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

Tuesday, November 26, 2013 - 10:56 AM

Look at this baby.


Robert Krulwich/NPR

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.

Human Chemistry

Robert Krulwich/NPR

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

Baby with water

Robert Krulwich/NPR

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.

Water content

Robert Krulwich/NPR

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.

Older people

Robert Krulwich/NPR

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.


Robert Krulwich/NPR

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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Comments [4]

Shoju from Earth

Are you saying that Bones was wrong when he announced the tricorder analysis of those little piles of dust on the abandoned space ship to be all that's left of a human when the water is removed? My recollection is that he said humans were something like 93% water. They were pretty small piles of dust.

Dec. 10 2013 06:59 PM
Patty Pearson from Tucson AZ

It's no surprise to me that babies are 75% water. Water oozes out of them constantly in the form of drool, tears, and diaper contents. Babies are so soft and squeezable - I can't think of any fruit quite that soft yet firm.
Great show!

Dec. 09 2013 11:09 AM
Tony Hartshorn from Bozeman, Montana

Fantastic; I'd never thought of our "fraction water" as a function of age.

I use your (Shubin's) "human recipe" in my Soils class--to introduce the concept of stoichiometry.

James Elser (Arizona State) has a wonderful series of 3 slides, geared to Homer Simpson's makeup. A lot of ecologists get excited about what is called the Redfield ratio (the ratio of N atoms to P atoms, for example, and that Elser boils down to--for Homer--84 atoms of carbon for every 6 atoms of nitrogen for every single atom of phosphorus). Elser not only highlights how not *every* part of a human has this ratio (our teeth are essentially rocks without the silicon), but pushes the value (which I can vouch for as a teacher) for an "extended Redfield ratio"--your recipe.

More info here, including Elser's 3 slides, but skip to 38:30--and your recipe shows up in slightly different form at 40:25.

Dec. 08 2013 02:37 PM
Kent Neff from Sisters, Oregon

Great programs. Thanks.

Dec. 04 2013 11:01 PM

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