Tear film - Tear film layers > Human eye facts

One of the fundamental conditions for natural functioning of the human eye is an appropriate moistening of the eye surface by tears. It does not lead to the accumulation of tears or their flowing down the cheeks. We can simply define tears as the liquid which flows out of our eyes when we are crying or when our eyes “water” – for example when irritated. They are the liquid product of lacrimal and meibomian glands and conjunctival goblet cells. In the case of irritation of the anterior segment of the human eye or emotional excitement the production of tears increases by couple of times.

The main function of tears, as it was mentioned earlier, is to moisten the surface of the anterior segment of the eye. The type of tears, which participates mostly in the process of moistening, is called the tear film. It keeps the human eyes permanently moist.

Tear film layers

The tears create the tear film on the cornea surface. Its primary function is the protection of the human eye and cleaning of the eye surface and thus it prevents the eye from getting infected. The tear film is important for the metabolism of the cornea and its nourishment. It is thick from 4.5 to 8.7 μm depending on the time after a blink. The tear film has large surface tension which prevents the aqueous layer of the film from rolling down. It is composed of three layers which are functionally closely related to each other: exterior – lipid, middle – aqueous, anterior – mucous.

Lipid layer

It is the most outer part of the tear film and its main function is the protection of aqueous layer from evaporation. Also it moistens the eyelids when they are moving across the eye bulb.

Aqueous layer

It supplies the corneal epithelium with atmospheric oxygen, has antibacterial effects, makes the cornea surface optimally smooth, enables the production of sharp image on the retina and finally washes away the remaining of dead cells and bacteria from the conjunctival sac.

Mucous layer

Mucous layer has a great extending ability, considerably decreases the surface tension and thus the hydrophobic characteristics of the epithelium change to hydrophilic. Thereby it permits the easy spread of aqueous layer on the cornea surface and its moistening.

The imbalance in quality and quantity of the components of the tear film or the blinking disorders worsen the distribution of tears and can present a substantial problem – for example the dry eye syndrome.