lacrimal fluid


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Related to lacrimal fluid: lachrymation, tears, tares

lacrimal fluid

a watery physiologic saline, with a plasmalike consistency, but also contains the bacteriocidal enzyme lysozyme; it moistens the conjunctiva and cornea, providing nutrients and dissolved O2 to the cornea.
Synonym(s): tears

film, precorneal

The field covering the anterior surface of the cornea which consists of lacrimal fluid and of the secretion of the meibomian and conjunctival glands. Its total thickness was thought to be about 9 μm but recent investigations have questioned that value and point to a much larger figure. It is composed of three layers: (1) The deepest and densest is the mucin layer (or mucous layer) which derives from the conjunctival goblet cells, as well as some secretion from the lacrimal gland. (2) The watery lacrimal fluid is the middle layer, called the lacrimal (or aqueous layer). It is secreted by the lacrimal gland and the accessory glands of Krause and Wolfring. It forms the bulk of the film and contains most of the bactericidal lysosyme and other proteins, inorganic salts, sugars, amino acids, urea, etc. (3) The oily layer (or lipid layer) is the most superficial and is derived principally from the meibomian glands in the lids as well as some secretion from the glands of Zeis. It greatly slows the evaporation of the watery layer and may provide a lubrication effect between lid and cornea (Fig. F6). Note: Some authors have suggested that the precorneal film is made up of only two layers; an innermost aqueous and mucin gel layer and an outer lipid layer. Syn. lacrimal layer; preocular tear film; tear film; tear layer. See hyperlacrimation; mucin; tear secretion; Tearscope; break-up time test.
Fig. F6 Diagram of the three layers of the precorneal film attached to the squamous epithelial cellsenlarge picture
Fig. F6 Diagram of the three layers of the precorneal film attached to the squamous epithelial cells

tears 

The clear watery fluid secreted by the lacrimal gland which, together with the secretions from the meibomian glands, the goblet cells, the gland of Zeis, as well as the accessory lacrimal glands of Krause and Wolfring, helps to maintain the conjunctiva and cornea moist and healthy. Periodic involuntary blinking spreads the tears over the cornea and conjunctiva and causes a pumping action of the lacrimal drainage system, through the lacrimal puncta into the nasolacrimal duct. Approximately 25% of the tears is lost by evaporation, the remaining 75% is pumped into the nasal cavity and over 60% of the tear volume is drained through the lower canaliculus. Tears contain water (98.2%), salts, lipids (e.g. wax esters, sterol esters, hydrocarbons, polar lipids, triglycerides and free fatty acids), proteins (e.g. lysozyme, lactoferrin, albumin, IgA, IgE, IgG, complement proteins C3, C4, C5 and C9, and beta-lysin), magnesium, potassium, sodium, calcium, chloride, bicarbonate, urea, ammonia, nitrogen, citric acid, ascorbic acid, and mucin. Tears have a pH varying between 7.3 and 7.7 (shifting to a slightly less alkaline value when the eye is closed) and the quantity secreted per hour is between 30 and 120 ml. Syn. lacrimal fluid. See alacrima; blink; epiphora; precorneal film; hyperlacrimation; keratoconjunctivitis sicca; lacrimal apparatus; lacrimal lake; lysozyme; mucin; fluorescein staining; break-up time test; non-invasive break-up time test; phenol red cotton thread test; Schirmer's test.
artificial tears Any eye drop solution that can replace tears by approximating its consistency in terms of viscosity and tonicity and may contain many of the substances found in tears. The most common agents found in artificial tears are cellulose derivatives, such as methylcellulose, hydroxymethylcellulose, hydroxypropylcellulose, hypromellose (hydroxypropylmethylcellulose), hydroxyethylcellulose, and polyvinyl alcohol, povidone (polyvinyl pyrrolidine), sodium hyaluronate and sodium chloride, which have low viscosity. Carbomer (polyacrylic acid), carmellose (carbomethylcellulose), liquid paraffin and yellow soft paraffin have medium to high viscosity. Acetylcysteine, a mucolytic agent prepared with hypromellose is used when the tear deficiency is associated with threads and filaments of mucus to soften and make the mucus more fluid, as well as shrinking the mucous membranes (astringent). Syn. ocular lubricant. See alacrima; recurrent corneal erosion; ectropion; dry eye; hypromellose; keratoconjunctivitis sicca; methylcellulose; Bell's palsy; wetting solution; xerophthalmia.
crocodile tears Copious secretion of tears occurring during eating in cases of abnormal regeneration of the seventh cranial nerve after recovery from Bell's palsy. Syn. paradoxic lacrimation.

lac·ri·mal flu·id

(lak'ri-măl flū'id)
Watery physiologic saline that also contains the bacteriocidal enzyme lysozyme; it moistens the conjunctiva and cornea.
Synonym(s): tears.