What are tears made of?

They rise up in our eyes when we're mourning the loss of a loved one, getting the "feels" from a touching scene in a rom-com or simply chopping onions for dinner: Tears, whether you shed them often or hardly at all, are a ubiquitous part of the human experience. 

You may think of tears as just water — after all, as Justin Timberlake demonstrated in his hit song "Cry Me a River," these droplets of sadness are often likened to bodies of water, raindrops or even storms. 

But what are tears actually made of?

It turns out that there are three distinct types of tears, which vary in their composition, Daniela Oehring, an associate professor of optometry at the University of Plymouth in the U.K., told Live Science.

Related: Vlad the Impaler may have cried tears of blood, chemical analysis of his letters finds 

You're probably most familiar with the emotional tears that people shed in sadness after a breakup or in joy when seeing a friend after years spent apart. But humans produce two other types of tears: basal tears, which cushion the eye, supply the tissue with nutrients and remove debris; and reflex tears, which are produced in response to irritants, such as smoke or the chemicals released when you chop an onion.

Basal tears have lots of salt ions and other electrolytes, as well as proteins with antimicrobial properties, Oehring said. These proteins include lipocalin, which binds and disrupts certain compounds in microbes, and lysozyme, an enzyme that kills bacteria by breaking down their cell walls, she said. 

Reflex and emotional tears, on the other hand, have a higher water content, and a lower concentration of fats and proteins, than basal tears do. Some scientists have reported that, compared with reflex tears, emotional tears carry higher concentrations of hormones typically released when the body is under stress.

Upon being made, all these tears are added to a thin film that covers and nourishes the cornea, the transparent tissue over the front of the eye. This tear film has three distinct layers. On the outside is an oily layer produced by the meibomian glands in the eyelids that stops the eyes from drying out. The next layer is watery and lubricates the eye; it's made mostly by the lacrimal gland, which also adds proteins, oxygen and electrolytes. The inner layer, closest to the cornea's surface, contains a slime-like protein called mucin, which helps the tear film stick to the eye's surface.

According to Oehring, people produce about 1 to 4 microliters of tears a minute — or about 1.44 to 5.76 milliliters per day — on average. These are primarily basal tears, which shed constantly, rather than emotional and reflex tears made in response to stimuli. However, this volume can vary significantly depending on what you're doing.

"If you drive a bike, for example — and you have a lot of air, so you have a high evaporation rate — your tear production changes," she said. "If you are in a steam room, then your tear production decreases, because generally, there is no need to produce the water." Tears drain from the eyes via tear ducts. But because emotional and reflex tears are typically released in a flood, not in a trickle, these can often spill out of the eye rather than draining normally. 

If the quantity or quality of your tears declines such that your eyes don't stay lubricated, you may develop dry eye, which affects 16 million Americans. 

Many factors raise the risk of this common condition, Oehring said, including smoking, poor nutrition, and excessive time spent staring at computer screens and not blinking enough. As we age, we also become more susceptible to developing the condition, because our tear glands stop working as well. Some diseases can also increase the likelihood of having dry eyes, such as Sjögren's syndrome, an autoimmune condition in which the body's own immune cells damage the lacrimal glands that moisturize the eyes. 

People with Sjögren’s syndrome cannot even produce reflex tears. "Even if they get something in their eye, like an eyelash or something," Oehring said, "there is just not enough capacity to produce tears." 

This article is for informational purposes only and is not meant to offer medical advice.