Overview
Tears are a clear fluid produced by glands around the eye to keep the surface moist, smooth, and healthy. Beyond simple lubrication, the tear film is a dynamic layer that protects the cornea, supplies oxygen and nutrients to avascular eye tissues, and contains molecules that help control microbes.
Composition and structure
The tear film is commonly described as three interacting layers that together maintain clarity and stability. These layers include:
- an outer lipid layer from the meibomian glands that reduces evaporation,
- a middle aqueous layer produced mainly by the lacrimal glands containing water, electrolytes and antimicrobial proteins,
- and an inner mucous layer from conjunctival goblet cells that helps the film adhere to the eye's surface.
Types of tears
Physiologically, three broad categories are recognized:
- Basal tears continuously coat the eye to maintain comfort and transparency.
- Reflex tears are produced in response to irritation from wind, onions, or foreign bodies, and often have a different composition to flush out particles.
- Emotional tears are associated with strong feelings and are observed in humans; they may differ chemically and play social and psychological roles.
Production, drainage, and examples
Tears are generated by the main lacrimal gland and several accessory glands. After spreading across the eye, fluid drains through tiny puncta into the nasolacrimal duct, which explains why crying can cause a runny nose. Common clinical issues include dry eye (insufficient or poor-quality tears) and epiphora (excessive tearing).
Functions and cultural notes
Functionally, tears lubricate and nourish, trap and remove debris, and provide antimicrobial defense through enzymes and immune proteins. In humans, emotional tearing also serves as a nonverbal signal that can facilitate social bonding. For further clinical or physiological details, see more information.