10:56am

Tue November 26, 2013
Krulwich Wonders...

Born Wet, Human Babies Are 75 Percent Water. Then Comes Drying

Look at this baby.

Lovely, no? Now think of this baby abstractly — as a sack of hundreds of millions of atoms. Here's the atomic formula for a new human being, arranged by elements, according to scientist Neil Shubin.

Notice that the two most plentiful atoms are H (hydrogen) and O (oxygen) which shouldn't be a big surprise, since 2 H's and an O make water, and we humans are very moist, especially when we're born.

It turns out, a brand new human baby is 75 percent water.

We're born as wet as a fresh potato. Tomatoes are wetter (93.5 percent water). Apples, too, but only slightly (80 percent). Check out this fruit vs. baby comparison.

OK, we aren't as wet as watermelons (who'd want to be?), but still, we begin our lives as noisy dewdrops that will one day learn to crawl, then walk. As science writer Loren Eiseley once put it, people "are a way that water has of going about, beyond the reach of rivers."

Aging = Drying

But then, with every step we take, we begin to dry. The longer we live, the drier we get. One year after birth, a human baby is only 65 percent water – a ten percent drop, says the U.S. Geological Survey.

Babies are wetter than children. By the time we're adults, the USGS says, adult men are about 60 percent water, adult women 55 percent. Elderly people are roughly half water.

There are variations, of course. The more buff you are (muscle tissue stores more water) the wetter you are. Because women generally have more fat cells, they tend to be a bit drier. Fat cells aren't as moist. The water that lubricates your joints, flushes your waste (I'm talking about pee), assists seminal reproduction, and absorbs shocks to your bones — as you age, the moisturizer in you slowly dwindles.

And the odd thing is, our wet parts aren't where you'd think. I figured if some giant fist were to plunge out of the sky and squeeze a human like a sponge, the wettest bit would be our blood. That's wrong.

High Tide

Our brains, lungs, heart, liver and kidney contain the wettest tissue — between 65 and 85 percent water. Bones, of course, are dryish, (but still 31 percent water.) There's also water between cells, but only a small fraction of our H2O is blood. Most of the water inside us is stored, not in our veins, but in our 100 trillion teeny cells. We are an assemblage of water packets, slightly salty, like the sea we came from. As Loren Eiseley put it, we're a "concentration" of water, "that indescribable and liquid brew which is compounded in varying proportions of salt and sun and time," and on the day we're born, we're at high tide. After that, very quietly, the sea within us ebbs and ebbs, and as it goes ... so do we.


Paleontologist Neil Shubin writes about water and the human body in his book The Universe Within: the Deep History of the Human Body. Loren Eiseley wrote about water in his classic, The Immense Journey.

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