The Chemistry of Tears

Composition of ...While all tears contain salts (NaCl and KCl), water (H20), and varying amounts of antibodies and proteins, different tears have different proportions. They all vary slightly, according to what kind of tear they are: Basal, Reflex, or Emotional (psychic).

• Basal tears are composed primarily of salts, water, and a little bit of mucus. The salts are sodium chloride (NaCl) and potassium chloride (KCl). The mucus helps the tears adhere to the eye. These tears are key in protecting the cornea of the eye.

• Reflex tears are very similar to basal tears (containing salts, and water). The primary difference between the two is that reflex tears contain more antibodies to fight foreign chemicals. This is why reflex tears are the eyes’ response to things like onions.

• Emotional (psychic) tears contain the most proteins of the three kinds. The proteins are emotional or stress hormones. They cause stress and depression, which is why it’s important for our body to get rid of them. When high enough levels of these stress hormones build up, they can become toxic, affecting the body’s immune system and mood.

Some people have a condition that limits tear production - keratoconjunctivitis sicca, also known as dry eyes. This condition can also result in dryness or irritation of the eye. A common treatment for dry eyes is artificial tears. The most common components of artificial tears are: polyvinyl alcohol, glycerin, and carboxymethylcellulose.

Main Chemicals, Compounds, Components

The main components in naturally occurring tears are:

• Salts (sodium chloride - NaCl and potassium chloride - KCl)
• Proteins - stress hormones (All 3 contain proteins, but psychic contain the most)
• Water - H20
• Antibodies - Immunoglobulin A (IgA)

The two main components are:

• Water - H20
  • Two hydrogen (H) atoms bonded to one oxygen (O) atom.

• Salts - NaCl and KCl
  • Sodium (Na) and Chloride (Cl) bonded together
  • Potassium (K) and Chloride (Cl) bonded together

For artificial tears, the two most common components are:

• Polyvinyl alcohol - C2H40
  • Two carbons, four hydrogens, and one oxygen
• Carboxymethylcellulose - molecular formula C8H15NaO8
  • Eight carbons, fifteen hydrogen, sodium, and eight oxygen

Chemistry's Role

Tears are naturally produced by the lacrimal or tear ducts in eyes.

Water, a primary ingredient of tears is found naturally (not man-made), and is covalently bonded hydrogen and oxygen atoms.

Sodium chloride (NaCl) and potassium chloride (KCl), other common components, are found naturally in the ground. Sometimes both are mixed together in a mineral called sylvinite. Sodium and chloride are ionically bonded, and potassium and chloride are ionically bonded.

In artificial tears, polyvinyl alcohol is used as a lubricant. It is a synthetic polymer that is water soluble. It is commonly used in various textiles and contact solution. To create polyvinyl alcohol, ethylene (a gas given off by plants) is synthetically produced (man-made), then turned into vinyl acetate using a chemical reaction between oxygen (O2) and acetic acid (CH3COOH). From there, vinyl acetate is polymerized (made to form a molecule with repeating parts, also known as monomers) before finally being dissolved into alcohol to get polyvinyl alcohol.

The second common ingredient in artificial tears is carboxymethylcellulose. This is a synthetic component created using cellulose and chloroacetic acid in an alkali-catalyzed reaction. Though more commonly found in ice packs and detergents, it is also used as a lubricant in artificial tears.

Background Research

While we all know that crying happens as a result of sadness and sometimes joy, what about crying over an onion? You aren’t sad about cutting an onion - right? So how do tears actually work, and why do they occur?

Though you may not realize it, your body goes through about 1.4 ml of tears everyday, if you don’t cry that is. Tears are important to keep your eye healthy and functioning. Tears also help get rid of stress and relieve depression. While everyone assumes that tears are mainly composed of salty water or a saline solution, they can also contain lysozymes, proteins, and manganese. There are 3 types of tears: basal, psychic, and reflex tears. Basal tears are the tears your eye uses everyday to help keep the cornea of your eye functioning. Psychic tears are the emotional tears that occur when you feel deep emotions such as sadness, joy, anger, frustration, and emotional pain. Reflex tears are the kind that accompany things like onions. Reflex tears are designed to protect your eye from irritants and toxins.

Resources

http://www.sciencealert.com/watch-why-do-we-cry-the-chemistry-of-three-types-of-tears

• Outlines kinds of tears and why they happen.

https://www.vsp.com/tear.html

• More details on make-up of tears.

http://www.lifebuzz.com/tears/

• Pictures of different kinds of tears.
• Person who analyzed 100 slides of dried tears.

http://www.scienceiq.com/facts/scienceoftears.cfm

• Mentions a few chemicals in tears.

http://articles.mercola.com/sites/articles/archive/2015/09/10/different-emotions-reveal-changes-in-tears.aspx

• How tears affect emotions and can (in some case) be a way for people to communicate (like babies).
• Also starts to go into the structure of the tear duct.

https://en.wikipedia.org/wiki/Tears

• Basic information on types of tears.

http://psychcentral.com/blog/archives/2012/11/18/the-science-of-tears/

• This mentions why fake tears don’t work as well.
• Manganese and protein are found in tears.

http://www.thenakedscientists.com/forum/index.php?topic=19782.0

• Explains more specific details on the three types of tears.

http://time.com/4254089/science-crying/

• Additional information on the science of crying.

https://en.wikipedia.org/wiki/Artificial_tears

• Explains the effects of dry eyes and how artificial tears help.

https://en.wikipedia.org/wiki/Polyvinyl_alcohol

• This explains polyvinyl alcohol, a lubricant used in artificial tears.

https://en.wikipedia.org/wiki/Carboxymethyl_cellulose

• This explains carboxymethylcellulose, a lubricant in artificial tears.

https://blog.honest.com/polyvinyl-alcohol/#

• The process in creating polyvinyl alcohol is laid out here.

carboxymethyl cellulose structural formula

• This shows the molecular formula for carboxymethylcelllulose.

About the Author

Maddie Fink is a homeschool sophomore who takes chemistry and orchestra at Senior High School. She is active in 4-H and loves animals of all kinds. When she isn’t doing homework, she enjoys hanging out with friends, riding horses, and playing cello. Her favorite classes are Latin, Omnibus, and Chemistry. She loves school and hopes to pursue a degree in science or government.