The human eye is an amazing organ that enables us to see the world from near and far at excellent quality, in spectacular colors and in 3D. Thanks to its amazing abilities, vision has become the most dominant sense through which we experience the world.

As the leading sensory organ in the human body, the eye is equipped with a multi-layered defense system designed to keep it safe. At the top of this defense system is the eye socket that protects it from blunt mechanical damage. Immediately after it are the eyelids, which close in response to any potential injury or penetration of a foreign object into the eye. The third and final part of the eye’s defense system is probably the most wondrous and interesting of them all: tears.

An in-depth look will reveal the chemical complexity that confers tears with an efficiency worthy of the eye’s last line of defense. Further exploring reveals that tears are much more than a layer of defense – not all tears are identical in terms of their composition and function, and often  they play a role as a tool for conveying social and chemical signals, overt or covert, between humans.

Each eye contains a tear gland (lacrimal gland) which continuously produces new tear fluid, while the old fluid is drained into small openings called “lacrimal punctum”. One could in fact say that the eye tears constantly in small amounts. Although these tears do not flow from the eye in a continuous stream, even if we ignore the tears shed onto the cheeks while crying, we shed about 120 liters of tears a year.

We do not usually feel the tears that are continuously produced, which are termed basal tears. These consist of three layers: the first layer is a mucous layer, above it is an aqueous layer that maintains a moist environment, rich in enzymes and antibacterial substances that repel and destroy bacteria, while the last layer is the lipid, or fatty layer, which creates a smooth surface area that facilitates sharper vision and acts as a kind of a cover, preventing the first two layers from evaporating.

Reflexive tears and emotional tears

The tears that we are more conscious of are reflexive tears and emotional tears. The reflexive tears are the eye’s washing mechanism and they are secreted in response to an attempt of a foreign or dangerous object to penetrate the eye. In response to an emergency situation, when the eye is at risk of being exposed to dangerous chemicals, malicious microorganisms or solid particles, the glands secrete large amounts of reflexive tears. In such situations, similar to what happens in response to the gas released from an onion, we may find ourselves with watery eyes and tears coming down our cheeks.

Why does an onion make us cry? Created by Bytesize Science

The third, and perhaps the most intriguing type of tears, are the emotional tears. The lacrimal glands secrete them in large amounts in response to our mood. They may be shed in response to stress, tension, anger, sadness, joy or helplessness. Sociologists argue that these tears are a social behavioral mechanism designed to gain sympathy from other members of the group and to express the individual’s emotional state, but in recent few years, more and more evidence is emerging to support the claim that tears are a physiological and chemical tool designed to assist the individual in coping with his or her emotional state.

A number of research studies have found that emotional tears, shed in response to an acute emotional state such as stress, anxiety or sadness, contained particularly high levels of toxins and hormones known as stress hormones. Some argue that these tears form part of a mechanism that aids in quickly removing these substances to improve a person’s emotional state. The high concentration of magnesium in tears, along with the release of natural painkillers, is a finding that supports this claim, since low levels of magnesium and the presence of pain killers in the body indeed contribute to mood improvement.

Prof. Noam Sobel from the department of neurobiology at the Weizmann Institute discovered an additional role of tears, when he found that through them chemical signals are transmitted to the environment, affecting the behavior of the people around us.

The aesthetics of tears

Since each of us continuously sheds basal tears, as well as reflexive and emotional tears at times, the subject of tears fascinates many people and has become the subject of scientific research and even a basis for works of art. An example of this is an exhibition presented at the Smithsonian museum by photographer  Rose-Lynn Fisher, who collected tears shed under different circumstances, dried them and photographed them under a microscope.

The first image in the exhibition is that of a tear shed during a long-awaited emotional encounter; the second one presents tears of change; after which are tears of ending, onion tears, tears of sadness as well as basal tears, ending with tears of happiness. Since different tears are composed of different substances and different concentrations of the same substances, a beautiful variety of structures was obtained.

Lastly, an interesting question is whether it is possible to cry in space. Watch the Canadian astronaut Chris Hadfield use water to simulate crying in a space station.