Understanding aqueous tears and the lacrimal glands. When we hear the word tears, most of us picture the watery fluid that appears when we cry — but your eyes produce and maintain tears all day long, usually without you noticing them at all.
This watery, or aqueous, component makes up most of the volume of the tear film. It helps keep the ocular surface hydrated, carries important substances across the surface of the eye, and contributes to the environment that allows the cornea and conjunctiva to remain healthy.
So where does this fluid actually come from?
A major source of the aqueous component of tears is the lacrimal gland. The main lacrimal gland is located above and toward the outer side of each eye, tucked behind the upper outer part of the eye socket. There are also smaller accessory lacrimal glands within tissues around the eye that contribute to tear secretion.
Together, these structures help supply the aqueous fluid that becomes part of the tear film covering the ocular surface.
But producing tears is only one part of the story. Your tear volume at any given moment reflects a balance between tear production, distribution, evaporation and drainage.
The word aqueous can make it sound as though this part of the tear film is simply water. It isn’t.
Tear fluid contains electrolytes, proteins and many other molecules that contribute to the normal function and protection of the ocular surface. Among these are substances involved in antimicrobial defense, lubrication, tissue maintenance and communication between cells.
So tears are not simply there to “wet” the eye. They create a complex biological environment at the ocular surface.
Yes. Tear secretion occurs continuously as part of normal ocular-surface function. The tears you notice when you cry are only one form of tear production.
It is useful to distinguish between the tears that help maintain the eye under normal everyday conditions and reflex tearing — the increased tear production that can occur when the ocular surface is irritated.
Think about what happens when you cut an onion, get something in your eye or walk into strong wind. Your eyes may suddenly flood with tears. That is very different from the relatively small amount of tear fluid normally needed to maintain the ocular surface throughout the day.
This is where the system becomes particularly interesting.
The lacrimal glands do not work independently from the rest of the eye. Sensory nerves in the ocular surface can detect changes and irritation. Information from the eye travels through neural pathways, which can then influence responses including tear secretion and blinking.
In other words, there is communication between the ocular surface, nervous system and tear-producing structures. This feedback is part of what helps the eye respond to changing conditions. If the surface becomes irritated, for example, increased sensory input can trigger reflex tearing.
Your Corneal Nerves: how your eyes sense, respond and communicate
Tears do not simply accumulate on the eye.
Blinking helps distribute them across the ocular surface, where they become part of the tear film. Some tear fluid is lost through evaporation. Some is removed through the eye’s natural drainage system.
Near the inner corner of the upper and lower eyelids are tiny openings called the lacrimal puncta. Tears can enter these openings and travel through small drainage channels before eventually passing into the nose.
This explains something you may have noticed when you cry heavily: your nose starts running too. Your tear and nasal drainage systems are physically connected.
It is tempting to think that dry eye simply means:
“not enough tears are being produced.”
But tear-film balance is more complicated than that.
The amount of aqueous tear fluid available on the ocular surface depends not only on how much fluid is produced, but also on how it is distributed, how quickly it evaporates and how it drains away.
And aqueous fluid itself is only one part of the tear-film system. The lipid component, mucins, ocular surface, eyelids and blinking all influence how the tear film behaves.
This is why two people can both experience “dry eyes” while the biological problem behind their symptoms may be very different.
When the amount of aqueous tear fluid available to the ocular surface is reduced, this can contribute to aqueous-deficient dry eye. There are different reasons why aqueous availability may become reduced, and understanding the underlying reason matters.
Aqueous Deficiency: when there isn't enough aqueous tear fluid
For now, the important thing is simply to understand how the healthy system is supposed to work.
TFOS DEWS III: Digest.
An updated evidence-based overview of dry eye disease and the systems involved in maintaining tear-film and ocular-surface homeostasis.
TFOS DEWS III: Diagnostic Methodology.
The current TFOS framework for diagnosing and subclassifying dry eye, including assessment of aqueous tear deficiency.
TFOS DEWS II: Tear Film Report.
A detailed scientific review of tear-film composition, proteins, electrolytes, mucins and tear-film physiology.