Tears are a complex bodily fluid essential for maintaining the health and function of the eye. They provide constant lubrication and protection for the delicate ocular surface, preventing damage and supporting clear vision. When tears are tasted, the immediate sensation is a distinct saltiness, confirming they are a balanced solution of various chemical components.
The Salty Truth: Electrolyte Composition
The salty taste of tears originates from electrolytes, which are minerals that carry an electric charge when dissolved in water. The primary electrolyte present is sodium chloride, which accounts for most of the perceived salinity. Other significant electrolytes include potassium, which is found in tears at a higher concentration than in the blood, and bicarbonate.
These charged particles regulate tear osmolarity, which is the concentration of dissolved particles in the fluid. The tear film must be isotonic, meaning its salt concentration must match the concentration of the surrounding cells in the cornea and conjunctiva. This balance is necessary to prevent water from rushing into or out of the eye’s cells, which would cause them to swell or shrink.
A normal tear osmolarity value averages around 302 milliosmoles per kilogram (mOsm/kg). Maintaining this precise salinity ensures the eye remains properly hydrated without causing cellular damage. When the balance is disrupted, such as in dry eye conditions, the electrolyte concentration can increase, leading to an unstable tear film and irritation.
Beyond Salt: The Full Chemical Makeup of Tears
Tears are structured as a three-layered film, where each layer is composed of specific non-electrolyte components. The innermost layer is the mucin layer, produced by goblet cells in the conjunctiva, which acts like a foundational adhesive. This mucus helps the tear fluid spread evenly and firmly attach the tear film to the eye’s surface.
The middle layer is the aqueous layer, which is primarily water secreted by the lacrimal gland. This layer contains the bulk of the hydrating fluid, along with nutrients to feed the avascular cornea. The outermost layer is a thin lipid or oily film, produced by the Meibomian glands located in the eyelids.
The lipid layer forms a barrier that slows the evaporation of the underlying aqueous layer, keeping the eye moist for longer periods. The aqueous layer is also rich in protective proteins that act as the eye’s first line of defense against pathogens. These include lysozyme, an enzyme that breaks down the cell walls of bacteria, and immunoglobulins, such as secretory IgA, which neutralize foreign invaders.
The Different Roles of Tears
The body produces tears for different reasons, which are broadly classified into three functional types based on their trigger mechanism. Basal tears are constantly produced at a low level to keep the eye lubricated and nourished throughout the day. These tears ensure the smooth movement of the eyelid over the cornea and maintain a clear visual surface.
Reflex tears are produced rapidly and in large volumes when the eye encounters an irritant, such as dust, smoke, or the fumes from chopping onions. Their function is to quickly flush the foreign substance from the eye to protect the ocular surface from harm. Reflex tears are typically more watery than basal tears to facilitate this rinsing action.
Emotional tears are triggered by strong feelings, whether sadness, joy, or stress. While their composition is mostly the same as other tears, they are hypothesized to contain higher concentrations of specific proteins and stress-related hormones. These may include prolactin and adrenocorticotropic hormone, suggesting that crying during emotional distress might serve a biological function by helping to excrete built-up stress chemicals.