Tears

Tears are the liquid product of a process of lacrimation to clean and lubricate the eyes. In human, the tear film coating the eye, known as the pre-corneal film, has three distinct layers, from the most outer surface: 1. The lipid layer contains oils secreted by the meibomian glands (or tarsal glands). The outer-most layer of the tear film coats the aqueous layer to provide a hydrophobic barrier that retards evaporation and prevents tears spilling onto the cheek

2. The aqueous layer contains water and other substances such as proteins (e.g. tear lipocalin, lactoferrin, lysozyme and lacritin) secreted by the lacrimal gland. The aqueous layer serves to promote spreading of the tear film, control of infectious agents and osmotic regulation

3. The mucous layer contains mucin secreted by the conjunctival goblet cells. The inner-most layer of the tear film, it coats the cornea to provide a hydrophilic layer that allows for even distribution of the tear film, as well as mucus covering of the cornea.

Drainage of tear film The tear gland of the eye of humans and other vertebrates. It lies beneath the upper eyelid. The lachrymal gland is controlled by the autonomic nervous system. It continuously secretes small amounts of sterile, slightly antiseptic, tears which keep the cornea moist. Tears drain away through two small openings (puncta) at the inner corner of the eye into the lacrimal caniculi. From there the tears pass into the nasal cavity via the lacrimal sac and nasolacrimal duct, or tear duct.

Types of Tears 1. Basal tears: In healthy mammalian 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, lacritin, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection as a part of the immune system. It is a typical body fluid with a salt content similar to blood plasma. Usually, in a 24-hour period, 0.75 to 1.1 grams of tears are secreted, this rate slows with age.

2. Reflex tears: The second type of tears results from irritation of the eye by foreign particles, or from the presence of irritant substances such as onion vapors, tear gas or pepper spray in the eye's environment. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting. These reflex tears attempt to wash out irritants that may have come into contact with the eye.

3. Crying or weeping (psychic tears): The third category, generally referred to as crying or weeping, is increased lacrymation due to strong emotional stress, suffering or physical pain.

Emotional tears contain more of the protein-based hormones prolactin, adrenocorticotropic hormone, and leucine enkephalin (a natural painkiller) than basal or reflex tears

Composition of human tears: • Water • Electrolytes, the electrolytes are principally Na+, K+, Cl-, and HCO-, with lower levels of Mg2+ and Ca2+ •Proteins – secreted from the lacrimal gland, (lysozyme, lactoferrin, lipocalin, and sIgA). • Lipids - from the meibomian glands  •Mucins

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). 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.

Function of tears: 1) Tears act as both a delivery and an excretory route for nutrients and metabolic products of the corneal epithelium. 2) The presence of the tear film improves the quality of the retinal image by smoothing out irregularities of the cellular surfaces. 3) It acts as a lubricant preventing eye dryness which is provided by the tear film coating the cornea. 4) It plays a protective role against infection carried out mainly by its protein content of lysozymal enzymes, lactoferrin, secretory IgA

Diseases and disorders "Crocodile tears syndrome” : Shedding tears while eating

Keratoconjunctivitis , more commonly known as dry eye, is a very common disorder of the tear film

Dry eye - symptoms: Pain Light sensitivity A gritty sensation A feeling of a foreign body or sand in the eye Itching Redness Blurring of vision

Tears of grief

TEARS FROM ONION