What is in a tear?

Ava Yr8

Tears raise many questions, the answers to which are very interesting. For example, how does our body receive signals of sadness- or extreme happiness- to trigger tears, and what is even in them?

Firstly, what substances are found in a tear? Well, they mainly consist of water, but also contain salt, fatty oils, and over 1, 500 different types of proteins. A tear has three different layers: the mucous, or continuous layer, the aqueous layer and the oily layer. The continuous layer keeps the tear attached to the eye while the aqueous layer which is the thickest layer, hydrates your eye and shields it from bacteria, as well as protecting your cornea (the transparent layer that covers the front of your eyeball). The oily layer precludes the other layers from evaporating.

However, you actually have three different types of tears. Basal tears are permanently in your eyes and keep them lubricated. Emotional tears are the kind that we are most familiar with and they are produced when we experience intense and severe emotions. The final one is the reflex tear, which is emitted when our eyes are exposed to irritants, like onions or smoke.

Where do tears go after they have been created? There are lachrymal glands stationed above your eyes. As you blink, tears spread on the surface of your eye and then drain into tiny holes in the corners of your lower and upper lids. Then, they travel through small channels and down your tear ducts to your nose.

Generating emotional tears requires the amygdala, the area of brain that controls the processing of emotions, to send a signal to the hypothalamus, a small, spherical gland in the brain that is connected to the autonomic nervous system. The autonomic nervous system handles functions that we cannot control, like body temperature, hunger, thirst and crying. The hypothalamus then starts the sympathetic nervous system and accelerates the response to this. This results in your glottis (the opening between the vocal cords in your throat) to swell, making your throat feel tight. All of your emotions tell the hypothalamus to produce the compound, acetylcholine. Acetylcholine links to receptors in your brain which send signals to your lachrymal glands, and tears are produced.

When your eyes are exposed to irritants tears are often produced. While chopping onions Syn-propanethial-S-oxide is the gas that triggers tears. This gas is an irritant because as onions grow, they mix with sulphur in the ground and create amino sulphides which turn into a gas. Then, this gas combines itself with the enzymes released from the onion when you chop it, creating sulfenic acid. This sulfenic acid reacts with the onion enzymes to form syn-propanethial-S-oxide and your lachrymal glands attempt to protect your eyes and produce reflex tears. Other examples of eye irritants are smoke, bright lights and reading small print for an extended period of time.

Why do we get a runny nose when we cry? When some of your emotional tears run through your tear ducts, also known as nasolachrymal ducts, they end up in your nasal passage. The tears mix with the mucus in your nose, causing you to have a runny nose.

As you age your basal tear volume decreases. Your gender can similarly regulate your tolerance for tears. The assumption that women cry more than men is often viewed as sexist, however, there are a few studies to prove this stereotype as actually true. This has not been thoroughly investigated but scientists believe that if that is the case it is down to two theories: that men may have smaller tear ducts than women, or that it is because of the higher quantity of prolactin in women’s emotional tears. Prolactin is a hormone that promotes breast milk production in females and women have 60% more prolactin than men.

Tears are clearly a multi-layered biological response. A good cry indisputably makes us feel better and this is likely to be because emotional tears contain stress hormones and other toxins that are flushed away and removed when we cry. So now the next time you cry, you can think about how and why your tears are forming.