If you are trying to understand dry eye, one of the best places to start is with something every healthy eye needs: the tear film.
Most of us think of tears as the watery liquid that appears when we cry. But your eyes are actually covered by a microscopic tear film all the time — even when you are not crying.
Every time you blink, this thin film is redistributed across the surface of your eye. It helps keep the eye comfortable and protected, creates a healthy environment for the ocular surface, and even plays an important role in clear vision.
And it is much more complex than simply “water.”
The tear film is the extremely thin layer of fluid that covers the exposed surface of the eye, including the cornea and conjunctiva. It contains water, lipids (oils), electrolytes, proteins, mucins and many other substances that work together.
The tear film is also constantly changing. It is spread and redistributed when you blink, changes during the seconds your eye remains open, and is renewed again with the next blink.
So rather than thinking of tears as liquid that simply sits on the eye, it is more useful to think of the tear film as a dynamic biological system.
For something so thin, the tear film has a remarkable number of jobs.
Every day, your eyelids move across the ocular surface thousands of times. The tear film provides lubrication between the eyelids and the surface of the eye, reducing friction and helping blinking remain comfortable.
Tears contain proteins and other molecules involved in protecting the eye from microorganisms and supporting the health of the ocular surface. The tear film also helps maintain the environment that the cells of the cornea and conjunctiva need to function normally.
The cornea is unusual because it has no blood vessels running through its central transparent tissue. The tear film contributes oxygen and nutrients to the corneal epithelium and helps remove waste products from the ocular surface.
This is one of the most interesting functions of the tear film. The tear film forms the first optical surface that incoming light encounters as it enters the eye. By creating a smooth interface over the microscopic irregularities of the corneal surface, a stable tear film helps produce good optical quality.
If the tear film becomes irregular or breaks up between blinks, optical quality can deteriorate. This is why some people notice fluctuating or blurry vision that temporarily becomes clearer immediately after blinking.
You may have seen diagrams showing the tear film as three separate layers:
lipidaqueousmucin
This traditional model is useful for understanding the basic components of tears, but the real structure is more complex. Today, the tear film is better understood as an interconnected system rather than three perfectly separate sheets stacked on top of one another.
At the air-facing surface is a very thin lipid-rich component. Below it is a much thicker mucoaqueous component, containing water, electrolytes, proteins and soluble mucins. At the surface of the corneal and conjunctival epithelial cells is a specialized interface containing membrane-associated mucins and a structure known as the glycocalyx.
All of these components interact.
The lipids at the surface of the tear film come primarily from the meibomian glands, which are located inside the upper and lower eyelids. These glands produce an oily substance called meibum. Blinking helps distribute meibum across the surface of the tear film.
The lipid component contributes to tear-film stability and is involved in the complex processes that influence evaporation. You may sometimes hear this described simply as “oil that stops your tears from evaporating.” That is a useful mental picture, but the biology is more complicated than that. For example, research has not established a simple relationship in which a thicker lipid layer automatically means a lower rate of tear evaporation.
Meibomian Glands & Lipids: where the oil in your tears comes from
The aqueous part makes up most of the volume of the tear film. It is supplied largely by the lacrimal glands and contains much more than water. It includes electrolytes, proteins and many other substances involved in maintaining and protecting the ocular surface.
This is the part we usually imagine when we think about “tears,” but it is only one component of the tear-film system.
Mucins are molecules that play important roles in lubrication, protection and the interaction between the tear film and the ocular surface.
Some mucins are secreted into the tear film, including mucins produced by goblet cells in the conjunctiva. Other mucins are associated with the surface of the epithelial cells themselves and contribute to a specialized interface called the glycocalyx. This interface helps the tear film interact properly with the ocular surface beneath it.
The tear film does not remain unchanged while your eyes are open.
A blink redistributes the tear film across the ocular surface. Then your eye opens. During the interblink period — the time between one blink and the next — the tear film gradually changes. Then another blink redistributes and renews it again.
This continuous cycle is one reason blinking is so important to the health and stability of the ocular surface.
blinktear-film redistributioninterblink periodnext blink
And this happens thousands of times every day, usually without us thinking about it at all.
There is one final idea that is important for understanding both healthy eyes and dry eye disease: the tear film is part of a much larger system.
The lacrimal glands supply aqueous tears. The meibomian glands supply lipids. The ocular surface and its mucins help create the interface with the tear film. The eyelids and blinking repeatedly redistribute it. And sensory nerves in the ocular surface participate in feedback mechanisms that influence responses such as tearing and blinking.
These structures do not operate independently. They continually interact to help maintain a stable environment on the surface of the eye.
Scientists often describe the maintenance of this balanced state using the word homeostasis. In simple terms, homeostasis means that the different parts of the system are working together to keep conditions within a healthy range, even as the environment around the eye constantly changes.
When the system is functioning well, the tear film remains sufficiently stable between blinks to support a healthy ocular surface, comfortable eyes and clear vision.
When that balance is disrupted, tear-film and/or ocular-surface homeostasis can be lost. This loss of homeostasis is one of the central concepts in our modern understanding of dry eye disease.
But before looking at everything that can go wrong, it helps to understand each part of the healthy system individually. That is what we will do next.
TFOS DEWS III: Digest.
An updated overview of the current scientific understanding of dry eye disease, including tear-film biology, ocular-surface homeostasis and neurosensory mechanisms.
TFOS DEWS III: Diagnostic Methodology.
The current TFOS framework for dry-eye diagnosis and etiological subclassification.
TFOS DEWS II: Tear Film Report.
A detailed scientific review of tear-film structure, composition, function and dynamics.