What are tears made of and what function do they have?

Tears have a vital function to protect and keep the surface of the eye well lubricated. The tear film is the interface between the ocular surface and the external environment. Although very thin, the tear film has a highly complex composition containing water, proteins, lipids, metabolites and electrolytes with different structures and functions that act in synergy to maintain a healthy and well-functioning refractive state and to protect key vision structures. The volume of the tear film tends to decrease with age. It performs mainly four functions: optical, mechanical, trophic, defensive.

How tears are made

The tear film that lines the surface of the cornea is formed by three layers:

Lipid outer layer (rich in fat) secreted by the sebaceous glands of the eyelids (meibomianglands). It is a thin layer (100 nanometers) and is formed by wax and cholesterol esters. It performs three main functions:
- Retards/inhibits the evaporation of the aqueous layer of tears.
- Lubricate the eyelids.
- Keep the tear film within the eyelid margins preventing it from leaking.
Watery layer, secreted by the lacrimal glands. More often than the previous one (7 microns), it also performs important functions:
- moistens and lubricates the eyeball.
- Provides oxygen to the epithelium of the cornea.
- Performs antibacterial activity.
- Maintains a smooth and mirrored surface.
- Sweeps away foreign bodies and debris.
Mucinic inner layer, composed of mucin, a glycoprotein produced by the goblet cells of the conjunctival (conjunctival goblet cells) and other proteins. It has a thickness of 1-2 microns:
- Contributes to the lubrication of the cornea.
- It has a protective function, both mechanical and antibacterial
- Reduces the surface tension of the cornea.

For a proper functioning of the surface of the eye, in addition to a precise and physiological composition of the various layers of the tear film must be distributed evenly. This is facilitated by:

A regular blinking (the rapid and momentary closure of the eyelids)
Perfect closure of the eyelids on the surface of the eye
Epithelium (lining tissue) of the cornea healthy and homogeneous

Any factor that disturbs the balance of the superficial system of the eye can disrupt the stability and osmolarity of tears, leading to tissue damage by means of osmotic, mechanical, and inflammatory mechanisms.

Blinking spreads, the tear film

The blink, or the momentary closing of the eyelids, produces wave of liquid that distributes tear film regularly and evenly over the surface of eye to moisten and cleanse the cornea, the conjunctiva, and the anterior segment of the eye. When the eye is slightly wet, from the receptors of the cornea and conjunctiva, a nerve signal reaches the brain and this commands the basal secretion of tears. In case of inflammation of the lacrimal glands or the surface of the cornea, despite nerve stimulation, the production of tears is insufficient.

Blinking frequency is usually 12-16 beats per minute with individual variations. In 80% of cases, the blinking is partial meaning that it does not cover more than 70% of the surface of the cornea. This frequency decreases when you carry out some activities such as reading, staying at the computer, or driving for extended periods, and in any case in all those situations that require extreme visual attention and keep your eyes “wide open”, almost wide open for a long time, even without realizing it.

The tear film makes vision clear.

The matrix of proteins, glycoproteins and lipids that form the tear film maintains a stable, well-lubricated and smooth ocular surface, promoting clear vision. Tear factors also promote wound healing, counteract inflammation, fight free radicals, and protect against microbial infections.

Cleanliness as a habit. Tears lubricate the surface of the conjunctiva, eyelids, and eyeball; they wash and cleanse, eliminating foreign bodies, dead cells, bacteria.
Distributed over the cornea, the tear film eliminates irregularities, keeps the corneal surface mirrored, homogeneous, smooth, and optically perfect.
The tear film carries oxygen and glucose for nourishment of the cornea (tissue without vessels).
The tear film represents a real shield, a first line of defense against pathogens and dry eye.