Encyclopedia - Tears

What do tears do?
What causes an eye to water too much?

Tears are the salty fluid that lubricates and helps protect the cornea, the membrane that covers the front of the eye. Tears flow into the eye through ducts from tiny glands located under the upper eyelids and are spread over the eye each time a person blinks, which is about every six seconds. Tears keep the eye moist and free of dust and other eye irritants.

What do tears do?

Tears drain into tiny openings on the edge of the upper and lower eyelids that lead to the nasolacrimal tear ducts near the bridge of the nose. From there, they are channeled into the nasal cavity where they are either swallowed or drain through the nostrils. That is why your nose runs when you cry. When your eyes are irritated, they produce additional tears called reflex or irritant tears. Emotional tears are produced when you are either happy or sad. Excess tears often overflow the lower lid and run down the cheeks.

Tears provide five functions:

1. Keeping the epithelium moist, thereby protecting the outer covering of the eye from damage because of dryness
2. Creating a smooth optical surface on the front of the cornea
3. Acting as the main supplier of oxygen and other nutrients to the cornea
4. Carrying waste products away from the cornea
5. Providing enzymes that destroy bacteria that can harm the eye

Normal tears that cover the corneal surface comprise three layers:

• the lipid, or oil layer, which is the outer layer of the tear film and helps prevent the lacrimal layer beneath it from evaporating or overflowing the lower eyelid
• the lacrimal, or watery layer, which is the middle layer and contains salts, proteins, and an enzyme called lysozyme that actually protect and nourish the eye
• the mucoid, or mucus layer, which is the bottom layer of tears. It contains cells called goblet cells that cause the tears to adhere to the eye.

All three layers of tears are necessary for proper lubrication.

What causes an eye to water too much?

When an eye produces too few tears or tears of faulty composition, a condition called dry eye results. Ironically, one of the symptoms of dry eye can be excessive watering of the eyes. The watering is a natural reflex caused by irritation to the eye because the composition of the tears is wrong. In addition, if you have dry eye, your eye can feel scratchy, dry, irritated, or generally uncomfortable.

An eye doctor diagnoses dry eye by measuring production, thickness, and chemical makeup of the tear film. To treat dry eye, use a tear substitute, also called artificial tears. Administered several times a day, artificial tears bring relief in most cases. If left untreated, dry eye can damage eye tissue and possibly scar the cornea.

When artificial tears do not sufficiently lubricate your eye, your eye care professional may insert a small plug in the corner of the eye to slow drainage and loss of tears. In rare cases, your eye doctor may recommend surgery.