The aqueous humor It is a transparent liquid contained in the compartment located in the anterior internal region of the eye, between the cornea and the lens, and which together with the vitreous humor helps to 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 lens and the vitreous humor..
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 appropriately 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..
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Among the functions of the aqueous humor, two of a physical nature and one of a nutritional or biochemical nature can be highlighted. Physics involve its contribution to the conservation of the size, shape of the eyeball and the contribution of a refractive medium that, added to the others, allows the focus of light..
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..
Being 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" of the same 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 liquid and its pressure (up to between 12 and 20 mm Hg). At this pressure, the appropriate shape and size for optimal 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.
This condition, in addition to causing pain, can initially occur with alteration of the eye's refractive parameters and blurred vision. If the pressure is very high and stays that way for a while, loss of vision may occur due to damage to the vessels, the retina and / or the optic nerve..
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 on 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 corneal interfaces..
To understand this, one can consider the fact that if the lens were extracted and put in contact with air (index 1.00), its refractive power at the same curvature would be about 120 diopters. This doubles the total normal power of 60 diopters of the system, and the images produced by it would project well in front of the retina.
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..
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.
When speaking of anatomy in relation to aqueous humor, reference is made mainly to the description of the compartment that contains it and to the structures that participate in its production and drainage, processes that will be described in the next sections..
Aqueous humor occupies the delimited space:
- from behind by the anterior aspect 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.
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 made up of epithelial cells endowed with a high secretory activity. The area of the processes 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 + ATPases pumps that actively transport this ion to the lateral intercellular 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.
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.
In the anterior chamber, the liquid 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 containing only 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.
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