Tear Fluid

Tear fluid, also known as *lacrimal fluid*, is a transparent liquid produced mainly by the lacrimal glands. It forms the tear film that covers the ocular surface and is essential for eye health and vision.

Tear fluid is composed of three distinct layers:

• Lipid layer (outermost): Secreted by Meibomian glands; prevents evaporation of the underlying aqueous layer.
• Aqueous layer (middle): Secreted by lacrimal glands; contains:
  • Water
  • Electrolytes (Na⁺, K⁺, Cl⁻)
  • Proteins (e.g., lysozyme, lactoferrin, secretory IgA)
  • Glucose and urea
• Mucin layer (innermost): Secreted by conjunctival goblet cells; facilitates even spread of tears over the cornea and improves adherence.

• Lubricates the ocular surface
• Provides nutrients to the avascular cornea
• Offers antimicrobial protection (lysozyme, lactoferrin, IgA)
• Removes debris and foreign particles
• Maintains a smooth refractive surface for optimal vision

• Dry eye disease (keratoconjunctivitis sicca): Results from reduced quantity or altered quality of tear fluid.
• Tear film breakup time (TBUT): A test to evaluate tear film stability; shorter times suggest instability.
• Tear osmolarity: Elevated osmolarity is a hallmark of dry eye disease.
• Tear biomarkers: Emerging research investigates the diagnostic potential of tear components for systemic and ocular diseases.

1. Pflugfelder SC, Stern ME. *Tear film dysfunction: A commonly unrecognized cause of ocular surface inflammation*. Am J Ophthalmol. 2020.
2. Bron AJ, et al. *TFOS DEWS II report*. Ocul Surf. 2017.