Tear Secretion

Through the Middle Ages, tears were thought to be excrements of the brain. Today, we know them to be secretions of a special set of glands. The Latin word for tear is "lacrima". Tears are composed of about 90% water; however, 10% of the tear film is composed of minute quantities of fatty oils. The glands that combine to secrete tears are therefore called the lacrimal glands or apparatus. These glands are found in or near the eyelids.
Of course, the body occasionally secretes an additional supply of tears, due to: sensory stimulation (irritants, inflammation, foreign bodies), retinal stimulation (sudden or intense light), and psychological stimulation (strong emotions). As the eye blinks, it spreads both types of secretion across the eye, forming the tear film. This film needs constant replenishing, for three reasons: absorption into the conjunctiva, normal drainage into the nasal passages, and evaporation.

Functions of the Tear Film

The tear film provides a number of different functions for the eye:

Moisturization This is extremely important. Preventing the outer layer of the eye from drying is fundamental to normal eye function. Should the tear film fail to do so, the resulting dry spots can generate not only gritty or burning sensations, but epithelial cell damage and potentially the loss of sight.

Lubrication
Easy blinking requires thorough lubrication. Without proper lubrication, friction can lead to abrasive damage, with results similar to ocular drying.

Nourishment
Filled with nutrients, the tear film directly nourishes the cornea. It also furnishes oxygen and removes carbon dioxide. In cases of corneal malnourishment, blood vessels may actually grow into the cornea, in order to keep the cells from starving. These blood vessels obstruct vision, and may be characteristic of a very severe manifestation of "Dry Eye Syndrome".

Protection and Cleansing
Tears serve to expel foreign particles, dilute unwelcome liquids, and remove waste products from corneal metabolism.

Defense
To minimize the possibility of eye infections, the tear film has both antibacterial and antiviral substances.

Refraction
The refraction of light occurs best from a smooth, clear optical surface, which is exactly what the tear film provides. Without it, light would meet a vast number of small surface irregularities in the cornea's outer layer resulting in blurred or distorted vision.

Among the most fragile tissues in the human body are those composing the external surface of the eye: the conjunctiva and cornea. The body constantly monitors the state of the eye surface. It provides, as the eye's chief defense mechanism against intrusion, a continual bath of tear fluid. This thin layer of fluid at once lubricates, protects, nourishes, and cleanses the ocular surface. Tear film disorders can cause insufficient lubricating moisture, resulting in Dry Eye Syndrome. Tear secretion is critical in preventing this condition.

Components of the Tear Film

An outer (oily) lipid layer, produced by the meibomian glands.
A middle (watery) aqueous layer, produced by lacrimal and accessory lacrimal glands.
An inner mucin layer, produced by goblet cells.

This is an impressive feat of nature, considering that the tear film is only 7-10 microns thick. At 100 microns, a human hair is ten times thicker than the tear film.

The Outer Lipid Layer

This layer of the tear film is only two molecules thick. The oily quality of this layer causes it to "rise to the surface of the water." This keeps it on the outermost portion of the tear film, where it serves two purposes:

• Lubricating the blink.
• Limiting evaporation.

Without this outer lipid layer, the tear film could lose water up to 20 times faster. So, if you were to experience a deficiency in the lipid layer, a Dry Eye condition could develop swiftly and severely, threatening eye damage.

The Middle Aqueous Layer

Comprising over 90% of tear volume, the middle aqueous layer is by far the largest component of the tear film. While mostly water, small portions of dissolved organic materials and electrolytes (as salts) are present. Nutrients in the aqueous layer include glucose, oxygen, vitamins, and salts. Proteins such as lactoferrin and lysozyme fight bacteria. Interferon helps ward off viral infections. The aqueous layer is isotonic with the epithelial cells in the cornea, which means it shares the same salt concentration. As a result, the tear film maintains a normal fluid balance with the rest of the eye. Salts are composed of certain electrolytes. Should the aqueous layer become deficient in any way, the fluid balance between parts of the eye may be disrupted. For example, if the tear layer contains fewer salts than the cells of the cornea, water may be drawn "inward." This could cause depletion of the tear film. Conversely, if the tear film is too concentrated with salts, the cornea may lose water outwardly, which could damage the corneal tissue. Such conditions could also harm the lubrication, nourishment, and protection of the ocular surface, leading to Dry Eye Syndrome.

The Inner Mucin Layer

The mucin layer is composed of glycoproteins. It is the thinnest and innermost layer of the tear film, and serves two primary functions: First, since it is "water attracting" near the aqueous layer, it enables that layer to spread smoothly over the surface of the cornea. Second, since mucin is "water repelling" near the corneal surface, it protects the delicate structure of the corneal epithelium.