Lacrimation is the production and secretion of tears by the lacrimal glands, and it happens constantly, not just when you cry. Your eyes produce tears around the clock to keep the cornea moist, nourished, and protected from infection. The process involves a surprisingly sophisticated system of glands, nerves, and drainage channels, and tears themselves turn out to be far more chemically complex than simple saltwater. What most people think of as “tearing up” is actually just one mode of a system that has at least three distinct operating states, each producing tears with a different chemical profile.
What Tears Are Actually Made Of
The tear film coating your eye is traditionally described as having three layers. The outermost is a thin lipid (oil) layer produced by tiny glands along the eyelid margins called meibomian glands. Beneath that sits the aqueous layer, the watery bulk of the tear film, secreted by the main and accessory lacrimal glands. The innermost layer, closest to the eye’s surface, is a mucous layer produced by cells in the cornea and conjunctiva. Each layer does different work. The lipid layer slows evaporation. The aqueous layer delivers antimicrobial compounds and nutrients. The mucous layer keeps the surface hydrated, provides lubrication during blinking, and acts as a barrier against pathogens trying to latch onto the eye’s surface.1PubMed Central. Tear film mucins: front line defenders of the ocular surface; comparison with airway and gastrointestinal tract mucins
The aqueous layer itself is not just water and salt. It contains lysozyme, lactoferrin, immunoglobulins, and other antimicrobial proteins that function as a frontline immune defense.2PubMed Central. The tear film and ocular mucins The lacrimal gland secretes this complex cocktail under the control of at least five different neurotransmitter receptors and three separate signaling pathways, which is unusually elaborate for a secretory organ.3PubMed. The Lacrimal Gland and Its Veil of Tears That complexity hints at how finely tuned tear production needs to be. The gland doesn’t just turn on and off; it adjusts the specific mix of water, ions, and proteins depending on conditions.
How Your Nerves Control Tear Production
Lacrimation is a reflex, and the sensors that trigger it are scattered across the surface of your eye. The cornea is one of the most densely innervated tissues in the body, packed with sensory nerve endings that detect temperature shifts, chemical exposure, and mechanical pressure. These nerves rely on a family of ion channels called TRP channels to convert environmental stimuli into electrical signals. One channel in particular, TRPM8, is a key regulator of baseline tear production. It responds to cooling and helps maintain the steady trickle of tears your eyes need between blinks.4PubMed. Corneal sensory nerve regulation of tear production through stimulation of transient receptor potential melastatin 8 (TRPM8) channel
A different channel, TRPA1, is the one that fires when something chemically irritating hits the eye. Researchers have shown that all major tear gas agents work by activating TRPA1 in sensory neurons, causing a flood of calcium into the cells that triggers the intense tearing, pain, and inflammation tear gas is designed to produce.5PubMed Central. Transient receptor potential ankyrin 1 antagonists block the noxious effects of toxic industrial isocyanates and tear gases Onion fumes provoke a similar reflex through related irritant pathways. In all these cases, the sensory signal travels from the cornea to the brainstem and then back out along parasympathetic fibers to the lacrimal gland, which rapidly ramps up secretion. The whole loop takes a fraction of a second, which is why your eyes start watering almost immediately when you chop an onion or walk into a dusty room.
Three Kinds of Tears and Why They Differ
Your tear system operates in three modes. Basal tears are the constant background film, produced all day and night to keep the cornea lubricated. Reflex tears are the surge that occurs in response to an irritant, whether it’s wind, smoke, bright light, or a foreign particle. Emotional tears are the ones tied to feelings, from grief to joy. These three types are not just different in trigger; they are chemically distinct.
Reflex tears are relatively dilute and loaded with antimicrobial compounds like lysozyme and defensin peptides. Their job is to flush out whatever irritant triggered them and kill anything infectious that might have hitched a ride. Emotional tears, by contrast, contain higher concentrations of protein. They are more viscous and sticky, which is why they tend to cling to the skin and roll slowly down the face rather than sheeting off quickly.6PubMed Central. Non-targeted Metabolomics Analysis Reveals Distinct Metabolic Profiles Between Positive and Negative Emotional Tears of Humans A proteomic study comparing emotional tears to standard tears identified over 900 proteins across all samples. Fifty-two of those proteins were significantly enriched in emotionally stimulated tears, many of them involved in immune defense or inflammatory signaling, including members of the S100A and neutrophil defensin protein families.7PubMed Central. A Comparison of Emotionally Stimulated and Conventionally Collected Tears Using Bottom-Up, Label-Free Quantitative Proteomic Analysis
Researchers are even investigating whether tears shed during positive emotions differ chemically from those shed during negative emotions. Early metabolomics work suggests the two contain distinct substances, though the field is still young and the sample sizes are small.8PubMed Central. Non-targeted Metabolomics Analysis Reveals Distinct Metabolic Profiles Between Positive and Negative Emotional Tears of Humans If the findings hold up, tears may carry something closer to a chemical fingerprint of emotional state, not just a generic overflow response.
Why Humans Cry and Other Animals Don’t
Most mammals produce basal and reflex tears. Emotional crying, though, appears to be uniquely human. The leading evolutionary explanation is that tears evolved as a social signal. Visible tears on the face communicate distress to others in a way that is hard to fake, prompting empathy and prosocial behavior from people nearby.9PubMed. Why Only Humans Shed Emotional Tears: Evolutionary and Cultural Perspectives A complementary hypothesis frames tears as a handicap signal: because crying blurs vision and makes aggressive action harder, showing tears is an honest signal of vulnerability that can function as appeasement or an attachment cue.10Evolutionary Psychology. Emotional Tears as Biological Signals
Brain imaging research backs up the idea that tears carry a strong, context-independent signal. When people in an fMRI scanner viewed faces with or without tears against positive, negative, or scrambled backgrounds, a region called the lateral occipital gyrus responded to the presence of tears regardless of what the background showed. The brain appears to process visible tears as an unambiguous cue, separate from the situation they appear in. That said, tears did help observers interpret ambiguous facial expressions, particularly when paired with a positive context, suggesting tears can clarify rather than just intensify emotional communication.11PubMed Central. The power of tears: Observers’ brain responses show that tears provide unambiguous signals independent of scene context
When Too Few Tears Are the Problem
Dry eye disease is one of the most common eye complaints, and it comes in two broad forms. In one, the lacrimal gland doesn’t produce enough aqueous fluid. In the other, the meibomian glands in the eyelids malfunction, reducing the oil layer and letting tears evaporate too quickly. The second type, evaporative dry eye, is actually more common. When meibomian glands become blocked or atrophied, the quality and quantity of their oily secretion drops, and the consequences cascade: faster tear evaporation, increased salt concentration in the remaining tear film, inflammation of the eye surface, and blurred vision.12PubMed Central. Meibomian Gland Disease: The Role of Gland Dysfunction in Dry Eye Disease
Autoimmune conditions can attack the lacrimal gland directly. Sjögren’s syndrome is a chronic inflammatory disease in which the immune system infiltrates the lacrimal and salivary glands, gradually destroying them.13PubMed Central. Ophthalmologic Manifestations of Primary Sjögren’s Syndrome Research in animal models has shown that the damage involves both T cells and macrophages: autoreactive T cells provoke macrophage infiltration into the lacrimal glands and cornea, where those macrophages play a direct role in driving the dry-eye disease forward.14PubMed Central. Critical involvement of macrophage infiltration in the development of Sjögren’s syndrome-associated dry eye People with Sjögren’s can experience severe dryness that goes well beyond the everyday irritation of occasional dry eyes, sometimes leading to corneal damage if untreated.
Medications are another underappreciated cause of reduced lacrimation. Drugs can interfere with tear production through several routes: suppressing lacrimal gland secretion, disrupting meibomian gland function, or damaging the mucous-producing cells on the eye surface. Systemic retinoids like isotretinoin (used for severe acne) are well known for causing meibomian gland changes. Cancer drugs including certain chemotherapy agents, immune checkpoint inhibitors, and targeted therapies have also been linked to lacrimal gland dysfunction or scarring of the conjunctiva.15PubMed Central. A review on drug-induced dry eye disease Even everyday eye drops can contribute to the problem. Preservatives in topical eye medications, particularly the widely used preservative benzalkonium chloride, can damage the ocular surface over time, and the drying effects of multiple medications likely add up.16PubMed Central. The role of medications in causing dry eye
When Too Many Tears Are the Problem
Excessive watering of the eyes, called epiphora, is sometimes a tear overproduction issue but more often a drainage problem. Tears normally drain through small openings (puncta) at the inner corner of each eyelid, travel down the nasolacrimal duct, and empty into the nose. A blockage anywhere along that path causes tears to pool and spill over the eyelid. Nasolacrimal duct obstruction is the most common congenital disorder of the tear drainage system, occurring in roughly 11% of newborns. The reassuring news for parents: about 90% of these cases resolve on their own by the child’s first birthday.17PubMed Central. Epiphora: A Structured Diagnostic Approach for the Otolaryngologist
In adults, acquired duct obstruction is more common in women and tends to result from gradual narrowing of the duct over time. It can also be caused by inflammatory conditions, tumors, radiation therapy, or even nasal polyps pressing on the duct from the outside. Beyond the annoyance of constantly watery eyes, stagnant tears sitting in a blocked system can become infected, leading to painful swelling of the tear sac called dacryocystitis.18Disease-a-Month. Nasolacrimal duct obstruction as an important cause of epiphora
One of the stranger tearing disorders goes by the evocative name “crocodile tears syndrome.” It occurs when the facial nerve regenerates abnormally after injury or Bell’s palsy, and nerve fibers that should have reconnected to the salivary glands instead grow into the lacrimal gland. The result is that eating triggers tears. Your brain sends the signal to salivate, but the misdirected wiring delivers it to the tear gland instead.19PubMed. Crocodile Tears Syndrome For severe cases, injecting botulinum toxin into the lacrimal gland can reduce the unwanted tearing by temporarily blocking the misrouted nerve signals.20Eye. Treatment of gustatory hyperlacrimation (crocodile tears) with injection of botulinum toxin into the lacrimal gland
Contact Lenses and Tear Film Disruption
If you wear soft contact lenses and feel like your eyes get drier as the day goes on, you’re not imagining it. Placing a lens on the cornea physically splits the tear film and creates a new surface the tear layer has to coat. Research using dynamic imaging of the eye surface has found that tear film breakup time drops significantly after soft lens insertion, with the tear layer thinning faster and becoming more irregular over the lens compared to the bare cornea.21PubMed Central. Tear Film Surface Quality with Soft Contact Lenses Using Dynamic Videokeratoscopy The lens absorbs some of the aqueous layer and disrupts the even spread of the lipid layer, which is why contact-lens-related dryness is one of the top reasons people stop wearing them. Rewetting drops help temporarily, but they don’t fix the underlying mechanical disruption of the tear film.
How Other Animals Handle Their Tears
While all land-dwelling vertebrates produce some version of a tear film, different species have evolved strikingly different solutions to the same basic problem of keeping the eye surface healthy. Seals and sea lions, for example, have had to adapt their tear chemistry to an aquatic environment where the eye alternates between saltwater immersion and air exposure. Studies of pinniped tear glands show both structural changes in the glands themselves and a different biochemical makeup of the tear film compared to purely terrestrial mammals.22PubMed Central. Characterization of ocular gland morphology and tear composition of pinnipeds
Some animals possess an additional tear-producing structure called the Harderian gland, which sits behind the eye and secretes lipids to stabilize the tear film. Rabbits have a particularly active Harderian gland, and the unusual lipids it produces appear to protect against evaporative dry eye. Those lipids have attracted research interest as potential supplements for human artificial tears, since the evaporative component of dry eye disease is so common and current treatments for it remain limited.23PubMed Central. Novel Lipids of the Rabbit Harderian Gland Improve Tear Stability in an Animal Model of Dry Eye Disease
Tears as a Window into Broader Health
One of the more interesting frontiers in tear research has nothing to do with eye disease. Because tears are easy to collect non-invasively, scientists are exploring whether their molecular contents can reveal information about conditions elsewhere in the body. Tears contain tiny particles called extracellular vesicles that carry proteins and RNA from the cells that produced them. Researchers have found that these vesicles can serve as biomarkers not only for inflammatory eye diseases but also for neurodegenerative conditions like Parkinson’s disease and Alzheimer’s, since the molecular cargo in tear vesicles reflects changes in the nervous system.24PubMed Central. Tear-derived extracellular vesicles as diagnostic biomarkers for ocular and neurodegenerative diseases
The appeal is obvious. Collecting tears is painless and quick, especially compared to blood draws, lumbar punctures, or brain imaging. If tear-based diagnostics prove reliable in larger studies, they could eventually offer a simple screening tool for diseases that currently require expensive or invasive testing to detect early. The science is still preliminary, with most studies using small sample sizes, but the concept has enough biological grounding that multiple research groups are pursuing it.
Engineering New Tear Glands
For people with severe dry eye who don’t respond to drops, plugs, or anti-inflammatory medications, the long-term hope is regenerative medicine. Several approaches are being explored to restore lacrimal gland function. Three-dimensional tissue engineering has been used to grow gland-like structures in the lab by mimicking the cell-to-cell interactions that occur during gland development in the embryo. A separate line of research has shown that human stem cells can be coaxed toward producing markers associated with lacrimal gland development, raising the possibility of growing replacement gland tissue from a patient’s own cells. There are also bioengineering efforts aimed at creating implantable scaffolds that could support the gland’s complex three-dimensional architecture, and neurostimulation devices designed to electrically trigger the tear reflex when the gland still has some residual function.25PubMed. Advances in Functional Restoration of the Lacrimal Glands
None of these approaches are available as standard treatments yet. Neurostimulation devices have gotten the farthest commercially, with some already on the market that deliver a mild electrical pulse to the nasal cavity to activate the tear reflex. The tissue engineering and stem cell work remains at the proof-of-concept stage, though the pace of progress over the past decade has been real. For the millions of people with severe aqueous-deficient dry eye who cycle through artificial tears all day, a functional replacement for a damaged lacrimal gland would be transformative.