The aqueous humour is a transparent water-like fluid similar to plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball. It fills both the

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

and the posterior chambers of the eye, and is not to be confused with the

vitreous humour The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as the vitreous humo ...

, which is located in the space between the

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'' ...

and the retina, also known as the posterior cavity or vitreous chamber.

Blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in th ...

cannot normally enter the eyeball.

Structure

Composition

* Amino acids: transported by ciliary muscles *98% water * Electrolytes ( pH = 7.4 -one source gives 7.1) **Sodium = 142.09 **Potassium = 2.2 - 4.0 **Calcium = 1.8 **Magnesium = 1.1 **Chloride = 131.6 **HCO3- = 20.15 **Phosphate = 0.62 ** Osm = 304 *

Ascorbic acid Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) a ...

Glutathione Glutathione (GSH, ) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources such as reactive oxygen species, free radicals, pe ...

Immunoglobulins An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

Function

*Maintains the

intraocular pressure Intraocular pressure (IOP) is the fluid pressure inside the eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response fu ...

and inflates the globe of the eye. It is this hydrostatic pressure that keeps the eyeball in a roughly spherical shape and keeps the walls of the eyeball taut. *Provides nutrition (e.g. amino acids and glucose) for the avascular ocular tissues; posterior

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...

, trabecular meshwork,

, and anterior vitreous. *May serve to transport ascorbate in the anterior segment to act as an antioxidant agent. *Presence of

immunoglobulins An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

indicates 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 In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...

. *Prevents eye dryness.

Production

Aqueous humour is secreted into the posterior chamber by the

ciliary body The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...

, specifically the non-pigmented epithelium of the ciliary body ( pars plicata). 5 alpha-dihydrocortisol, an enzyme inhibited by 5-alpha reductase inhibitors, may be involved in production of aqueous humour.

Drainage

Aqueous humor is continually produced by the ciliary processes and this rate of production must be balanced by an equal rate of aqueous humor drainage. Small variations in the production or outflow of aqueous humor will have a large influence on the intraocular pressure. The drainage route for aqueous humor flow is first through the

posterior chamber The posterior chamber is a narrow space behind the peripheral part of the iris, and in front of the suspensory ligament of the lens and the ciliary processes. The posterior chamber consists of small space directly posterior to the iris but anteri ...

, then the narrow space between the anterior iris and the posterior lens (contributes to small resistance), through the pupil to enter the anterior chamber. From there, the aqueous humor 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"eye, human" (see 'Cornea') Encyclopædia Britannica -from

Encyclopædia Britannica 2006 Ultimate Reference Suite DVD An encyclopedia (American English) or encyclopædia (British English) is a reference work or compendium providing summaries of knowledge either general or special to a particular field or discipline. Encyclopedias are divided into articles ...

2009) It flows through 25–30 collector canals into the episcleral veins. The greatest resistance to aqueous flow is provided by the trabecular meshwork (esp. the juxtacanalicular part), 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 the 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 (approx. 10% of the total drainage whereas by trabecular meshwork 90% of the total drainage). The fluid is normally 15 mmHg (0.6 inHg) above atmospheric pressure, so when a syringe is injected the fluid flows easily. If the fluid is leaking, the hardness of the normal eye is compromised, leading to collapse and wilting of the cornea.

Clinical significance

Glaucoma Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye re ...

is a progressive optic neuropathy where retinal ganglion cells and their axons die causing a corresponding visual field defect. An important risk factor is increased

(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 due to obliteration of the meshwork resulting from injury or disease of the iris. However, increased interocular 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 Visual impairment, also known as vision impairment, is a medical definition primarily measured based on an individual's better eye visual acuity; in the absence of treatment such as correctable eyewear, assistive devices, and medical treatment� ...

. Uveoscleral outflow of aqueous humour can be increased with

prostaglandin The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are der ...

agonists, while trabecular outflow is increased by M3 agonists. Fluid production can be decreased by

beta blocker Beta blockers, also spelled β-blockers, are a class of medications that are predominantly used to manage abnormal heart rhythms, and to protect the heart from a second heart attack after a first heart attack ( secondary prevention). They are ...

s, alpha2-agonists, and

carbonic anhydrase inhibitor Carbonic anhydrase inhibitors are a class of pharmaceuticals that suppress the activity of carbonic anhydrase. Their clinical use has been established as anti-glaucoma agents, diuretics, antiepileptics, in the management of mountain sickness, g ...

Additional Images

File:Three Main Layers of the Eye.png, Structures of the eye labeled File:Three Internal chambers of the Eye.svg, Another labeled view of the structures of the eye File:Blausen 0390 EyeAnatomy Sectional.png, Sectional Anatomy of the Eye in greater detail, showing Anterior and Posterior Chambers in Anterior Cavity, and Posterior Cavity; also Canal of Schlemm, Ciliary body, and Ora serrata

References

External links

* {{Authority control Body fluids Human eye anatomy