How Come?: Why do eyes turn red and swell when we cry?

You may not notice it, but even as you read this, your eyes are watering. Conveniently located above the outer corner of each eye, almond-shaped lacrimal glands are churning out cleansing tears. Each time you blink, your eyelids spread those tears over the eye's surface. Like windshield wiper fluid that never needs refilling, cleansing tears wash away dust, debris and microbes from the eye's surface.

The oceanlike saltiness of cleansing (or lubricating) tears also helps the salt concentration inside and outside the eye stay balanced, keeping eyes moist as they are blasted with drying air. Besides salty H2O, tears contain glucose (sugar), lubricating oils and mucus, minerals like potassium and manganese, and enzymes and antibodies that protect eyes from infection. Tears also contain traces of hormones.

When our eyes become "red," they may look suddenly bloodshot. But blood is always there. Our eyes are fed by many tiny blood vessels, normally almost invisible in the white part of the eye (a covering called the sclera). But when these small vessels swell, engorging with blood and becoming leaky, we see them as red.

Crying can make eyes red in several ways. Lacrimal glands that must churn out a flood of emotional tears suddenly need a bigger supply of watery blood. So blood vessels around the eyes obligingly dilate (enlarge), prompted by the nervous system. As we cry harder, blood vessels on the surface of the eyes widen, too, and spring tiny leaks. The result: red-rimmed, bloodshot eyes.

Why do crying eyes swell? The tears we cry when we're upset also have less saline (salt) than everyday lubricating tears. So our eyes are suddenly awash in tears that are a mismatch for the higher-salt fluids of the eye and the rest of the body. The result: Salt in the lubricating fluid bathing our eyes attracts water in the emotional tears. Tissues in and around our eyes plump with water. Presto: puffy, swollen eyes.

Crying hard strains muscles of the eyes and face, adding to the swelling. Eyes can become even more irritated if we rub them while we're crying, triggering extra redness and swelling of delicate, thin skin. A cold compress can make eyes feel and look better, shrinking dilated vessels as well as bloated tissues in and around the eye. But it's best to avoid those "get the red out" drops, which use chemicals to force blood vessels to shrink. The drops can cause a redness rebound, leaving eyes looking even more bloodshot hours later.

Sometimes, prolonged crying (or sneezing, coughing hard, or throwing up) can cause an alarming-looking "blood spot" to appear in the eye. Also known as a subconjunctival hemorrhage, the red splotch is caused by tiny blood vessels bursting under strain. While the vessels will quickly repair themselves, it can take a week or two before the spilled blood is absorbed by the body and the spot disappears completely.