====== 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.

===== Composition =====
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.

===== Functions =====

  * 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

===== Clinical Relevance =====

  * **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.

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