Aqueous humour

Aqueous humour
Schematic diagram of the human eye.
Details
Identifiers
Latin	humor aquosus
MeSH	D001082
TA98	A15.2.06.002
TA2	6791
FMA	58819
Anatomical terminology [edit on Wikidata]

The aqueous humour is a transparent water-like fluid similar to blood plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball.^[1] It fills both the anterior and the posterior chambers of the eye, and is not to be confused with the vitreous humour, which is located in the space between the lens and the retina, also known as the posterior cavity or vitreous chamber.^[2] Blood cannot normally enter the eyeball.^[3]

Structure

Composition

• Amino acids: transported by ciliary muscles
• 98% water
• Electrolytes (pH = 7.4 -one source gives 7.1^[4])
  • 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
• Glutathione
• Immunoglobulins

Function

• Maintains the intraocular pressure 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, trabecular meshwork, lens, and anterior vitreous.
• May serve to transport ascorbate in the anterior segment to act as an antioxidant agent.
• Presence of immunoglobulins 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.
• Contributes refraction, with a refractive index of 1.3335^[5]
• Prevents eye dryness.

Production

Aqueous humour is secreted into the posterior chamber by the ciliary body, 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.^[6]

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, 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^[7]). 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.^[7] 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.^[7]

Clinical significance

Glaucoma 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 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 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.

Uveoscleral outflow of aqueous humour can be increased with prostaglandin agonists, while trabecular outflow is increased by M3 agonists. Fluid production can be decreased by beta blockers, alpha2-agonists, and carbonic anhydrase inhibitors.^[9]

Additional Images

• 
  Structures of the eye labeled
• 
  Another labeled view of the structures of the eye
• 
  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

See also

• Vitreous Humour

References

External links

• "Anatomy diagram: 02566.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2012-07-22.

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Anatomy of the globe of the human eye

Fibrous tunic
(outer)

Sclera	Episcleral layer Schlemm's canal Trabecular meshwork
Cornea	Limbus layers Epithelium Bowman's Stroma Dua's layer Descemet's Endothelium

Uvea / vascular
tunic (middle)

Choroid	Capillary lamina of choroid Bruch's membrane Sattler's layer
Ciliary body	Ciliary processes Ciliary muscle Pars plicata Pars plana
Iris	Stroma Pupil Iris dilator muscle Iris sphincter muscle

Retina (inner)

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
Cells	Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Retina ganglion cell (Midget cell, Parasol cell, Bistratified cell, Giant retina ganglion cells, Photosensitive ganglion cell) → Diencephalon: P cell, M cell, K cell, Muller glia
Other	Macula Perifoveal area Parafoveal area Fovea Foveal avascular zone Foveola Optic disc Optic cup Ora serrata

Anatomical regions
of the eye

Anterior segment	Adnexa (Eyebrow, Eyelid, Conjunctiva, Lacrimal system, Orbit) Fibrous tunic Anterior chamber Aqueous humour Iris Posterior chamber Ciliary body Lens Capsule of lens Zonule of Zinn
Posterior segment	Vitreous chamber Vitreous body Retina Choroid

Other

• Keratocytes
• Ocular immune system
• Optical coherence tomography
• Eye care professional
• Eye disease
• Refractive error
• Accommodation
• Physiological Optics
• Visual perception