Tear film

Tear film is the thin fluid coating over the exposed eye surface. Its interacting layers lubricate and protect the cornea and conjunctiva while supporting clear vision.

Lipid layer of tear film: The outer tear-film layer, rich in lipids secreted chiefly by the meibomian glands. It slows evaporation and helps stabilize the exposed tear surface.
Dry eye disease: A multifactorial disorder in which tear-film homeostasis is lost, causing ocular symptoms and surface damage. Tear-film instability and deficiency are central features of this disorder.
Cornea: The transparent front surface of the eye that helps focus incoming light. The tear film coats the cornea and smooths its optical surface.
Visual acuity: The ability to resolve fine spatial detail. An uneven tear surface can blur the first refracting interface of the eye.
Aqueous layer of tear film: The watery tear-film layer containing water, salts, proteins, and antimicrobial substances. It supplies much of the film’s volume and carries dissolved protective molecules.
Tear breakup time: A clinical measure of the interval between a blink and the first detectable break in the tear film. It estimates how well the film remains intact between blinks.
Conjunctiva: A thin membrane covering the front of the eye and lining the inside of the eyelids. Its surface cells and glands contribute to the tear-film environment.
Corneal epithelium: The outermost cellular layer of the cornea. The tear film protects this exposed tissue from drying and mechanical stress.
Mucin layer of tear film: The mucin-rich inner region that helps tears wet and adhere to the ocular surface. Its surface chemistry lets the watery layer spread across the corneal epithelium.
Schirmer test: A test that estimates tear production by measuring wetting of filter paper placed at the lower eyelid. It assesses aqueous tear supply, one contributor to tear-film volume.

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