Lacrimal fluid

The lacrimal fluid is transparent 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 is protein, urea, sugar, sodium, potassium, chlorine, epithelial cells, mucus, fat. The tear also contains lysozyme, which has a bactericidal effect.

During a person's waking hours, the accessory lacrimal glands secrete 0.5-1 ml of tears per 16 hours, i.e. as much as is required to moisturize and cleanse the surface of the eye; the orbital and eyelid parts of the gland are activated only when the eye or nasal cavity is irritated, when crying, etc. The secretory function of the lacrimal glands has the ability to quickly and intensively increase under certain circumstances, such as in the wind, when a foreign body gets on the cornea, when there are diseases of the cornea, etc. With strong crying, up to 2 teaspoons of tears can be secreted.

Secretory nerve fibers enter the lacrimal gland as part of the lacrimal nerve, which they join only in the orbit. Lacrimal secretory fibers from the pons are part of the lacrimal nerve, which they join only in the orbit. Lacrimal secretory fibers from the pons are part of the facial nerve, and then go as part of the second branch of the trigeminal nerve.

Normal tear drainage is based on the following factors:

• capillary suction of fluid into the lacrimal puncta and lacrimal canals;
• contraction and relaxation of the orbicularis oculi muscle and Horner's muscle, creating negative capillary pressure in the lacrimal tube;
• the presence of folds in the mucous membrane of the lacrimal ducts, which act as hydraulic valves.

Tears secreted by the primary and accessory lacrimal glands flow across the surface of the eye. The amount of aqueous component of the tear film decreases with evaporation. This is related to the size of the palpebral fissure, blink rate, ambient temperature and humidity. The remaining tear fluid is drained as follows:

1. The tear passage runs along the upper and lower edges of the eyelids, and through the puncta, tears enter the upper and lower canaliculi by means of a capillary and suction mechanism. Approximately 70% of the tears drain through the lower canaliculi, the remainder through the upper.
2. With each blink, the orbicularis muscles compress the ampulla, contract the horizontal canaliculi, and move the lacrimal punctum medially. At the same time, the lacrimal part of the orbicularis muscles, attached to the fascia of the lacrimal sac, contracts, expanding it, thus creating a negative pressure that sucks the lacrimal fluid from the canaliculi into the lacrimal sac.
3. When the eyes open, the muscles relax, the sac collapses, and positive pressure is created, which pushes the tears through the nasolacrimal duct into the nose. Gravity also plays a role in this process. The lacrimal punctum shifts laterally, the canaliculi lengthen, and fill with tears again.