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Friday, July 25, 2014

The Amazing Design of Human Tears

Posted by jlwile on January 12, 2011

Tears do some amazing things!
(click image for credit)

You probably don’t think about them very often, but tears are amazing. They are produced continually by your body’s lacrimal glands in order to lubricate your eyes as well as various tissue membranes associated with your eyes. They generally drain away through two structures called the lacrimal punctua. This is why you normally don’t notice your tears. However, if your lacrimal glands start producing tears too quickly for them to be drained away, they collect in your eyes until they eventually fall down your cheek. At that point, you (and other people) notice them, because you are crying.

There are two reasons for crying: eye irritation and strong emotions. If dust or other debris gets into your eyes, your lacrimal glands start producing a lot of tears in order to flush out the debris. All creatures with moveable eyes can cry because of irritation. I will call the tears produced by this kind of crying “irritant tears.” The chemical content of irritant tears is not all that surprising. In addition to oils for lubrication, water, and salt, they contain a powerful enzyme called lysozyme. This broad-spectrum antibiotic helps to prevent eye infections.

The second reason for crying has inspired today’s blog. A friend of mine sent me a news story regarding some new research that has been done on tears that are the result of emotion. Interestingly enough, she I and disagree strongly on what should trigger emotional tears (I am an old sap – she rarely cries for emotional reasons), but she knew the story would be of interest to me. When I looked a the study that generated the news story, it reminded me of some old research that was done on tears. Together, the old and new research tell us a lot about how amazing tears are.

Let’s start with the old research. Back in 1981, William Frey and his colleagues produced a very interesting study that compared the contents of “irritant tears” to those of “emotional tears.” They had subjects cut onions in order to produce irritant tears, and then they had those same subjects watch sad movies (Beaches is one of my favorites). When they chemically analyzed the tears, they found that irritant tears are chemically quite different from emotional tears.1

Specifically, Frey and his colleagues found that emotional tears contained 24% higher protein concentrations than irritant tears. Among those proteins is the adrenocorticotropic hormone, mercifully abbreviated ACTH. This protein is produced in high concentrations when the body is stressed, and it stimulates the body’s adrenal glands to produce a series of hormones that regulate the body’s response to stress.

Frey and his colleagues suggest in their research that removing ACTH from your body through emotional tears will help reduce your body’s stress response. In other words, having a good, long cry will produce a physiological response that will make you feel better. According to Frey, then, there is not just a psychological benefit to crying, there is a biochemical benefit to it as well.

I mention this fact in my general science course (for seventh graders) and the human anatomy and physiology course I co-authored (for high school students). In those texts, I note that emotional tears are generally accepted as something that only humans can produce.2 As a result, you can think about emotional tears as a gift from God – something your Creator gave you to help you cope with stress.

Now on to the new research. Shani Gelstein and Yaara Yeshurun (along with five other colleagues) studied how men respond to women’s tears. They trickled saline down the cheeks of two donor women. This saline served as a control sample. It had nothing to do with actual tears, but it should contain any chemicals that tears would “pick up” as they traveled down the women’s cheeks. They also had the two women watch sad movies and collected their tears of sadness. They then tested the responses of 24 men to the saline and the tears.3

The results were interesting. First, the men could not smell any difference between the tears and the saline. As a result, the effects noticed in the rest of the study were not the result of a perceived odor. Second, the researchers had the men sniff the saline and the tears and rate them according to intensity, pleasantness, and familiarity. There was no difference.

Here’s the interesting part: They then showed the men pictures of women and had them rate the women according to how sad each woman was and how sexually attractive each woman was. While the smell of tears had no effect on how the men rated the sadness of the women, it made the men view the women as slightly less sexually attractive.

Even though the effect that smelling tears had on sexual attraction was very slight, when the experimenters directly measured physiological conditions associated with arousal, the results were more pronounced. For example, the researchers measured testosterone levels in the men after sniffing tears and saline. The testosterone levels were significantly reduced after sniffing tears.

The researchers also measured how well the men’s skin conducted electricity, which is recognized as one measure of psychological or physiological arousal. They found that the during the actual sniffing process, the tears produced an increase in the skin’s conductance, but afterward, the tears produced a long-term decrease. This result is consistent with the idea that detecting emotional tears triggers a response by the body (hence the quick, initial increase in skin conductance), but that response is to reduce arousal (hence the long-term decrease).

So the bottom line seems to be that just the detection of tears produces a signal to men to reduce arousal. Remember, the experiment did not involve seeing women who were crying. Even in the pictures the men rated, the women were not crying. They just had rather ambiguous expressions. The experiment involved nothing more than smelling tears, which have no perceptible odor. As the authors state:

…the current results conclusively demonstrate a chemosignal in human tears. In this, we illustrate a novel functional role for crying.

I am not sure that a study with 24 men and two women’s tears “conclusively” demonstrates anything. However, the results are intriguing.

Lubricating the eyes, disinfecting the eyes, getting rid of stress-related proteins, and most likely containing a chemical message that reduces arousal. Tears are designed to do some truly amazing things!

REFERENCES

1. Frey WH 2nd, DeSota-Johnson D, Hoffman C, McCall JT., “Effect of stimulus on the chemical composition of human tears,” American Journal of Ophthalmology, 92:559-567, 1981.
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2. Silvia H. Cardoso and Renato M.E. Sabbatini, “The Animal That Weeps,” Cerebrum, 4:7-22, 2002.
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3. Shani Gelstein and Yaara Yeshurun, et al., “Human Tears Contain a Chemosignal,” Science Express, Science DOI:10.1126/science.1198331, 2011. (Abstract available online)
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Comments

5 Responses to “The Amazing Design of Human Tears”
  1. Josiah says:

    I wonder whether this is specific to women’s tears, or whether tears in general have this inhibiting effect.

    “which have no perceptible odor.”
    Or no consciously perceptible odor?

  2. jlwile says:

    You are absolutely right, Josiah. No CONSCIOUSLY perceptible odor. That’s why the authors say its a chemosignal. The nose obviously detects it, but there is no conscious perception of odor.

    The news story I was sent makes a glib comment about men not crying. They obviously haven’t watched me during Beaches, four separate scenes in Les Miserables, or the “For Good” number in Wicked. In any event, the news story says the investigators are now looking at male tears because they “finally have one good male crier” (John Boehner, perhaps?).

    The earlier study by Frey saw no difference in the chemical makeup of men’s and women’s tears, but the process used is sensitive to proteins. If the chemical producing the response is not a protein (or is a small protein), it would have been missed. They did look for manganese using a separate process, but it would not have been sensitive to anything else.

  3. Robot's Husband says:

    What an interesting topic. I am glad I don’t see female tears very often!

  4. jlwile says:

    Robot’s husband, I read your comment and THEN your name. “Laugh out loud” doesn’t even begin to describe what I did.

  5. The Black Sheep says:

    I’m just imagining the looks on the faces of the men when the scientists brought them in and said, “Here, we’d like you to smell these tears so we can test how it affects the testosterone levels in your body.”

    On another note, sign my dad up for the “good male crier.” You guys can watch Les Mis together. And Beauty and the Beast.

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