Understanding Dry Eye

What Are Tears?

Most of the time, you probably only think about your tears when you're sad – or cutting up an onion. But, your tears are hard at work all the time, keeping your eyes healthy and your vision clear. We may think of tears as little more than salt water, but they're actually made up of a complex mix of components that must be in perfect balance to keep your eyes properly moistened and comfortable. If that balance is upset, your eyes may begin to feel dry and irritated.

Tear Film Structure

You see, every time you blink, a thin layer of tears is spread across the cornea. This layer of moisture, or tear film, forms a protective coat, lubricating your eyes and washing away any foreign bodies that might cause harm or obscure your vision.

Three main components are crucial in forming the tear film that coats the cornea:

The largest portion of the tear film is made up of a water-based element called aqueous. Produced by the lacrimal gland located on the upper, outer portion of each eye, aqueous supplies all of the moisture your eyes need to feel healthy and comfortable.

By itself, the aqueous would quickly evaporate off the eye or run off as tears. Luckily, on top of the aqueous, forming the outermost layer of the tear film, you'll typically find a layer of lipids. Formed by oil-secreting glands embedded along the edges of the upper and lower eyelids, the lipids seal in the aqueous and prevent it from evaporating too quickly.

Finally, below the aqueous, you'll find a layer of proteins called mucins. Created by cells on the eyelids and on the surface of the eye, mucins coat the eye and allow the aqueous to "stick" to the otherwise water-repellent cornea.

All three of these layers are necessary in creating tear film for moist, healthy eyes. If any layer becomes depleted, the tear film will be unable to properly coat your eyes, and they'll feel gritty and uncomfortable.

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