Tears are clear liquids that are secreted by tear glands (lacrimal glands), located at the eyes of all land mammals including humans. Tear reflexes are said to be in the control of the sensory pathways Trigeminal nerve (cranial nerve V). Studies have shown that when the trigeminal nerve is cut, the reflex tears cease to be secreted but not the emotional tears, as the emotional tears are triggered by the limbic system that is responsible for emotions.
Chemical composition of tears:
Tears are made up of three layers – lipid, aqueous and mucous. In general, tears are composed of water, salts, antibodies and lysozymes (enzymes that kill bacteria). Also, different types of tears are different in composition as they are triggered by different centers of the brain.
In general, most of the fluids of the body have salt in them. The salt content in the tears is almost the same as the salt content in the blood plasma.
Types of tears:
There are three basic types of tears, based on their composition and function.
This type of tears is secreted in small quantities to keep the cornea continuously wet. They lubricate the eyes and keep out the dust particles. Tear-fluid is made of water, lipids mucin, immunoglobulins, glucose, urea, sodium, potassium and a few enzymes. Lysozyme is an enzyme that helps fight against bacterial infection in the eyes. Further, basal tears also contain antioxidants like Ascorbate, Cysteine, Urate, Tyrosine and Glutathione.
They are formed in the eye in response to an irritation of the eye by foreign particles. They are also released due to irritants like onion vapors, perfumes, tear gas or pepper spray. Stimulation of nasal cavity with substances like pepper or the tongue with hot or peppery substances can release tears. The reflex tears are secreted in an attempt to wash out the irritants.
They are secreted as a result of crying or weeping and other emotions like pleasure, anger, mourning or physical pain. Emotional tears are usually accompanied by reddening of face or sobbing, whether the person is happy or sad. Tears brought about by emotions usually are comprised of protein-based hormones prolactin, adrenocorticotropin (stress hormone) and leu-enkephalin (a natural pain killer). Emotional tears are usually triggered by compassion, empathy, sentimental or moral feelings.
People have generally experienced the tears coming out of the nose. This is because, the lacrimal duct is a drainage system that carries tears from the eyes to the nose and also down the throat. This is the reason people experience a distinct taste when they put eye drops in their eyes.
Lacrimal gland infections, dry eye, pseudobulbar effect, crocodile tears syndrome and other disorders of the eye are related to lacrimal glands.