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Tears suddenly start flowing out of our eyes when we find our long lost friend or when someone unexpectedly decides to break up with us. Do tears really save us from harm? Or are they just the reflex shown?

What biological purpose do tears serve for the body?

As I understand it, crying causes endorphin release, specifically leucine-enkephalin. This serves as a painkiller for us. Are tears the only way to accomplish this? It seems I am missing something because oozing out of our eyes is an inferior pain distribution system to say; the circulatory system.

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  • $\begingroup$ Homeostasis is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant. Your concluding sentence makes sense (what is the function of tears), but linking it to homeostasis per se draws the question specifically to the realm of fluid balance and ion balance. You should clarify this question I think. $\endgroup$
    – AliceD
    Feb 22 '16 at 9:16
  • $\begingroup$ I've edited it. If they don't account for homeostasis then why we consider them as reflex? $\endgroup$ Feb 22 '16 at 10:55
  • $\begingroup$ I would read the wiki page on Tears first $\endgroup$
    – CKM
    Feb 22 '16 at 21:13
  • $\begingroup$ @علیآفاق I've made some big edits to the question to move it away from homeostasis. Feel free to roll back. $\endgroup$
    – James
    Feb 23 '16 at 7:32
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    $\begingroup$ Emotional-tears (tears of cry) are different in biochemical composition than other-sorts of tears. Emotional tears contain higher concentration of neurohormones and it is believed that unwanted (toxic) level of neurohormones drained out in this way. view en.wikipedia.org/wiki/Tears#Chemicals_in_tears and en.wikipedia.org/wiki/Crying#Function. $\endgroup$ Sep 8 '16 at 7:58
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Lets break this question into parts and answer them one by one.

Do tears really save us from harm?

Well, yes. Tears, mucus and saliva contain an enzyme lysozyme that breaks down the cell wall of many bacteria. Those that are not killed immediately are trapped in mucus and swallowed.1

You can, obviously, search on google for another reference if you do not believe this one.

Or are they just the reflex shown?

It depends on what type of tears you are talking about. Tears are divided into 3 categories:

  1. Basal tears
  2. Reflex tears
  3. Psychic tears

In "reflex action" part, the cause is reflex tears. Reflex tears result from irritation of the eye by foreign particles, or from the presence of irritant substances in the eye's environment, which trigger TRP (Transient Receptor Potential) channels in the ophthalmic nerve. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth... These reflex tears attempt to wash out irritants that may have come into contact with the eye.

These are, of course, different from those tears which "suddenly start flowing out of our eyes when we find our long lost friend or when someone unexpectedly decides to break up with us". These are psychic tears i.e. increased tearing due to strong emotional stress, pleasure, anger, suffering, mourning, or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy such as during times of intense humour and laughter.2

But the actual relation of tears with emotions can be summarized as:

  • There is an area of your brain specifically to deal with your emotions, called the limbic system (specifically the part of it called the hypothalamus), which is hard-wired into your autonomic nervous system (that’s the part you don’t have any control over).
  • This system, via a neurotransmitter called acetylcholine, has a degree of control over the lacrimal ‘tear’ system; and it is this tiny molecule which then stimulates tear production.
  • So in short, your emotional reaction to the break-up triggers your nervous system, which in turn, orders your tear-producing system to activate.3

Moving on to next part:

crying causes endorphin release...Are tears the only way to accomplish this?

Certainly not. Even when you exercise, your body releases endorphins. These endorphins interact with the receptors in your brain that reduce your perception of pain. So, tears are not the only way to accomplish this.4

At last, I would answer this part

What biological purpose do tears serve for the body?

as tears are necessary for the continued health of the ocular surface. Normal constituents include water, mucin, and lipids, electrolytes, non-electrolytes, and proteins. Retinoids are also important for ocular health and prealbumin may be a carrier for vitamin A in the tears to supply corneal epithelium with its requirements. Changes in tear constituents may cause certain ocular disorders.5

It is important to know here that by the above part, I meant basal tears and not psychic ones. Basal tears lubricate the eye, and help to keep it clear of dust. Tear fluid contains water, mucin, lipids, lysozyme, lactoferrin, lipocalin, lacritin, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection as a part of the immune system. Lysozyme does this by dissolving a layer in the outer coating, called peptidoglycan, of certain bacteria. It is a typical body fluid with a salt content similar to blood plasma.

EDIT 1: Now, as you asked in the comments

what if we don't maintain our ocular surface?

I can tell you a very common example of dry eye syndrome. Dry eye syndrome is a common tears and ocular surface multifactorial disease, described by changes in the ocular surface epithelia related to reduced tears quantity and ocular surface sensitivity, leading to inflammatory reaction. It is believed that decrease in the tear volume on the corneal and conjunctival surface caused by either a decreased tear secretion or accelerated evaporation plays the main role. The clinical features of dry eyes include ocular discomfort, feeling of dryness, feeling of eye fatigue, hyperemia, kerato-conjunctival epithelial disorders and abnormalities of vision.6

EDIT 2: When it comes to function of psychic tears, then how could one leave talking about why we feel better after crying (except headache). It is now proven that psychic tears contain a higher amount of hormones than basal or reflex tears. See this article7:

Emotional tears are composed of more protein-based hormones, such as prolactin, adrenocorticotropic, and leucine enkephalin (a natural pain killer), which is suggested to be the mechanism behind the experience of crying from emotion making an individual feel better.

Though these hormones do not have any direct relation with emotions (excluding prolactin which counteracts the effects of dopamine), they can be related to emotions as long as another biological reason for their secretion through tears is discovered.

Bonus: From the same article 7, here is a very unusual function of tears i.e. controlling arousal! See this paragraph:

On a study conducted by the Weizmann Institute of Science in Rehovot, Israel, emotional tears from women have been found to reduce sexual arousal in men. Also, emotional tears are made up of a different chemical component than those evoked by eye irritants and can relay chemical messages to others. The change in sex drive could be attributed to a drop in testosterone provoked by the tear chemicals, reducing aggression. In the animal world, it has been found that some blind mole rats rub tears all over their bodies as a strategy to keep aggressive mole rats away.

This way, we can soon have a long list of functions. I hope these many functions are enough to answer your question.

References:

1: The body’s first line of defence

2: Tears - Wikipedia

3: Why do we cry? The science of tears

4: Exercise and Depression

5: Eyelid secretions and the prevention and production of disease

6: Recent developments on dry eye disease treatment compounds

7: Chemicals in tears - Wikipedia

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  • $\begingroup$ In the last part you've talked about"Tears are necessary for the continued health of the ocular surface" but we do cry hardly a year ( males) what if we don't maintain our ocular surface? can you elaborate it little more ( last part) ? $\endgroup$ Feb 26 '16 at 18:10
  • $\begingroup$ This does not seem to actually answer the question: you didn't explain what is the biological reason of what you called "psychic tears". It is not obvious why emotional stress should trigger a response which basically serves to "wash" the eyes. $\endgroup$
    – glS
    Feb 26 '16 at 19:19
  • $\begingroup$ @علی آفاق as I have told in the answer, crying is actually psychic tears, which is only one type of tear. Ocular health is maintained by basal tears. I will describe them too in the answer. $\endgroup$ Feb 27 '16 at 6:24
  • $\begingroup$ @gIS I have added that part too in the answer, hope this helps :) $\endgroup$ Feb 27 '16 at 6:38

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