My Morning Musings:  Tears

We’re nearing the final months of 2020, a year punctuated with joy and sadness.  Tears streaking down our faces likely marked those highs and lows.  Emotional tears are yet another physiological event that underscores our shared humanity: We are the only mammals who secrete emotional tears.  While other mammals do feel emotions, they secrete tears only to lubricate the eyes.  (Some research suggests that elephants and gorillas may secrete emotional tears, but data are still inconclusive.)  We too, secrete such lubricating tears.  But, did you know that the composition and function of tears changes depending on what’s happening to us?  Let’s dig a little deeper.

Tears are produced by lacrimal glands, these tiny structures located under the upper eye lids.  The lacrimal bone is a small bone forming part of the eye socket and the word lacrimal comes to us from the Latin word lacrima meaning…you guessed it, tear. When you cry, you might notice that you also get a runny nose.  The reason is because these tears that wash across the eye surface drain into the nasolacrimal canal, which leads to the nose.  Another thing you might notice when you have a good cry is that those tears streaming down your face cause your skin to burn (thanks to urea and lysozyme) or taste salty (thanks to sodium).  So, let’s break it all down.

When our eyes are open, they are exposed to everything the world has to offer us, both literally and figuratively.  Thus, tears serve a protection function.  And here’s a fun fact:  Our eyes are also windows to the body interior because this is the only place where blood vessels are not covered by skin, allowing clear viewing using an ophthalmoscope.  In addition to water, urea, and lysozyme (an antibiotic), tears contain chemicals such as glucose, potassium, immunoglobulins (antibodies), and hormones.  The list is longer, but we’re not in anatomy and physiology class.

A pre-COVID experiment we do in microbiology class shows just how potent tears are.  To do this, we inoculated nutrient agar Petri dishes with common bacteria, dropped tears onto the plates, and then incubated them for a day or so to see if the substances in tears prevented bacterial growth on the plates.  What you’ll see on the surface are so-called zones of inhibition, which are circles (think mini crop circles) created by the teardrops where the bacteria didn’t grow.  How did we get tears for the experiment?  Good question, since most people don’t cry on demand.  In a sterile environment using sterile instruments, we plucked nose hairs or placed onions close to the eye and used medicine droppers to collect tears.

It’s interesting to note that there are people out there who actually study tears, and we know that humans have three types of tears: basal tears, reflex tears, and emotional tears.  Basal tears are lubricants that keep the cornea (the transparent outer layer) moist.  Your eyelids don’t stick to your eyeballs because of basal tears.  Reflex tears are secreted when something irritates the eye (dirt, bright light, irritating chemicals) or pain.  Reflex tears are also secreted in greater amounts than basal tears and they contain more antibodies and antibiotics than basal tears.  Emotional tears release with happiness, sadness, anger – anything evoking feelings.  These tears also contain stress hormones (cortisol) and endorphins (natural pain killers). It is thought that these chemicals are released to help stabilize our mood.

Although this varies by culture, emotionalcrying also sends a message to those around us that perhaps we need physical contact, such as a hug, backrub, arm stroke, or whatever.  But with coronavirus, going without a hug or other forms of physical contact is leading to “affection deprivation” or “touch starvation.”  Sure, we have virtual hugs and e-wave conversations, but it’s not the same as human touch.  The same holds true with dogs:  we may verbally praise them, but they prefer petting.  Preliminary research is showing that just 10 minutes of petting a dog or cat reduces stress hormones in both of you.  Thus, the antidote to pandemic stress may just be your canine or feline – provided the dander isn’t an eye irritant.

Signing Off for Science