It is a mismatch between tear production and drainage. Either the drainage is excessive or the production is deficient. Usually the blame lies on inadequate production.
Tears are composed of three components – Lipid or oils, Aqueous or water and Mucin. Oil normally floats on water. Mucin layer helps keep the oily layer mixed with watery layer. There are also minerals, lysozymes and antibodies. Minerals are important for the nutrition of the cornea. Lysozymes and antibodies are the natural defense mechanism. The proportion and interaction of these three components is as important as the total volume.
Each layer of tear is produced by a different part of the eye. The water is secreted by the main lacrimal and accessory lacrimal glands. The former is located behind the outer part of the upper lid. The helper lacrimal glands are smaller and are in the lids. The meibomian glands produce the oily layer. They are present near the root of eyelashes and they pour their oil out from openings just behind the eye lashes. Numerous small goblet cells are embedded on the membrane lining the inner surface of the lids adding to the tear film.
The tears are spread across the front surface of the eye by the blinking action of the lids. The eyelids blink around 16 times per minute. The upper lid acts as a windshield wiper. With each blink it mixes the tears and coats the cornea with this thin protective layer. In a few seconds this film breaks up, to be restored with the next blink.