Amniotic Fluid: Structure, Function & Diseases

The aqueous humor in the eye chamber has important functions to fulfill for the supply of the eye. In doing so, it is subject to a constant balance between inflow and outflow. Disturbances of this balance can lead to severe eye diseases and even blindness.

What is aqueous humor?

Amniotic fluid is a clear liquid found in the anterior and posterior chambers of the eye. It has a similar composition to blood plasma. However, it contains fewer proteins and bilirubin. Therefore, it also appears colorless. As its main component, it is 98 percent water. The remaining two percent includes amino acids, electrolytes, lactic acid, ascorbic acid (vitamin C), gluthation, immunoglobulins and traces of hydrogen peroxide. The aqueous humor is formed at the ciliary body (corpus ciliare) and has the task of supplying the cornea, lens, retina and vitreous body with nutrients. Furthermore, it provides for the generation of intraocular pressure for the form stability of the internal structures in the eye. The enzymes alpha-carbanhydrases, which catalyze the conversion of carbonic acid to water in the ciliary body, are responsible for the production of aqueous humor.

Anatomy and structure

The formation of aqueous humor occurs in the posterior chamber of the eye at the ciliary body. Its production occurs in the epithelium of the greater ciliary process. Three processes are involved. Thus, aqueous humor is formed by ultrafiltration, diffusion, and active transport processes. Its chemical nature is similar to that of blood plasma. Nevertheless, it has a highly specific chemical composition that requires its separation from the bloodstream. This is ensured by a blood-chamber water barrier, which has a similar structure to the blood-brain barrier. The various cells of the ciliary epithelium are firmly connected to each other via “gap junctions” and “tight junctions”. This creates a highly specific layer of connective tissue that allows only specific substances to pass between the blood and aqueous humor. From the corpus ciliare it flows between the iris and the lens into the anterior chamber. Via the trabecular meshwork in the chamber angle and Schlemm’s canal, the main outflow (85 percent) of the aqueous humor then occurs into the venosus sclerae plexus, a venous plexus in the cornea. From there, the fluid re-enters the bloodstream. A small percentage of 15 percent is discharged unconventionally through the vessels of the ciliary body. The trabecular meshwork represents a connective tissue-like basic framework covered with epithelium. In the eyeball, aqueous humor creates intraocular pressure, which maintains the internal structures in the eye.

Function and tasks

The function of aqueous humor is to provide nutrients to the cornea, lens, retina, and vitreous humor. To this end, it provides amino acids, electrolytes, vitamins (vitamin C), and other substances. Furthermore, it also contains immunoglobulin G for the defense against pathogens. Immunoglobulin G is a mixture of class G antibodies. These antibodies are responsible for fighting viruses and bacteria in the eye. Furthermore, aqueous humor is also necessary for the form stability of the eyeball and the internal structures via the generation of an intraocular pressure. However, the pressure must not be too high or too low. Therefore, it is regulated by a finely tuned mechanism of control of aqueous humor production and outflow. In this process, regulation takes place via stimulation of beta receptors. The production of aqueous humor depends on blood pressure and oncotic pressure. Increased blood pressure leads to its increased production. The amount of aqueous humor production is subject to constant fluctuations and decreases with age or diabetes mellitus.

Diseases

The aqueous humor plays a major role in eye diseases caused by changes in intraocular pressure. In most cases, these are disorders related to an increase in intraocular pressure. In rarer cases, the intraocular pressure is decreased. This can happen, for example, due to a loss of aqueous humor during eye surgery or accidents. If the pressure loss is not compensated quickly, choroidal swelling occurs. More significant than a drop in pressure, however, is an increase in intraocular pressure. In the long term, this leads to damage to the eye with possible blindness as a result. The resulting disease is called glaucoma.However, glaucoma is not a uniform disease, but represents a group of disorders related to or influenced by intraocular pressure. As a rule, the intraocular pressure is between 10 and 21 mmHg. If there is a permanent pressure above 21 mmHg, it can lead to long-term damage to the optic nerve. Whether and when this leads to eye disease varies from individual to individual depending on predisposition. Generally speaking, however, the longer the elevated pressure persists and the higher it is, the more likely eye damage will occur. Elevated intraocular pressure is caused by a disturbance in aqueous humor outflow. Outflow disorders of aqueous humor can be caused by arteriosclerosis, diabetes, age-related diseases, but also by specific eye diseases. Thus, the risk of developing glaucoma increases with age. A special feature is the glaucoma attack. In this case, a sudden reduction in aqueous humor outflow occurs due to the obstruction of the angle of the ventricle. In addition to the rapid deterioration of vision, eye pain, nausea, vomiting and cardiac arrhythmias occur. However, there are also forms of the disease with normal intraocular pressure. Here, too, however, an improvement of the symptoms can be achieved by lowering the intraocular pressure with medication. Depending on the drug used, either the production of aqueous humor is reduced or its outflow through the vessels of the ciliary body is increased.

Typical and common eye diseases

  • Eye inflammation
  • Eye pain
  • Conjunctivitis
  • Double vision (diplopia)
  • Light sensitivity