kyynelneste

Kyynelneste (tear fluid) is the clear liquid secreted by the lacrimal glands that coats the front surface of the eye, forming the tear film essential for ocular health. It is produced by the lacrimal glands and supplemented by secretions from conjunctival goblet cells; blinking distributes tears across the cornea and conjunctiva, and excess tears drain through the puncta into the nasolacrimal system.

The tear film consists of three layers with distinct roles. A superficial lipid layer, produced by the Meibomian glands, reduces evaporation. Beneath it lies an aqueous layer containing water, electrolytes, and proteins such as lysozyme, lactoferrin, and secretory IgA, along with nutrients. A mucin-rich layer from goblet cells helps the tear film spread and adhere to the ocular surface. Tear fluid also contains osmoprotectants and small amounts of glucose and lactate. Normal tear osmolarity is around 300 mOsm/L.

Functions of kyynelneste include lubrication of the ocular surface, maintenance of a smooth refractive interface for clear vision, delivery of nutrients and oxygen, removal of debris, and antimicrobial protection.

Clinical aspects: Disruption of tear production or evaporation can lead to dry eye disease (keratoconjunctivitis sicca) and tear film instability, causing irritation and fluctuating vision. Excess tearing, infections, allergies, and inflammatory conditions can also affect kyynelneste. Diagnostic tests include the Schirmer tear test and tear breakup time (TBUT); management may involve artificial tears, anti-inflammatory therapies, and addressing underlying causes.

See also: Lacrimal apparatus, Meibomian glands, Tear film, Conjunctiva.