The film of tears on the eye is a complex system, consisting of three layers—oil, water and mucus. The water layer is the thickest and contains electrolytes (such as sodium and potassium), proteins (mostly enzymes), glucose and other substances. Various fats in the outer oil layer slow evaporation. But the specific composition of tears may change day to day and depend on your general health and other conditions.
Tears are produced mostly by the lacrimal glands located just above the outer corner of each eye; blinking spreads the tears across the eye surface. Tears drain into the tiny openings in the eyelids, called puncta (one on the inside corner of each lid), and then through ducts to the nasal cavity, where you either swallow them or they become part of nasal fluid—which is why you also get stuffy when you cry. If production of tears outpaces drainage, the excess will spill from the lower eyelids, as when you cry.
Besides lubricating the eye and flushing away debris, tears serve the same purpose in the eye that blood does elsewhere in the body, delivering oxygen and nutrients and removing waste products. Tears also improve the image that forms on the retina by smoothing any irregularities on the surface of the eyeball—that’s why people with dry eye may have blurry vision. Tears are a defense mechanism as well: their antibodies, enzymes and other immune components help protect eyes against microorganisms.
There are three types of tears. Continuous (or basal) tears are produced for basic eye function, such as lubrication. Reflex (or irritant) tears occur when the eye is exposed to excessive light, cold, wind, a foreign body or irritating gas (as from cut onions). Psychogenic tears are shed for emotional reasons.
Crying is a way to communicate needs and elicit attention from others, at least in early life, and may be a means of releasing emotional tension. According to research from the 1980s, psychogenic tears differ in composition from other tears, and it’s theorized that crying may be a way to rid the body of stress-related chemicals. Boys and girls cry in equal frequency; women reportedly cry more than men. But how people express emotions, in general, depends largely on cultural and psychological factors.
If tear production declines or tears evaporate too quickly, the result is dry eye. Inflammation may play a role. Besides causing burning or grittiness, dry eye syndrome can cause blurry vision, excessive tearing and potentially eye damage. Women, especially at menopause, and older people are at higher risk. Other contributors: environment (low humidity, wind, smoke, dust), certain medications (such as diuretics, antidepressants, and antihistamines), some diseases (including Parkinson’s and diabetes), smoking and LASIK surgery. If artificial tears don’t help, see your doctor.