When you take a sip of water it doesn’t just slake your thirst. It literally becomes you. The water that runs down your gullet will, within minutes and without processing of any kind, become some of the dominant fluid in your veins and your flesh. Most of your blood is simply tap water with cells, salts, and organic molecules floating in it. Some of the rubbery squishiness of your earlobe poured out of a bottle or a can just a short time ago. And much of the moisture in your eyes only recently fell from rainclouds. Your mouth is the portal through which water normally enters your body, but you are quite a leaky vessel. A hydrogen isotope study published in the British Journal of Sports Medicine reported that the sedentary men under examination consumed and lost about seven pints of body water per day, with four pints leaving through urine and two or three pints through sweat and breath moisture. Vigorous exercise can boost non-urine water losses to one or two pints per hour. Now let’s see what logic can do with those facts. Nearly two-thirds of your weight comes from water, and your body is an eddy in a stream of that common fluid. Surely the liquid that you slurp from a fountain is not alive, and you don’t consider it murder to stomp on a puddle of water. Therefore most of you is not alive at all, nor is it even permanent or unique enough to merit a personal name.
Long, Beautiful Hair
Next let’s consider your hair. It is a slow-motion shower of lifeless protein that sprays out of your head at roughly half an inch per month or six inches per year. Each filament is a tangle of carbon and oxygen atoms that are outnumbered two to one by hydrogen and sprinkled with nitrogen along with a dash of sulfur. The atoms in the roots are derived from meals that you ate within the last few days, along with some drinks, metabolic water, and your own recycled cells. Your fingernails are also full of keratin, and they roll out of your fingertips at three to four millimeters per month, on average. Your toenails grow half as fast but they, too, release atoms from their leading edges as you cut or wear them away, and you also shed millions of microscopic keratinous skin flakes every day. If you could fast-forward a video of your hair, nail, and epidermal growth, you would seem to smolder with skin dust while jungles of protein poured from your hairy parts and curly peels of keratin shot from your fingertips and toes. Even at the usual slow pace, that’s a lot of atoms, all of which must be replaced if you are to resemble yourself for very long. One way of summarizing this unusual time-lapse view of yourself is that you are a walking fountain of carbonated water vapor, liquid water, and protein. Much of it trails off behind you in an invisible mist of exhalations and exfoliations, ending up in the dust bunnies beneath your bed or, should you misbehave badly enough, in the nostrils of a bloodhound. If you ever do become a fugitive of that sort, then perhaps you might attempt a plea of atomic innocence should you be brought before a court of law. “It wasn’t I, Your Honor,” you can truthfully say (and perhaps with excessive grammatical propriety), thanks to the rapid turnover of matter in your body.
The Sum of Your Parts
There is a lot more to your body now than there was when you could still fit inside your mother’s womb, and that fact alone makes it obvious that most of your body is younger than you are. But imagining yourself as a temporary collection of cells can also make the transitory nature of your body more apparent, too. A study by the Italian researcher Eva Bianconi and her colleagues recently put the average number of cells in an adult human body at thirty-seven trillion. They vary wildly in shape and size, with diameters ranging from eight microns for a red blood cell and about twenty-two microns for a liver cell to roughly one hundred microns for a mature egg cell (five hundred microns would span a grain of salt). Some of them may last for a few days or weeks before being recycled and replaced, while others may last a lifetime. How can you tell which is which? One way to estimate the turnover rates of human cells is to measure the amount of carbon-14 in them. During the Cold War, atmospheric testing of thermonuclear weapons turned atmospheric nitrogen atoms into radioactive carbon-14 that still contaminates air and oceans today. Moving into plants as carbon dioxide, the unstable atoms of bomb carbon have worked their way through food chains and lodged in the bodies of everyone on Earth, including you. Since 1963 when above-ground testing was banned, radiocarbon concentrations have declined as carbon-rich organic matter has been buried in ocean sediments, and the change is reflected in our bodies. If there is any bright side to thermonuclear pollution, it may be this shifting concentration of bomb carbon that provides a global isotopic tracer for determining the ages of our cells. Olaf Bergmann, a cell biologist at the Karolinska Institute in Stockholm, recently coauthored a paper in Science that used this technique to document the growth of new cells in heart muscle. His approach resolved a long-standing conflict between experts who believed that the heart is renewed as many as four times during a lifetime and those who believed that we die with essentially the same heart we were born with. By measuring the radiocarbon contents of cardiomyocyte cells from which heart muscle is made, Bergmann’s team found that the cardiac tissues of relatively young people who spent their entire lives amid the earth’s contaminated carbon reservoir were far more radioactive overall than those of older people who were born before the nuclear tests began. From these and similar findings, it appears that you do continue to form some new heart tissue throughout your life, and that you do so at different rates as you age. According to Bergmann’s calculations, you replace about 1 percent of your cardiac muscle cells per year at age twenty and half as many at age seventy-five. Nevertheless, you still keep most such cells with you throughout your adult life. Similar radiocarbon tracer studies suggest that the average replacement rate of most cells in your body is between seven and ten years, but some cells fall well outside that range. Your heart, for example, is full of connective tissue, blood vessels, and other structures that are replaced more often than your cardiomyocytes. A median annual turnover rate of 18 percent for those components suggests that most of your heart is less than five years old. In a follow-up paper in Science, Bergmann and the biologist Jonas Frisén reported that nerve cells within the olfactory bulb and hippocampus of a human brain are continuously regenerated. This means that when a whiff of something sparks a memory, be it a smoky campfire or a familiar perfume, the neurons that originally encoded those sensations may no longer be with you, and the memories may now be preserved by cells that never experienced them. Most of your other brain cells date back to your infancy, but tracer studies now show that some fresh neurons can also appear within your cerebral cortex, perhaps registering new experiences from day to day.
Cellular Churn
Cells that line your digestive tract are replaced every few days, which is not surprising considering the abuse they take from stomach acids, bile, and erosion by the passage of food and waste. Work by the physiologist Bernd Lindemann posits a lifespan of about ten days for the taste cells in your mouth, and the dermatologist Gerald Weinstein and his colleagues estimate a mean turnover time of thirty-nine days for skin cells, which spend only a couple of weeks in your outermost layers before flaking off by the hundreds of millions. This continuous shedding gives you a new wrapper of skin once or twice a month and a steady supply of house dust to keep up with. The lives of your red blood cells are rather “nasty brutish and short.” After tumbling through hundreds of miles of aortic rapids and hard-to-squeeze-through capillaries, and after repeatedly swelling and shrinking in thousands of transits through the osmotic jungles of your kidneys, most of them wear out within four months or so and must be replaced by progenitor cells in your spleen and bone marrow. And according to the science journalist Nicholas Wade, the replacement times of three hundred to five hundred days that liver cells enjoy can grow you a whole new liver every year or two. The Swedish biologist Kirsty Spalding and others have found that your fat-storage cells persist for about a decade, which is good news for people who struggle to lose weight. It was long thought that starvation merely deflates fat cells rather than killing them off, leaving them to fill up again like grocery bags when a dieter tires of feeling hungry. But if you can stick to a healthy regimen for long enough, it seems that you can help to stabilize your weight by outliving some of your fat cells. Your bones and muscles are constantly remodeled. About 3 percent of the dense outermost layers of your skeleton and up to a quarter of the porous bone in the knobby parts of your limb joints are recycled every year, and experts calculate an average life cycle of a decade or so for your skeleton as a whole. The muscle cells between your ribs live for about fifteen years, according to Nicholas Wade, and the collagen cores of your tendons are essentially permanent once they finish developing during your late teens. Recent isotopic analyses by researchers in Denmark and Sweden show that the oldest easily identifiable structures in your body are the crystalline lens proteins of your eyes and the enamel of your teeth. If you carry healthy ovaries, then you may also carry thousands to millions of microscopic oocytes that formed while you were still in your mother’s womb, making the initial cells of your potential future children nearly as old as you are. And as for tattoos, although younger than you they are permanent because the ink is not cellular and therefore not recycled; it is more like the persistent pebbles in a cornfield than the ephemeral crops of skin. In sum, your tissues are a mishmash of newborn, persistent, and dying cells, most of which are relatively new. Therefore, whatever you have supposedly done to deserve credit or blame, it really wasn’t you after all, was it? In this mad worldview the most likely culprits would be your eyes, your teeth, some brain matter, and perhaps the seeds of your unborn children.
Images by markos86 / Shutterstock
Excerpted from YOUR ATOMIC SELF: The Invisible Elements That Connect You to Everything Else in the Universe by Curt Stager. Copyright © 2014 by the author and reprinted by permission of Thomas Dunne Books.
