The chemical composition of tears

Tears are a universal human phenomenon, both an essential physiological mechanism and a powerful emotional expression. Although they may appear to be simple drops of water, their chemical composition is remarkably complex and finely tuned to serve several critical functions.

Types of Tears

Before we delve into their composition, it is important to understand that there are three main types of tears: basal, reflex and emotional. Each type has a slightly different composition, adapted to its specific function.

1. Basal Tears: These tears are constantly present and serve to lubricate and nourish the eye.
2. Reflected Tears: They occur in response to irritants such as smoke or wind.
3. Emotional Tears: They are triggered by intense feelings such as sadness, joy or frustration.

Chemical Composition of Tears

Tears are composed primarily of water, but also contain a variety of other chemical components that allow them to perform their protective and lubricating functions.

1. Water (H2O)

Water is the main component of tears, accounting for approximately 98-99% of their volume. It provides the medium necessary to dissolve and transport other chemical components.

2. Electrolytes

Electrolytes in tears include sodium (Na+), potassium (K+), chloride (Cl-), bicarbonate (HCO3-), and calcium (Ca2+). These ions are crucial for maintaining the osmolarity and pH of tears, ensuring that they are compatible with the surface of the eye and preventing irritation.

• Sodium and Chloride: These ions are primarily responsible for the salinity of tears. The balance between sodium and chloride helps maintain proper osmotic pressure.
• Potassium: It is important for the health of the epithelial cells of the cornea.
• Baking soda: Helps maintain the pH of tears in a slightly alkaline range, around 7.4, which is optimal for the ocular surface.
• Calcium: Participates in the integrity and stability of the tear film.

3. Proteins

Tears contain a variety of proteins, each with specific functions:

• Lysozyme: An antibacterial enzyme that helps protect the eye from infections.
• Lactoferrin: A protein that binds iron and has antimicrobial properties.
• Lipoproteins: They help stabilize the lipid layer of the tear film.
• IgA: A type of immunoglobulin that plays a crucial role in the immune defense of the ocular surface.

4. Lipids

The outermost layer of the tear film is lipid and is produced by the meibomian glands. This layer helps prevent excessive evaporation of tears and keeps the ocular surface smooth.

• Cholesterol and Cholesterol Esters: They are key components of the lipid layer that help stabilize and extend the tear film.

5. Mucins

Mucins are glycoproteins that make up the innermost layer of the tear film, close to the surface of the eye. They facilitate the adhesion of tears to the ocular surface and provide a protective barrier against pathogens.

6. Vitamins and Other Bioactive Molecules

Tears also contain small amounts of vitamins (such as vitamin A) and other bioactive molecules that help maintain eye health.

Functions of Tears

The complex chemical composition of tears is designed to perform several essential functions:

• Lubrication: Tears allow the smooth movement of the eyelids over the surface of the eye.
• Protection: Components such as lysozyme and lactoferrin protect against infections.
• Nutrition: Electrolytes and vitamins in tears nourish the cornea and other parts of the eye.
• CleaningReflex tears help remove particles and irritating substances from the eye.