HOW COME? Our tears come with a dash of salt
Author Isak Dineson wrote that the "cure for anything is salt water -- tears, sweat, or the sea."
It's no accident that tears taste like the sweat that runs down our face on a hot day. Our flowing blood is salty, too. In fact, about 55 percent of the salt in the body is dissolved in fluids outside our cells, like blood plasma. Another 40 percent of the body's salt is inside our bones. The remainder is found in other cells and organs.
The salty total? According to researchers, each human adult body contains about 8 ounces, or half a pound, of salt.
Sodium and chloride, the two elements of a salt molecule, have a wide range of jobs. Sodium, chloride and potassium are our body's major electrolytes. Electrolytes dissolve easily in a solution and can conduct electrical currents. Positively charged sodium ions are crucial for transmitting nerve impulses, such as those that keep the heart beating. Sodium also helps transport nutrients from cell to cell, regulate the body's water balance and maintain our blood pressure.
All day long our eyes are wetted by a film of salty tears each time we blink (some 900 times an hour when we're awake). These "basal" tears wash away dust and bacteria and keep eye surfaces moist. If irritating pollen particles, windblown dirt or stinging onion fumes intrude, a flood of "reflex" tears streams to the rescue.
Tears flow from the almond-sized lacrimal glands at the upper outer corner of each eye. Leftover tears that don't evaporate (or run down our cheeks) flow away through handy drains in the eye's inner corner.
Tears are about 98 percent water. In addition to sodium, tears contain other minerals, like potassium, manganese, calcium and zinc. There's also glucose, and enzymes and antibodies that protect eyes from infection. And there are oils and mucus, which lubricate eyes, helping retain moisture.
Also in the tear ingredient list are hormones like corticotropin, made by the brain's pituitary gland when we're stressed. Scientists have also found at least one natural painkiller, leucine enkephalin. Both seem to show up in greater amounts in "crying" tears, the kind that spill out of our eyes at sad movies or after a bruising fall.
The actual concentration of salt in our tears, as in our blood plasma, is a little less than 1 percent. By contrast, seawater is about 3.5 percent to 4 percent salt by weight. Which is why a mouthful of ocean tastes overwhelmingly salty. But to our vigilant taste buds, tears register as pretty salty, too.