Appliance of science: Who do we cry?

By Dr Naomi Lavelle

This question comes in from a very curious girl called Kayla; she would like to know why we cry?

WHY DO WE CRY?

Crying (of the emotional kind) is a uniquely human thing and scientists are still researching why we do it. Is there an evolutionary advantage to letting tears stream down your face? Does it change the way we feel? Why do we sometimes cry for joy as well as sadness?

DIFFERENT TYPES OF TEARS

Crying is a complex process; we actually produce three different types of tears. Basal tears are produced constantly in order to keep the eye lubricated.

Reflex tears are produced in response to an irritant and to help clear that irritant from the eyes (this one explains why we cry when chopping onions). When we talk about crying we are usually referring to the third type of tears we humans produce, the emotional ones.

IS THERE AN ADVANTAGE TO CRYING?

So why do we cry in response to fear, sadness or pain? One strong theory is that we do it to communicate with others, letting people around us know we are sad or injured or to show submission, signalling that we are not a threat. As well as letting others know our intentions or needs, crying can also help us express empathy and increase our bonds with others. This theory does support the fact that there is an evolutionary advantage to shedding tears.

HOW DO WE MAKE TEARS?

We produce tears in tears glands (they are also called lacrimal glands) and they are located just above each eye. The tears then travel to each eye via little ducts that open behind our upper eye lids.

Our tear ducts are constantly producing tears, to keep our eyes clean and moist, but we don’t usually even notice.

When we blink we wipe the tear fluid over our eyes, keeping them moist. This fluid is then drained off from the eyes through more ducts. It is only when we start producing a lot more tears that we start to notice them. The ducts that drain the fluid away cannot cope with all the extra tears and they start to run down our cheeks.

The regions of the brain that control tear production in the tear glands are the hypothalamus and basal ganglia. Both of these areas play a role in controlling our emotional responses.

WHAT ARE TEARS MADE OF?

Different tears have different compositions. Reflex and basal tears are mostly made up of salty water. They also contain enzymes that kill bacteria and vitamins and minerals.

Emotional tears also contain proteins, called hormones that can change how we feel.

BACK TO WHY WE CRY

So why do we cry? Apart from cleaning our eyes, clearing irritants and helping to communicate feelings of sadness, happiness or empathy with others, crying probably has a direct effect on our emotional state as well.

Emotional tears contain several hormones that are produced at times of stress. Crying may be a way of removing these stress hormones, reducing their levels in the body and calming us down.

Crying can produce physical changes in our body too; when we cry our heart rate changes and our breathing is altered.

Sometimes we cry because it makes us feel better and that sounds like as good a reason as any.

- Naomi is a science communicator and mother to three inquisitive children. She can be found at sciencewows.ie

- Feel free to email your questions to [e-mail]