Tears• It is that fluid produced by the lacrimal gland that normally moistens the surface of

the cornea.• The lacrimal glands are paired almond-shaped glands one for each eye, that

secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit. The lacriminal gland produces tears and then flow into canals that lead to the lacriminal sac. From this sac, the tears drain through a passage into the nose.

a = lacrimal glandb = superior lacrimal punctumc = superior lacrimal canald = lacrimal sace = inferior lacrimal punctumf = inferior lacrimal canalg = nasolacrimal canal

Tear gland

• The lacrimal gland lies within the orbit on the outer portion of the upper eye. The gland continually secretes tears which moisten, lubricate, and protect the surface of the eye. Excess tears drain into small ducts which empty into the nasal cavity

TearsComposition of human tears;-Water,- Electrolytes, the electrolytes are principally Na+, K+, Cl-, and HCO-, with lower levels of

Mg2+ and Ca2+

- Proteins such as ;lysozyme(an enzyme capable of destroying the cell walls of certain bacteria and thereby acting as a mild antiseptic). Lactoferrinan(an iron-binding protein , by combining with iron, lactoferrin prevents microorganisms from combining with and using iron for their growth) , lipocalin, secretory IgA ,albumin, IgG (leakage from conjunctiva)

- Lipids which is thought to promote tear film stability by reducing evaporation from the open eye .

- Mucins - Defensins, collectins, other small molecules.

- The levels of the major tear proteins are known to decline with age at the same time the volume of tears also tends to decline leading to a dryer eye parallel to aging.

Tears• In humans, the tear film coating the eye, known as the precorneal film, has three

distinct layers, from the most outer surface:• 1- The lipid layer (0.11 µm thick), produced by the Meibomian glands, it coats the

aqueous layer, providing a hydrophobic barrier that reduces the evaporation of tears , and prevents tears spilling onto the cheek.

• 2- The aqueous layer, (7.0 µm thick), which is secreted by the lacrimal gland which Promotes spreading of the tear film, control of infectious agents ,and promotes

osmotic regulation.- 3- The Mucous layer , (7.0 µm thick) produced by the conjunctival goblet cells

made up mainly of mucin , coating the cornea with a hydrophilic layer which allows for even distribution of the tear film.

Type of tears.• There are three very basic types of tears:• 1-Basal Tears; In healthy mammilian eyes, the cornea is continually kept wet and

nourished by basal tears. They lubricate the eye, and help to keep it clear of dust. Tear fluid contains water, mucin, lipids,lysozyme, lactoferrin, lipocalin,lactirin, immunoglobulins, glucose , urea, sodium , and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection .

• 2- Reflex tears; They results from irritation of the eye by foreign particles, or from the presence of irritant substances such as onion vapors, tear gas etc… present in the in the eye's environment, including the cornea, conjunctiva, or nasal mucosa. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting, coughing or yawning.These reflex tears attempt to wash out irritants that may have come into contact with the eye.

• 3- Crying tears; Which is caused by increased lacrimation induced by strong emotional stress, or physical pain. Tears brought about by emotions have a different chemical make up than those for lubrication; emotional tears contain more of the protein-based hormonesprolactin, adrenocorticotropic hormone, and leucine enkephaline (a natural painkiller) than basal or reflex tears.

Function of the tears• Tears act as both a delivery and an excretory route for nutrients and metabolic

products of the corneal epithelium .

• The presence of the tear film improves the quality of the retinal image by smoothing out irregularities of the cellular surfaces.

• It acts as a lubricant preventing eye dryness which is provided by the tear film coating the cornea.

• It plays a protective role against infection carried out mainly by its protein content of lysozymal enzymes, lactoferrin, lipocalin, secretory IgA .

Diseases related to tear formation

• Keratoconjuctivitis sicca, more commonly known as dry eye is characterized by inadequate tear film protection of the cornea because of either inadequate tear production or abnormal tear film constitution, which results in excessively fast evaporation or premature destruction of the tear film.

Diseases related to tear formation

• Leamy Eye" is a condition whereby there is excessive watering of one eye, seemingly for no apparent reason,

• Familial dysautonomia ;is a genetic condition which can be associated with a lack of overflow tears (alacrima) during emotional crying.