What Are Tears Made Of?
Although it’s common to study the chemical composition of blood or urine, a lot of people forget about the composition of a teardrop, even though we’d be miserable without tears. Tears nourish and moisturize the eyes, plus they rinse away dust and foreign matter. Although you may just think of them as salt water, there’s actually a lot more going on, from a chemical perspective.
Three Layers of Tears
Tears aren’t uniform in composition. There are three layers:
- An outer hydrophobic oily layer protects tears from evaporating or spilling out onto your cheeks. The oils are produced by the Meibomian glands.
- A middle aqueous or watery layer transports vitamins, salt, and other minerals and nutrients to the cornea of the eye. The composition spreads evenly over the eye, promotes osmoregulation (salt balance), and helps prevent infection. This layer is mostly water, with electrolytes (sodium, chlorine, potassium, urea), glucose, and proteins (antibodies, lysozyme, lipocalin, and lacritin).
- A mucous layer coats the cornea. This inner layer helps the tear stay on the eye and keeps the tissue moisturized. The mucins are produced by goblet cells in the conjunctiva of the eye. Mucous is hydrophilic, so it helps ensure smooth distribution of the aqueous layer over the entire eye.
How the Composition of a Teardrop Changes
Tears that protect your eyes every day are slightly different from those that flush away irritants or those you cry in response to emotion. The watery tears your eyes produce to rinse away a bit of grit or a stray eyelash tend to have healing properties. Emotional tears carry protein-based hormones. Here are the three main types of tears and their properties:
Basal Tears: These are typical tears that keep the eye hydrated, clean, and nourished. The salt content of these tears is comparable to that of blood plasma. Lysozyme in basal tears protects against infection by dissolving the peptidoglycan layer of some types of bacteria. Basal tear production slows with aging.
Reflex Tears: Reflex tears are produced in response to irritation from chemicals, bright light, or foreign matter. Although the chemical composition isn’t appreciably different from basal tears, a much larger volume of tearing occurs. The same reflex that causes watery eyes is linked to coughing, yawning, and vomiting.
Weeping Tears: Crying as an emotional response or from pain is termed “weeping”. These tears contain the protein-based hormones prolactin, adrenocorticotropic hormone, and Leu-enkephalin. Leu-enkephalin acts as a natural painkiller.