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Tear fluid
Last reviewed: 20.11.2021
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The lacrimal fluid is clear or slightly opalescent, with a slightly alkaline reaction and an average relative density of 1.008. The lacrimal fluid has the following chemical composition: 97.8% water, the rest being protein, urea, sugar, sodium, potassium, chlorine, epithelial cells, mucus, fat. The composition of the tear also includes lysozyme, which has a bactericidal effect.
During wakefulness of the person for 16 h additional lacrimal glands give off 0.5-1 ml of a tear, i.e., as much as is required for moistening and cleaning the surface of the eye; orbital and secular parts of the gland are included in the work only with eye irritation, nasal cavity, with crying, etc. The secretory function of the lacrimal gland has the ability to rapidly and intensively intensify under certain circumstances, for example, in the wind, when a foreign body enters the cornea, in diseases cornea, etc. With strong crying, up to 2 teaspoons of tears can stand out.
Secretory nerve fibers enter the lacrimal gland in the lacrimal nerve, which is joined only in the orbit. The lacrimal secretory fibers from the variolium bridge are part of the lacrimal nerve, which are joined only in the orbit. The lacrimal secretory fibers from the variolium bridge are part of the facial nerve, and then go in the second branch of the trigeminal nerve.
At the heart of normal lacrimation are the following factors:
- capillary suction of fluid in lacrimal points and tear ducts;
- contraction and relaxation of the circular muscle of the eye and the muscle of Horner, creating a negative capillary pressure in the tear duct;
- presence of folds of the mucous membrane of the tear ducts, playing the role of hydraulic valves.
Tears secreted by the major and additional lacrimal glands pass through the surface of the eye. The amount of the aqueous component of the tear film during evaporation decreases. This is due to the size of the eye gap, the frequency of blinking movements, the surrounding temperature and humidity. The remaining lacrimal fluid is drained as follows:
- Passage of tears passes along the upper and lower edges of the eyelids, and through tear points, tears enter the upper and lower tubules by means of a capillary and suction mechanism. Approximately 70% of the tears are drained through the lower tubule, the rest is through the upper canal.
- With each blinking motion, the orcular muscles compress the ampoule, cut the horizontal tubules and move the teardrop medial. Simultaneously, the teardrop of the orbicular muscles attached to the fascia of the lacrimal sac, contracting, widens it, thus creating a negative pressure that sucks the lacrimal fluid from the tubules into the lacrimal sac.
- When the eyes open, the muscles relax, the bag falls off and a positive pressure is created, which pushes the tear fluid through the nasolacrimal canal into the nose. In this process, gravity also plays a role. The lacrimal point is displaced laterally, the tubules are lengthened and re-filled with a tear.