Tears begin in the lacrimal gland, a small almond-shaped structure tucked beneath the brow bone on the outer side of each eye. When stimulated by nerves, this gland secretes a watery fluid rich in proteins, electrolytes, and antimicrobial compounds that flows across the eye’s surface and eventually drains into the nose. But that watery secretion is only part of the story. Your tear film is a multilayered coating assembled by several different glands and cell types working together, regulated by an intricate loop of nerve signals and hormones. Understanding how it all fits together reveals why something as seemingly simple as keeping your eyes moist is, biologically, anything but.
The Lacrimal Gland and Where Tears Come From
The main lacrimal gland sits in a shallow depression in the upper outer rim of the eye socket. It is divided by a tendon into two lobes, an upper (orbital) portion and a lower (palpebral) portion, and its dozen or so tiny ducts empty into the upper fold of the conjunctiva, the thin membrane lining the inside of your eyelids. When those ducts release fluid, each blink spreads it across the cornea.
Inside the gland, clusters of secretory cells called acinar cells do the heavy lifting. They respond to signals from parasympathetic and sympathetic nerves by pumping out water, salts, and a cocktail of proteins onto the eye’s surface.1PubMed Central. Neural regulation of lacrimal gland secretory processes: relevance in dry eye diseases The process is not passive. These cells actively transport ions across their membranes, and water follows by osmosis. The result is a precisely calibrated fluid that matches the needs of the cornea moment to moment.
The main lacrimal gland is not alone. Smaller accessory lacrimal glands, known as the glands of Krause and the glands of Wolfring, are scattered in the conjunctival tissue of the eyelids. For a long time their contribution was unclear, but gene-expression studies have shown that the glands of Wolfring produce many of the same key tear proteins as the main lacrimal gland, including lysozyme, lactoferrin, and tear lipocalin. They also express the same receptor types for nerve and hormone signaling, which suggests they function as miniature versions of the main gland, secreting fluid and proteins under nervous and hormonal control.2PubMed Central. Gene expression in human accessory lacrimal glands of Wolfring These accessory glands likely contribute to the baseline level of moisture your eyes maintain between blinks, even when the main gland is not being strongly stimulated.
The Three-Layer Tear Film
The fluid on your eye is not a simple saline wash. It is an organized film with three functional layers, each produced by a different source. The innermost layer, sitting directly against the cornea, is a mucin layer. The middle layer is the aqueous (watery) portion. The outermost layer, exposed to the air, is a thin film of oil.
The mucin layer is secreted primarily by goblet cells, specialized cells embedded in the conjunctiva.3PubMed Central. Conjunctival goblet cells: Ocular surface functions, disorders that affect them, and the potential for their regeneration The corneal surface on its own is not naturally water-friendly, and without mucins, tear fluid would bead up and slide off rather than forming a smooth, even coat. Mucins transform that surface into one that water can spread across evenly, preventing dry spots from forming and giving the tear film stability.4IntechOpen. The Physiology of Tear Film
The aqueous layer, making up the bulk of the tear film’s volume, comes from the main and accessory lacrimal glands. It carries dissolved oxygen and nutrients to the cornea (which has no blood vessels of its own), flushes debris, and contains antimicrobial proteins that defend the eye against infection.
The outermost lipid layer is the product of the meibomian glands, a row of oil-producing glands embedded in the upper and lower eyelid margins. There are roughly 25 to 40 of them in each lid. Their oily secretion, called meibum, spreads over the aqueous layer with every blink and serves as an evaporation barrier, preventing the watery tears beneath from drying out too quickly.5PubMed. New Insights Into the Lipid Layer of the Tear Film and Meibomian Glands The meibum also acts as a surfactant and helps keep tears from spilling over the lid margin during normal blinking.6PubMed. Meibomian gland function and the tear lipid layer The synthesis of meibomian lipids depends on a mix of neural, hormonal, and vascular signals, which is one reason why conditions affecting any of those systems can destabilize the tear film.
How Nerves Control Tear Production
Your tear system operates on a feedback loop that begins at the eye’s surface and ends at the glands. Sensory nerve endings in the cornea and conjunctiva constantly monitor the state of the ocular surface. When they detect dryness, irritation, temperature change, or foreign particles, they fire signals along the trigeminal nerve to the brainstem. From there, efferent (outgoing) parasympathetic and sympathetic nerves relay the message to the lacrimal gland, triggering it to release water, electrolytes, and proteins.1PubMed Central. Neural regulation of lacrimal gland secretory processes: relevance in dry eye diseases
This reflex arc is essential for protecting the cornea. Its integrity depends on corneal sensation; without functioning sensory nerves, the brain never gets the signal that the eye needs moisture, and tear secretion drops.7PubMed Central. Corneal nerves and their role in dry eye pathophysiology This is exactly what happens after procedures like LASIK, which cut through corneal nerves during flap creation. The resulting disruption can cause transient or, in some people, persistent dry eye symptoms because the reflex loop is interrupted.8PubMed Central. Post-LASIK tear dysfunction and dysesthesia
Basal Tears Versus Reflex Tears
Not all tearing is the same, and the nervous system distinguishes between at least two modes. Basal tearing is the low-level, continuous secretion that keeps your cornea lubricated between blinks even when nothing is irritating your eye. Research indicates that basal tear production and blinking rate are regulated by a specific subset of corneal nerve endings called cold thermoreceptors, which sense the slight cooling that occurs as the tear film thins between blinks.9Acta Ophthalmologica. Conscious sensations and protective reflexes driven by eye surface nerves in health and disease When the film gets thin enough to cool the surface, these receptors nudge the system to blink and secrete a small amount of fresh tears.
Reflex tearing is the dramatic, high-volume response you get when something goes wrong: chopping onions, a gust of wind, a stray eyelash, or a whiff of smoke. This response is driven by a different group of nerve endings, polymodal nociceptors, which respond to chemical or mechanical irritation. When they fire, the lacrimal gland ramps up secretion far beyond basal levels, flooding the eye to dilute and wash away the offending substance. That same reflex arc controls the protective blink you can’t suppress when something flies toward your face.
Emotional Tears and Why Humans Are Unique
Humans are the only species known to shed tears in response to emotions. You cry at funerals, during movies, when overwhelmed with joy, and sometimes for no reason you can easily articulate. The neurobiology behind emotional crying is still not fully mapped. Much of what researchers know comes from animal studies and from observing patients with neurological conditions that cause uncontrollable crying or laughing. Autonomic measurements taken during emotional crying provide some clues about which brain circuits are involved, but the full picture remains incomplete.10PubMed Central. The neurobiology of human crying
What is clearer is the evolutionary angle. The leading hypothesis is that emotional crying evolved as a social signal. Visible tears advertise distress in a way that is hard to fake, which makes them a reliable cue for others to offer help or comfort.11PubMed. Why Only Humans Shed Emotional Tears: Evolutionary and Cultural Perspectives More recent comparative work frames emotional tears as a form of paedomorphic signaling, a pattern seen in other social species where adults strategically redeploy juvenile-typical behaviors to influence the responses of others. Infants cry to recruit caregiving; adults, according to this theory, retain that signal in a modified form because other humans have evolved predispositions to attend to and respond to juvenile-like cues.12PubMed. A comparative approach to emotional crying in humans (Homo sapiens)-Tears as paedomorphic signals
Interestingly, the tears themselves are not all chemically identical depending on what triggered them. Metabolomics studies have found measurable biochemical differences between reflex tears and emotional tears, and even between tears shed during positive versus negative emotional experiences. Over a hundred distinct metabolites have been identified that differ between negative and positive emotional tears.13PubMed Central. Non-targeted Metabolomics Analysis Reveals Distinct Metabolic Profiles Between Positive and Negative Emotional Tears of Humans: A Preliminary Study Paired exploratory studies have confirmed that these within-subject metabolic differences between emotional and reflex tears are reproducible, though which specific metabolites matter and what they do remains an open question.14Metabolites. Targeted Metabolomic Profiling of Emotional and Reflex Tears: A Paired Exploratory Study
Where Tears Go After They Cross Your Eye
Tears don’t just evaporate. The drainage system is a miniature plumbing network that shunts used tear fluid from the eye surface into the nasal cavity, which is why your nose runs when you cry. At the inner corner of each eye, two tiny openings called puncta lead into narrow tubes called canaliculi, which empty into the lacrimal sac, a small pouch sitting in a bony groove beside the nose. From there, the nasolacrimal duct carries the fluid down into the nose.
The mechanism that moves tears through this system is called the lacrimal pump, and it is powered by the same muscle you use to blink. When you close your lids, the orbicularis muscle contracts and compresses the canaliculi while simultaneously pulling the upper wall of the lacrimal sac outward, creating a low-pressure zone that sucks tears from the canaliculi into the sac. At the same time, the lower portion of the sac wall moves inward, generating positive pressure that pushes tears down the duct toward the nose. When you open your lids, the process reverses: the canaliculi spring open and draw fresh tears from the eye surface, while the sac resets for the next cycle.15PubMed. Tricompartment model of the lacrimal pump mechanism Every blink is a pump stroke. This is why people with weak blinks or lid-closure problems often develop watery eyes: the pump cannot move fluid as efficiently, and tears pool or overflow.
Tears As an Immune Defense
Beyond keeping the cornea moist, tears serve as a front-line immune barrier. The aqueous layer contains a suite of antimicrobial proteins. Lysozyme, one of the most abundant, attacks bacterial cell walls. Lactoferrin binds iron that bacteria need to grow. Other compounds include members of the cationic antimicrobial peptide family and surfactant protein-D. No single compound in tears is overwhelmingly powerful on its own; many are present in relatively low concentrations. The real defensive strength comes from cooperation between them. Studies have demonstrated synergistic and additive interactions among several tear antimicrobials, meaning that together they provide far more protection than any one of them would alone. This cooperativity is likely the primary way tears defend the ocular surface against infection in everyday life.
The accessory glands of Wolfring contribute to this defense as well: among the most highly expressed genes in those glands are those encoding lysozyme, lactoferrin, and other proteins with antimicrobial activity.2PubMed Central. Gene expression in human accessory lacrimal glands of Wolfring The fact that multiple gland types produce overlapping sets of defensive proteins suggests the eye has built-in redundancy for immune protection, an important safety margin for an organ constantly exposed to the environment.
How Hormones Influence Tear Production
Nerve signals are not the only regulators of the tear system. Hormones, particularly sex hormones, play a significant role. Androgens are important for maintaining the health and function of both the lacrimal gland and the meibomian glands. When androgen levels drop, whether from aging, certain medications, or medical conditions, both the watery and oily components of the tear film can deteriorate.16PubMed Central. Sex Differences in the Lacrimal Gland: Implications for Dry Eye Disease This partly explains why dry eye disease is considerably more common in women, especially after menopause.
The role of estrogens is less straightforward. Evidence suggests their effects on the lacrimal gland depend on dose and context, sometimes promoting inflammation and sometimes supporting glandular function.17PubMed Central. Hormones and dry eye disease Progesterone, thyroid hormones, and other endocrine factors have also been linked to tear film stability, though androgens remain the best-studied hormonal influence on tear production.
How Aging Affects the Tear System
The lacrimal gland does not age gracefully. Over time, its secretory acinar cells shrink and some are replaced by fibrous tissue. The nerve connections that drive secretion become less effective. Lipofuscin, a cellular waste product associated with aging, accumulates within the gland. Inflammatory cells, including mast cells, increase in number. The combined effect is a gland that responds more sluggishly to stimulation and produces a lower volume of tears with an altered protein composition.18PubMed Central. The aging lacrimal gland: changes in structure and function
Recent work has fleshed out the convergent mechanisms at play. Oxidative stress damages glandular cells. Mitochondria fragment and become less efficient at producing energy. Cellular senescence, in which old cells stop dividing but linger and release inflammatory signals, contributes to a low-grade chronic inflammation within the gland. Parasympathetic and sympathetic nerve signaling both decline, which further depresses tear secretion.19PubMed. Integrative Mechanisms and Emerging Therapies for Lacrimal Gland Aging The result is that many older adults experience drier eyes not because of a single disease process, but because multiple aspects of the tear-production system deteriorate in parallel.
When the System Breaks Down
Dry eye disease is one of the most common reasons people visit an eye doctor, and the condition comes in more than one form. When the lacrimal gland itself fails to produce enough watery tears, the result is aqueous-deficient dry eye, sometimes associated with autoimmune conditions like Sjögren syndrome. When the meibomian glands malfunction and the lipid layer is compromised, tears evaporate too quickly, producing evaporative dry eye. Many patients have elements of both.
These subtypes leave different physical signatures on the eye. Research comparing the corneal surface in each subtype found that aqueous-deficient dry eye tends to produce thinning of the corneal surface layer in the center and lower portions of the cornea, consistent with increased friction from inadequate lubrication. Evaporative dry eye from meibomian gland dysfunction, by contrast, shows a more preserved surface layer, shaped by the thickness of the remaining lipid film.20PubMed Central. Comparison of Corneal Epithelial Thickness Profiles Between Aqueous-Deficient and Evaporative Dry Eye Disease Proteomic analysis of tears from these different patient groups has also revealed distinct protein profiles, with aqueous-deficient patients showing elevated markers of inflammation and altered metabolic processes.21PubMed Central. Proteomics analysis of human tears from aqueous-deficient and evaporative dry eye patients
This matters practically because treatment differs. Aqueous-deficient dry eye often responds to approaches that increase tear volume or reduce inflammation of the lacrimal gland. Evaporative dry eye is better addressed by improving meibomian gland function through warm compresses, lid hygiene, or procedures that unclog blocked glands. Treating the wrong subtype, or treating both as if they were the same condition, is a common reason people find that over-the-counter artificial tears only partially help.
Tear Composition Across Species
Humans are far from the only animals that produce tears. Every land-dwelling vertebrate with eyes needs a tear film to keep its corneas hydrated, and the basic structure of that film is broadly conserved across mammals, birds, and reptiles. But the specifics vary. A comparative analysis of tears from humans, domestic mammals, reptiles, and birds found that concentrations of total protein, albumin, urea, glucose, and cholesterol differed across all groups studied.22PubMed Central. Comparative Analysis of Tear Composition in Humans, Domestic Mammals, Reptiles, and Birds
Even among mammals, the lipid layer is not always the same. In humans, dogs, and mice, meibomian lipids are dominated by non-polar components like wax esters and cholesteryl esters. Rabbits, on the other hand, produce a meibum with a substantially different composition, dominated by diacylated diols and certain sterol esters, with wax and cholesteryl esters present only in trace amounts.23The Veterinary Journal. Tear biomarkers in veterinary species: An updated review with special focus on dogs and cats These differences matter for researchers: the rabbit has long been a popular model for studying tear film and dry eye, but its meibomian chemistry is quite unlike ours. Findings from rabbit models may not always translate directly to human tear-film biology, a caveat that is easy to overlook when reading about new dry-eye treatments tested in animals.
What does appear to be unique to humans is the emotional component. While many animals produce copious reflex tears in response to irritants, and sea turtles famously excrete salt-laden fluid from glands near their eyes, no other species has been confirmed to shed tears as a result of emotional experience. That distinction makes emotional crying a genuinely human trait, layered on top of the protective tearing machinery that we share with most other vertebrates.