AQUEOUS HUMOR: FUNCTION, COMPOSITION, ANATOMY, PRODUCTION - BIOLOGY - 2025

The aqueous humor is a transparent liquid contained in the compartment located on the front inner region of the eye between the cornea and lens, and together with the vitreous helps maintain the shape and size of the eyeball in many animals.

Aqueous humor is the second component of the system of four refractive media (which refracts light) that must be traversed by light on its way to the retina. The first is the cornea and the last two are the crystalline lens and the vitreous humor.

A human eye «Juraj Varga» at www.pixabay.com

Its transparency, like that of the rest of the components of the refractive optical system of the eye, is an essential condition so that light can pass through with minimal attenuation and its rays can be properly focused on the retina.

It is, like the other components of the eye, essential for the vision process and has very particular characteristics, structure and functions.

Functions of the aqueous humor

Among the functions of aqueous humor, two of a physical nature and a nutritional or biochemical nature can be highlighted. The physical ones involve its contribution to the conservation of the size, the shape of the eyeball and the contribution of a refractive medium that, added to the others, allows the focus of light.

- Function relating to the preservation of the shape and size of the eyeball

This is a mechanical function related to the expansion pressure exerted by the aqueous humor on the walls of the compartment that contains it, and also has to do with its volume.

In the case of a closed compartment, with walls endowed with a certain elasticity, the pressure that this liquid exerts on these walls depends both on the degree of "expandability" thereof and on the volume that the compartment contains.

The walls of the chambers that form the compartment containing the aqueous humor are not very "stretchable" or "elastic". Once the compartment is filled to a volume corresponding to its relaxed capacity, the pressure depends on the additional volume it can hold when it "expands" its walls.

This additional volume increases the total volume of the fluid and its pressure (up to between 12 and 20 mm Hg). At this pressure, the appropriate shape and size for the optimum refractive parameters are achieved, without damaging delicate ocular structures.

The volume of aqueous humor is the result of the balance between its production (entry into the compartment) and its reabsorption (exit or drainage). When the input exceeds the output, a condition of intraocular hypertension (glaucoma) is established with values ​​above 20 mm Hg, even reaching 60 or more.

Glaucoma

This condition, in addition to causing pain, can initially occur with alteration of the refractive parameters of the eye and blurred vision. If the pressure is very high and stays that way for a while, loss of vision can occur due to damage to the vessels, the retina and / or the optic nerve.

- Refringent function

The refractive indices of the 4 refractive media of the eye, as well as the radii of curvature of two of them, the cornea and the lens, are of such magnitude that they provide the system with the necessary parameters to focus the images in the retina.

The refractive power of the lens at rest is about 20 diopters, it depends on the curvature of the lens and the relationship between the refractive indices of the lens and the aqueous humor, in addition, it is appropriate for the entire system to achieve focus on the retina.

The refractive index of aqueous humor is 1.33. That of the lens, with whose anterior face the aqueous humor interfaces, is 1.40. This small difference contributes, in just the right amount, to the additional deflection that the lens exerts on the light already deflected at the interfaces of the cornea.

To understand this, one can consider the fact that if the lens were removed and put in contact with air (index 1.00), its refractive power at the same curvature would be about 120 diopters. This doubles the normal 60 diopter total power of the system, and the images produced by it would project well in front of the retina.

Nutritional function

This function refers to the fact that it is the aqueous humor that provides the cornea and lens with the factors that these tissues need for their metabolic activity.

The cornea is a transparent structure, devoid of blood vessels, but with free nerve endings. It has a thickness of 1 mm, in which about 5 layers are arranged, the innermost of which is an endothelium that covers it from the inside and puts it in contact with the aqueous humor.

The lens does not contain vessels or nerves. It is made up of concentric layers of fibrous cells and is bathed in aqueous humor on its anterior face. The energy for the metabolism of both structures derives from the oxidation of glucose, and all the necessary factors derive from the aqueous humor.

Composition

Aqueous humor is a kind of “ultrafiltered” substance, with a composition very similar to that of blood plasma, except for the fact that it contains lower concentrations of proteins, molecules whose sizes do not allow their free passage through the filtration slits of ciliary processes.

Anatomy

Human eye scheme. 1. crystalline. 2. suspension ligament of the lens. 3. posterior chamber and 4. anterior chamber (filled with aqueous humor). 5. cornea. 6. pupil. 7. iris (anatomy). 8. ciliary body. 9. choroid

10. sclera. 11. retina. 12. fovea. 13. optic disc. 14. optic nerve. 15. vitreous humor. Source: See the source section

When speaking of anatomy in relation to aqueous humor, reference is mainly made to the description of the compartment that contains it and the structures that participate in its production and drainage, processes that will be described in the next sections.

The aqueous humor occupies the delimited space:

- from behind by the anterior face of the lens and its suspensory ligaments, - laterally by the ciliary processes and the iris and

- anteriorly on the posterior aspect of the cornea; compartment divided by the iris into a posterior chamber, where it is produced, and an anterior chamber where it is reabsorbed.

Production

The average production speed of this liquid is between 2 and 3 micro liters per minute, an amount produced by the ciliary processes, which are "folds" that protrude from the ciliary body to the space behind the iris, where the lens ligaments and muscle ciliary join the eyeball.

These processes are lined by an epithelium with a surface area of ​​about 6 cm square and composed of epithelial cells endowed with a high secretory activity. The process zone located below the epithelium is highly vascularized and provides the raw material for secretion.

The formation of aqueous humor begins as a secretion of sodium by the Na + / K + ATPase pumps that actively transport this ion to the intercellular lateral spaces. Anions such as chlorine (Cl-) and bicarbonate (HCO3-) are entrained behind sodium to maintain electroneutrality.

The accumulation of these ions has an osmotic effect that promotes the movement of water from neighboring capillaries. The solution thus formed accumulates, its hydrostatic pressure increases and flows through the intercellular junctions of the epithelium towards the posterior chamber.

In addition, many other nutrients pass through the epithelium by active transport or facilitated diffusion, including amino acids, glucose, glutathione, and ascorbic acid. Oxygen, on the other hand, passes by diffusion.

Sewer system

The aqueous humor that passes from the ciliary processes to the most peripheral recess of the posterior chamber, establishes a pressure gradient that determines the movement of the liquid towards the circular edge of the iris that limits the pupil, that is, it passes from the chamber after the previous one.

In the anterior chamber, the fluid moves towards the periphery, towards the angle formed by the union of the cornea with the iris, where it passes through a network of trabeculae to later penetrate into Schlemm's duct, a circular channel that empties, through of small veins that only contain aqueous humor, in the extraocular veins.

The balance between the posterior inlet and the anterior outlet that keeps the intraocular volume of aqueous humor constant is established when the internal pressure reaches, as mentioned, a value of between 12 and 20 mm Hg; Values ​​above these are considered pathological and harmful to visual function.

References

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