protein.

• Water: 99%^[4]
• Ions: HCO3^-, buffers metabolic acids; Cl^-, preserves electric neutrality; Na⁺; K⁺; Ca²⁺; PO₄^3-.
• Proteins: albumin, β-globulins. Very low density due to filtration.
• Ascorbate: anti-oxidative, protects against UV.
• Glucose
• Lactate: produced by metabolism of anaerobic structures of the eye.
• Amino acids: transported by cilary 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. 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 traveling 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^[5]) 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.^[6]

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 in 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.^[7]

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.^[8] 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.

References

External links

• Roche Lexicon - illustrated navigator, at Elsevier 02566.000-1

v • d • e Sensory system – visual system – globe of eye (TA A15.2.1-6, GA 10.1005) Fibrous tunic (outer) Conjunctiva Plica semilunaris Sclera Episcleral layer • Schlemm's canal • Trabecular meshwork Cornea Limbus • layers (Epithelium, Bowman's, Stroma, Descemet's, Endothelium) Uvea/vascular tunic (middle) Choroid Capillary lamina of choroid • Bruch's membrane • Sattler's layer Ciliary body Ciliary processes • Ciliary muscle Iris Stroma • Pupil • Iris dilator muscle • Iris sphincter muscle Retina (inner) Cells Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Retinal ganglion cell (Giant retinal ganglion cells, Photosensitive ganglion cell) Muller glia Layers Inner limiting membrane • Nerve fiber layer • Ganglion cell layer • Inner plexiform layer • Inner nuclear layer Outer plexiform layer • Outer nuclear layer External limiting membrane • Layer of rods and cones • Retinal pigment epithelium Other Macula • Foveola • Fovea • Optic disc (Cup) Anterior segment Anterior chamber • Aqueous humour • Posterior chamber • Lens (Capsule of lens, Zonule of Zinn) Posterior segment Vitreous humour Other Ocular immune system • Tapetum lucidum eye navs: anat/adnexa anat/pathways/physio/dev, noncongen/congen/neoplasia, eponymous signs, proc