The eye has a tear film which is refreshed each time you blink. The tear film hydrates, lubricates and protects the surface of the eye to maintain clear vision.
However, when the tears produced are insufficient to replenish the tear film, or the tear film is unstable, eyes can feel gritty, scratchy and irritated, an issue more commonly known as dry eye.
Tear deficient dry eye (poor tear film quantity) - where not enough tear film is produced or it is poorly distributed across the surface of the eye.
Tear sufficient or evaporative eye (poor tear film quality) - enough tears are produced but evaporation of moisture from the surface of the eye causes dry eye symptoms.
A healthy tear film consists of three distinct layers with each one working differently but in harmony to keep your eye moist and lubricated.
The inner and foundation layer of the tear film anchors the film to the cornea. It is produced from goblet cells within the conjunctiva. Its key functions are to facilitate film distribution evenly over the corneal surface and its adherence to the corneal epithelium.
The central layer of the tear film is produced by the lacrymal gland and accounts for about 98% of tear volume. It consists mainly of water but also contains other substances such as electrolytes and proteins, which nourish the eye. This layer also washes away irritants, keeping the eye healthy.
The lipid layer is composed of oils secreted by the meibomian glands. This is the outermost layer of the tear film covering the aqueous and mucin layers and providing a barrier which delays tear evaporation and retains moisture. The lipid layer also protects the eye surface from contamination.