5
Tears from the Earth
A man is an aqueous salty system in a medium in which there is but little water and most of that poor in salts.
—John Z. Young
The cure for anything is salt water: sweat, tears, or the sea.
—Karen Blixen (Isak Dinesen)
The entomologist Hans Bänziger is, to say the least, passionate about insects. He is lucky, too, in that he has been able to make a career following his passion in the forests of Southeast Asia. But Dr. Bänziger is not only lucky; he is renowned among his peers for going beyond the call of duty in his research by offering himself as bait.
In his landmark paper published in 1992 and titled “Remarkable New Cases of Moths Drinking Tears in Thailand,” the casual reader is given little warning of what is to come farther down in the dry scientific text. The opening sentence merely states that “10 cases of lachryphagous moths settling at human eyes were witnessed.” But when Hans Bänziger tells you that such behavior was seen, he means that it was observed from the closest possible viewpoint, giving new meaning to the term “eyewitness.”
Many insects land on larger creatures or objects in order to lick salty fluids from them. A species of sweat bee that was recently discovered in Brooklyn, New York (and appropriately named “L. gotham”), occasionally pesters city residents by landing on glistening arms, legs, and faces in hot weather. Colorful butterflies cluster like flower petals around the muddy rims of roadside puddles in order to lap salt and other substances. But eyes? Although tears certainly are salty, most of us consider our eyes to be no-go zones when it comes to insects.
Not, however, for one dedicated entomologist.
In an entry from November 1989, Bänziger described watching moths hover near some cattle in a forest clearing. It was just after sundown, and a half-moon was visible. One of the moths flitted over to him, landed on his wrist, and began to sweep its threadlike proboscis gently over his skin. Minutes later it landed on his bare leg where it continued to drink from the thin film of sweat. Then it moved to his cheek, and then to the lower edge of his right eye. “The perception I now felt was … comparable to that of a particularly edgy grain of sand being rubbed between eye and lid.”
At this point most of us would have swiped the pesky visitor away. Instead, Bänziger pointed a camera at his face and snapped a flash photo, which appears in the article as well as in this book.
A tear-drinking moth meets Hans Bänziger in Thailand. Photo, amazingly, by Hans Bänziger himself
The irritation was apparently coming from the moth’s tiny clawed feet as they scrabbled at his inner eyelid: Later investigations suggested that this was intended to stimulate the flow of more tears. In some species, the threadlike mouthparts are rough-edged enough to cause the requisite irritation, but this moth’s delicate proboscis could barely be felt as it swept back and forth over the smooth, curved surface of his eye.
“Unfortunately,” he reported with no apparent irony, “the flash scared the moth away…”
Nearly a year later Bänziger was again at work in the forest at night. Suddenly he noticed a dark object hovering in front of his face and felt something “like a fine straw” sliding between his lips. The moth then probed his nostrils, causing “almost unbearable tickling,” after which it rose higher to lightly sponge moisture from his eye.
This species was gentler in its approach, and Bänziger described the sensation as “surprisingly mild,” although he also admitted with what must have been no little understatement that having “one of the largest tear-drinkers hovering in front of the face with a 4-cm long proboscis aimed straight into the eye is somewhat trying.” The encounter ended abruptly when he lowered a collecting net over both his head and the moth.
More trying experiences occurred under a full moon in a pasture near a rural village. A moth that had been flying around several mules and horses “suddenly turned its attention to me.” This one followed him for a hundred yards before settling to drink from his left eye. “I closed the eye, pressing the lids tightly together, but the moth would not leave.” Another visitor on that same night was even more aggressive. “I felt pain due to clawing of the lid,” he reported. After two minutes of this, even Bänziger had had enough: “After having indulged in this tear-letting for the second time that night, I could not bear it any longer and caught the tormentor.”
It isn’t flattering to think of yourself as being a walking mud puddle to an insect, but your eyes and puddles do have much in common. Both contain water, and both are rich in biologically useful atoms that come from the rocks of the earth’s crust. In many ways your eyes are much like the mineral springs and salt licks that have attracted wildlife for millions of years, and you share a taste for salt with many other animals.
Of the five officially recognized tastes in the human mouth—sweet, bitter, pungent umami, sour, and salt—only one is aimed specifically at a single mineral element. The first three arise from relatively complex carbon compounds, and sourness arises from acidity produced by hydrogen ions. Only saltiness stems from an atom that comes to you from rocks and soils: sodium, which carries eleven protons and twelve neutrons in its nucleus and comprises half of the atoms in table salt. Sodium puts the tang in your sweat and blood, and it also helps you to think, move, and perceive the world around you. Without enough of it in your body fluids, most of your cells would swell up and die. With too much of it, they would shrivel into microscopic prunes. It makes nearly 98 percent of all liquid water on Earth undrinkable to you, but in doing so it also helps to determine what lives where on this ocean-dominated planet.
You have probably heard that most of us eat too much sodium in comparison to what our ancestors used to consume. This may or may not be true. Some historians point out that heavy salt use for food preservation dates back millennia, that salt mining was common in China four thousand years ago, and that Roman soldiers were once paid with salt, hence the term “salary.” In any case billions of dollars have been spent in developing and marketing foods that cater to our desire for salt, while medical professionals warn of associations between sodium and hypertension, heart disease, and other ailments. But it would also be a mistake to carry too little sodium inside you, for reasons that are both interesting and important to your health.
Technically speaking, sodium is only half of the atomic basis of saltiness. Normally a positively charged cation such as sodium travels in the company of a negatively charged anion of some sort, and in table salt and the brine in your tears the primary partner is chloride, an ionized version of the element chlorine, which is also the active ingredient in household bleach. But although other ions also play important roles within your body, sodium remains the star for several reasons. For one thing, you can taste it. And until today’s Western diet evolved around endless supplies of processed salt, sodium was harder to come by than another important dietary cation, potassium. Vegetables are full of potassium because plants use it for maintaining moisture balances in their cells and for operating the breathing holes in their leaves. Sodium, in contrast, is rare in most plants, simply because they don’t use it as we do. An apple, for instance, can contain a hundred times more potassium than sodium, and the imbalance in a mango or nectarine can be twice as large.
Because of the scarcity of botanical sodium, many animals seek out sources other than plants, often with the aid of sodium-sensitive taste cells. Whether you get yours from a shaker or a bloody red steak, you run your body on a relatively rare commodity in the food webs of the world. For many herbivores there is no such thing as too much sodium in the diet, and if the vegetable kingdom won’t supply enough of it they sometimes turn to geological sources.
There are many possible reasons why salt licks draw wildlife, but there has been surprisingly little definitive research in this regard. For many years scientists simply assumed that wild game was licking mud in order to stock up on sodium without bothering to analyze the mud itself. More recent work has shown that many so-called salt licks have surprisingly little salt in them, and that other substances may occasionally be the target instead.
South American parrots, for example, are now thought to eat riverbank clay simply because it helps to detoxify harmful chemicals in the seeds that they consume. Amazonian fruit bats visit clay deposits more often than insect-eating bats even though their diets are richer in sodium, again suggesting a digestive function. And a study at Ngorongoro Crater, Tanzania, recently concluded that salt may be a cue that helps wildlife to find even rarer elements such as cobalt or selenium, which can’t be tasted but are also important for nutrition.
Many people indulge in geophagy (earth eating) as well, though not so much for salt as for other reasons. A fascinating overview of this behavior that was compiled by the essayist Beth Ann Fennelly shows that more people eat clay than you might suppose. Although reluctance to admit to dining on dirt can make reliable numbers difficult to obtain, studies in rural regions of the southern United States have found that up to half of all women surveyed consume clay during pregnancy, and studies in Africa find rates as high as 90 percent. Most cases are associated with pregnancy but many men indulge, too, suggesting both nutritional and digestive needs.
In the Andes Mountains, potatoes are sometimes dipped in a slurry of wet clay as though it were gravy, and clay is often sold in marketplaces alongside the produce in Peru. Presumably this protects against the harmful effects of botanical toxins that can occur in potatoes, which belong to the same plant family as deadly nightshade. Clay can also be a famine food that fills empty bellies but unfortunately can also cause intestinal blockages and introduce parasites. And other studies suggest that people eat dirt simply because they like it.
Fennelly described a recent soil-tasting event in a San Francisco art gallery that was modeled on a high-end wine tasting. Soils were mixed with water in wine goblets to release aromas for the attendees to sniff and savor. According to Fennelly’s sources creamy white kaolinite clay “tastes like rain with a hint of peanuts and it melts in your mouth like chocolate.” One family in Mississippi would regularly “fry it and eat it warm,” and the author likened her own taste testing of Georgia kaolinite to gnawing on a chocolate Easter bunny. And if you ever consumed Kaopectate as a digestive aid before a recent reformulation removed the clay for which it was named, then you, too, are a geophage.
However, most of these examples do not really amount to dirt-eating in the strictest sense. When you ingest clay you don’t use its atoms to build or operate your own body as you would with a pinch of salt. Clay is indigestible, being made of minuscule flakes of glassy minerals, and its role in your digestive system is that of a transient sponge. Undesirable molecules found in some plant tissues tend to stick to the surfaces of the flakes, which renders them less harmful and easier to excrete.
The atoms in salt, on the other hand, quickly become important parts of you, and the licking behavior of butterflies and moths is a clear case of sodium hunting. In fact it has been investigated in such detail that we can now deduce with some confidence what happened to the sodium atoms that Hans Bänziger donated to the tear-drinking moths of Thailand. If related studies are a reliable guide, then Bänziger’s tear sodiums became wedding gifts to lady moths.
Although the moths of New York State don’t seem to care much for tears, many of them do drink from mud puddles at night as butterflies do during the day, and a study by the Cornell University scientists Scott Smedley and Thomas Eisner demonstrated that at least some of them gather large amounts of sodium in doing so. Smedley and Eisner found that most of the puddling by Gluphisia septentrionis, a rather plain-looking gray, tan, or cream-colored moth, is done by males, and that they spend many hours drinking at the local mud bar along with other males. A Gluphisia moth can pump five hundred times its body weight in mineral-rich water through its body at one sitting, squirting jets of filtered fluid through its backside over distances of more than a foot.
If you were to do such a thing yourself, however odd that might be, a single sitting would have you jet about nine thousand gallons of your own watery waste behind you in graceful arcs some two hundred feet long and you would also nearly double the sodium content of your body. Like you, however, moths don’t normally need so much sodium—unless they’re planning to share it, which is exactly what Gluphisia moths do.
In the Cornell study, pairs of male and female moths were allowed to mate in cages under controlled conditions. Females who mated with males who had recently puddled contained much more sodium than females who had consorted with nonpuddlers, and after mating, the puddling males contained less than two-thirds of the sodium that they had before. Sodium was as much a part of this lepidopteran lovemaking as the gametes, a nuptial gift akin to a suitor’s box of chocolates. But what would a female moth do with so much sodium?
The answer became clearer when the investigators studied the eggs of the moths. Eggs from salt-gifted mothers carried much more sodium than the others, showing that each courted female relinquished a third to half of her nuptial winnings to her young along with her genes. In other words, sodium atoms from puddles moved directly from father to mother to children. In contrast, females who did not mate with puddlers had to give up as much as 80 percent of their personal sodium supplies in order to properly stock their eggs.
What does this have to do with you and your own relationship to sodium?
For one thing, it shows that there is something special about this element as a commodity in the animal kingdom to which you also belong. It shows that most plant foods in your diet are a poor source of it, because they are built differently from animals. And finally, it shows that we are all so full of sodium atoms that insects flock to us in order to lick us. The sweat on our skin and the puddles in our eyes have close to one-quarter of the salinity of seawater.
All of your sodium atoms came from minerals in the earth’s crust. There is no escaping it: You are an earth eater, and you also consume organisms that are themselves geophages. Among your early human ancestors, most dietary sodium came directly from food, but today most of us also shake, spoon, and drizzle it directly onto our meals or have it done for us by those who produce and package our food. Table salt is an edible version of mineral sand that dissolves in your mouth, stimulates your taste buds, enters your bloodstream, and sometimes emerges from the pools of your eyes in times of joy and sorrow.
* * *
The stories of how sodium atoms end up in your tears could lead your imagination from fiery magma to high mountains, from deep mines to sun-baked plains of gleaming crystals, or from plants and animals to your dinner table. But at some point along the way, most of the sodium in your body spent millions of years adrift in oceans of the distant past.
Until shortly after the American Civil War, nearly all the commercial salt sold in the United States came from a region of central New York that once was a shallow sea. Between five hundred and four hundred million years ago, downwarping of the land allowed an arm of the ocean to fill a geological depression near the site of today’s Syracuse. Millions of years later the isolated sea dried out, leaving vast sheets of salt between blankets of mud that stretched as far west as today’s Ohio. Over the ages some of the salts dissolved into local groundwater and seeped upward through hundreds of feet of sedimentary rock to reach the surface as brine. In recent centuries indigenous people collected the oozings of marshy springs along the shores of Onondaga Lake, boiled or sun-dried the water away, and harvested the salt.
By the nineteenth century, industrial-scale exploitation of the salt deposits and springs gave Syracuse the nickname “Salt City,” and the oldest evaporation plant in the country still operates at nearby Silver Springs, now under the ownership of the Morton Salt Company. If you use Morton salt at your table, then there is a good chance that those tiny cubic crystals that famously pour even when it rains (thanks to an inert powder that keeps the grains from sticking together) came from Paleozoic ocean deposits beneath Silver Springs. But what were they doing in that old seawater in the first place?
Sodium clings to chlorine when seawater evaporates into rock salt, but these elements emerged from very different sources before they ever met one another in an ocean. Most of your chlorine atoms came from volcanic fumes and super-heated fluids, and your sodium was originally weathered out of solidified lava or magma.
When rain and groundwater degrade rocks, they extract sodium atoms and sweep them downhill where they may eventually reach the ocean or an enclosed basin such as Israel’s Dead Sea or Utah’s Great Salt Lake. They go willingly on that journey because water molecules find them very attractive. If you could shrink down small enough to watch the atoms of a basalt boulder as it crumbles ever so slightly during a rainstorm, you could see new cohorts of sodium atoms begin the same journeys that most of the sodium in your tears, sweat, and blood once made. And while you’re at it, why not also travel four hundred million years back in time to do this on the eroding flanks of a hillside in what is now upstate New York? If you do, then you might see exactly where many of your personal sodiums came from.
Each sodium atom in the weakening framework of a feldspar grain trembles in its fetters as though eager to join the water molecules that are tearing at the mineral surface. A water molecule carries slight charges, with the paired hydrogens on one end of it being slightly positive and the central oxygen being slightly negative. Because sodium atoms carry a positive charge in solution, the negative ends of water molecules coax them away from their mineral lattices through the attraction of opposite charges. Like prisoners released by a liberating army, your sodiums are pulled free after millions of years of bondage.
Half a dozen water molecules close in around each sodium cation, pressing their negative ends against it. Around that “first hydration shell,” a second layer of water molecules packs in as well, inflating the virtual size of the cation to several times its original volume. The ball of sodium and water then tumbles away into the general flow of runoff, along with other escapees from the same rock face.
Joining a stream that drains into the warm, shallow sea to the west, each sodium ion remains at the center of a huddle of water molecules as it wanders among ancestral sharks and trilobites whose fossils still lie entombed in stone. An ion’s watery companions are not reliably loyal, however. Any given water molecule soon skips away, opening a space for another one to replace it. But if the supply of replacements runs dry, so to speak, then sodium may find itself bereft. This is what happened when slow Earth movements isolated Paleozoic New York from the main ocean, causing the inland sea to dry out.
In the increasingly concentrated brine, other dissolved substances faced the same abandonment by their evaporating chaperones. The most numerous of these were chloride anions, which became increasingly attractive partners for increasingly desperate sodiums. Like sodium, these chlorides resembled popular celebrities while in solution, although their watery fans pressed positive rather than negative ends against them. One might imagine that this popularity would be even more surprising to chlorine than to sodium, because chlorine atoms have more trouble fitting into the close-knit atomic communities of mineral grains than sodium does.
Unlike sodium, chlorine is rarely locked into the crystal lattices of most minerals. Although you can find small quantities of it in igneous rocks such as basalt or granite, it usually resides in tiny pockets of fluid that are trapped within or between the mineral grains. Because of this, geologists sometimes measure the chlorine content of seemingly impermeable gems such as tourmaline by boiling them in water and then letting the liquid evaporate away.
Chlorine atoms usually escape from magma as hydrogen chloride, the same corrosive acid that your stomach contains. Most of it drifts off in wisps of gas, and geologists estimate that volcanoes release several million tons of hydrogen chloride into the atmosphere every year. It then dissolves into raindrops and eventually ends up in the oceans.
According to a report by Herbert Swenson of the U.S. Geological Survey, a cubic foot of typical seawater contains 2.2 pounds of salt, more than two hundred times more than a similar volume of typical lake water. About 85 percent of the dissolved material is sodium and chloride ions, with magnesium and sulfate ions making up most of the rest. Although rivers deliver four billion tons of these solutes to the oceans every year, the global average salinity of seawater remains fairly stable because similar quantities of salts are buried in sediments on the seafloor.
Today the oceans contain about fifty million billion tons of salt, enough to blanket the continents with a layer of snowy crystals five hundred feet deep. Something much like this actually happened, though on a smaller scale, in central New York about four hundred million years ago.
In the shrinking sea, sodium and chloride ions drew ever closer in the thickening broth of mineral elements. When the last waters evaporated, sodium chloride crystals piled up in sheets of rock salt dozens of feet thick. Over the ages the salt beds were buried under more than a thousand feet of shale, limestone, and other sedimentary deposits. It is refugees from that petrified ocean that escape into the evaporation chambers of the Morton Salt Company today, and that eventually make it to your table.
So as you shake that next dash of salt onto your food, think of the long-standing relationships that you are about to unravel. After long ages apart, ionic opposites were wed in a dying sea. When you toss them back into solution within your body the jostling of watery crowds will separate them again, but don’t weep for those ancient ions. They are about to begin one of the most interesting chapters of their existence inside the living sea of atoms that is you.
* * *
What happens to sodium and chlorine when you swallow them? For starters, don’t swallow either of them in pure elemental form. Purified sodium explodes on contact with water, and chlorine gas can destroy your lungs as it did to soldiers on the battlefields of World War I. You want them tamed in ionic form, which is easy to do when you encounter them in salt because the water in your saliva rapidly dismantles the crystals into their charged components.
One of the first things that sodium does upon dissolving in your mouth is to tell you that it has arrived. Tiny pores in thousands of taste buds on your tongue welcome some of the cations inside, where they trigger nerve impulses that your brain interprets as the taste of salt. Although the physiology of taste remains somewhat mysterious for now, sodium channels such as these are well known not only as portals to flavor, but also to the very act of sensation itself. More on that shortly.
Contrary to what you may have learned in school, taste is not restricted to discrete patches on your tongue. Most of us can actually taste salt all over the tongue as well as on the walls of the mouth, and although certain cells are especially sensitive to specific tastes, many of them can respond to more than one stimulus. Recent research shows that this is especially true when it comes to sodium, and for good reasons.
You can’t store sodium as you do the calcium in your bones, so your body has to monitor and regulate it carefully in dissolved form. Your kidneys, colon, and sweat glands manage most of the losses, while taste cells in your mouth help your brain to control how much sodium enters your body when you eat or drink. When sodium concentrations in a mouthful of food are low, the salt specialists among your taste buds urge you to eat more and enjoy. But if a meal becomes briny enough to pose an immediate risk of overindulgence, it also triggers other taste cells that normally respond most strongly to bitter substances.
Such interactions among salty and bitter sensors may also help to explain why low-sodium salt substitutes, such as potassium chloride, often deliver a bitter aftertaste that can reduce incentives to use them. The only other atom that triggers salt receptors as pleasantly as sodium is lithium, but unfortunately you can’t improve your health by coating your potato chips with lithium chloride. The latter could poison you if not properly administered.
The presence of taste sensors in your mouth hints at the importance of sodium to your body. But what, exactly, do you need it for? This can be answered by showing what happens when someone is deprived of it.
Noting in the Proceedings of the Royal Society of London that “no papers of any value” had yet been published on salt deficiency in humans, the King’s College Hospital physician Robert McCance made what he called a “direct experimental attack” on the topic during the 1930s by putting volunteers on a salt-free diet and subjecting them to daily bouts of heavy sweating.
The sweating was done while each subject lay upon a mattress inside a makeshift cylinder that was equipped with heat lamps, as an assistant swabbed up the drippings to measure the salt losses. After an uncomfortable four-and-a-half-hour trial, one subject reported losing more than half a gallon of body water. He wrote in his journal, “Out, feeling rotten few minutes, washed down. Then OK. Total loss of weight … 2150 gm.”
Cooking for themselves in their homes, the volunteers ate only bread, synthetic milk, thrice-boiled vegetables, and other sodium-free foods. All liquid and solid secretions were collected, dried, and weighed for analysis.
Signs of trouble began within days as sodium levels in body fluids plummeted. “As the deficiency developed all three subjects lost weight,” McCance noted. “Their cheeks fell in and they began to look ill.” All sense of flavor faded, which helps to explain why it is so difficult for many people to adjust to low-sodium diets. Cigarettes lost their taste, and fried onions produced only “greasy sweetness which was extremely nauseating.” Breathlessness became a problem, and energy levels fell so low that one man’s arm “got tired shaving” and his jaw “got tired eating toast.”
After ten days, sodium concentrations in sweat were less than a third of what they had been on day one, and the levels in urine fell to virtually nil. Blood samples became dark and viscous, but blood pressure, urine volumes, and pulse rates remained normal because the subjects drank water freely. This was not dehydration but a slow decline by desalination.
Fortunately the volunteers recovered quickly when the experiment ended, and no permanent harm was done. Their sense of flavor returned within minutes of eating salty bread, butter, and eggs, and within two days their energy levels were back to normal. One man was so relieved to feel normal again that he “jumped off the bus while it was still going and ran up the stairs.”
Dehydration can be even more harmful than running short on sodium, however, because water forms most of your body in addition to providing a medium for chemical processes to operate in. Eating, drinking, and releasing wastes are all part of a continuous balancing act, and you remain properly inflated and functional only by adjusting supply and demand on a regular basis.
A typical, relatively sedentary American adult loses close to thirty ounces (three or four cups) of sweat each day through several million microscopic glands that squeeze fluid out onto the surface of the skin like toothpaste from a tube. A square inch of your forehead can contain hundreds of sweat glands, but other parts of your body have considerably more or fewer. A similar-size patch of skin on your arm, for instance, contains nearly a thousand such glands but the fingerprint ridges of your palms have two or three times as many per square inch. Even the composition of your sweat varies with location and situation. The liquid that cools your legs can be less salty than the dew on your arms, and the odors emerging from your armpits are the work of bacteria that convert oily, protein-enriched “emotional sweat” into aromatic compounds that can act as social cues.
Along with evaporation from the lungs and bulk excretion through the kidneys and bowels, a sedentary adult typically loses five or six pints of water that must be replaced daily in order to maintain a net water supply of roughly eleven gallons. With heavy exercise and a hot climate added to the equation, you can easily lose two to three gallons over the course of a day, along with nearly an ounce of salt. This continual loss of moisture can make you more vulnerable to dehydration than to hunger, and if your water intake is interrupted for long enough the structural effects begin to mimic the dissipation of your body at death.
In 1906 the geologist-anthropologist W. J. McGee delivered a report to a medical conference that was titled “Desert Thirst as Disease.” During the summer of the previous year, McGee helped to rescue a man who had wandered for more than a week in the Arizona desert without adequate water. His account of the ordeal became a classic in the medical literature, and it illustrates how important the balance between salt and water is to your physical appearance in addition to your well-being.
A Mexican prospector named Pablo Valencia left McGee’s tent camp on horseback with a companion, heading for a remote gold claim. That night the companion returned with both horses, saying that Valencia had decided to continue on foot with only a small canteen of water. After a desperate search, the barely living “wreck of Pablo” was eventually found crumpled on the sand beneath an ironwood tree.
According to McGee, half of all waterless travelers succumb to the Arizona desert within a day and a half of running dry, but Valencia somehow managed to walk, stumble, and crawl more than a hundred miles over eight days and nights. His only moisture came from his canteen, an occasional insect or scorpion, and his own scant and concentrated urine.
During his recovery Valencia described how he progressively jettisoned clothing along with his tools and gold nuggets, and watched vultures come almost close enough for him to touch. He became so dehydrated that deep scratches left by thorns and stones didn’t bleed. His nose shrank to half its original length, his lips vanished, and his tongue was “a mere bunch of black integument.” Even the tissues surrounding his eyes retreated so far back into their sockets that the whites of his eyeballs were exposed.
Most sources report that we begin to feel severe thirst at the command of cellular sodium receptors after losing only 1 or 2 percent of our body water. Convulsions and delirium generally begin with a 10 to 15 percent loss, and higher deficits are usually fatal. Amazingly, Pablo Valencia lost a quarter of his original weight, mostly through evaporation from his skin and mouth. Nonetheless he was back on his feet within a week of his rescue, and lucky to be alive.
The examples of McCance’s volunteers and McGee’s miner illustrate some of the ways in which shortages of salt and water can affect you, but the processes that cause those effects are relatively simple. It all comes down to the random thermal motions of atoms.
The lethargy of the sodium-starved subjects, for example, arose from the relative movements of salt and water among cells. Osmosis and diffusion are the net migration of water and dissolved substances, respectively, from an area of high abundance to low. Both are powered by the heat-driven dancing of atoms, and your health depends on rather precarious relationships between these two processes. The delicate membranes of your cells allow water molecules to cross more easily than salt ions, which are fattened by shells of hydrogen-bonded water. Sodium therefore enters and leaves your cells mostly with the aid of protein channels and pumps. In the restless atomic realm, simply placing a selectively permeable membrane barrier between a cell and its surroundings can produce remarkable results.
If you were to place one of your red blood cells into a drop of pure water and watch it under a microscope, you would soon see it expand and then burst like an overinflated balloon. This is because water molecules can easily enter the cell by dancing across its thin membrane, but the salt ions are trapped inside. The imbalance of mobility between incoming water and the confined salts swells the cell. In very salty water, however, your blood cells would shrivel instead: Osmosis of water from the interior would not be matched by inward diffusion because the salt ions cannot cross the membrane barrier.
Such differential movements of water and salt among your cells can have powerful effects. For example, running short of sodium could cause your red blood cells to expand until they no longer pass as easily as they should through your narrow capillaries. The resultant blockages would quickly cause oxygen shortages all over your body, as happened to McCance’s lethargic patients. Unpleasant effects of dehydration that McGee described can also arise from changes in the sizes of cells due to osmotic imbalances, and you don’t have to wander for days in a desert in order to experience them. You can also trigger them by tipping the scales in the other direction with excessive hydration.
When my fellow college students and I foolishly competed in a water-drinking contest during the 1970s, we never dreamed that it could be dangerous. Our rules were simple: a glass per minute, and “one go, no blow.” Within less than an hour we all quit in trembling misery, rushing to the bathrooms to unload and feeling somewhat drunk as we ran. We thought that we shivered and staggered from cold, and that may have been partially correct. But it could also have been the start of something more sinister, which, unknown to us at the time, has ended the lives of more determined competitors.
There are many sad stories about people who have convulsed and died from drinking too much water, usually in some sort of contest. In 2007 a young mother of three perished shortly after drinking too much water for a radio station’s “Hold Your Wee for a Wii” contest, and thrill-seeking college students are occasional victims of this, as well. The swelling of brain cells can constrict the flow of blood inside your skull, and if your nerve cells swell too much they begin to malfunction. This is why athletes often drink Gatorade or other ion-rich electrolyte solutions after prolonged heavy sweating rather than glugging pure water, because such drinks better maintain their crucial osmotic balance.
Your tears harness similar processes in ways that help to protect you against infections. Lysozyme, an enzyme in tear fluids, attacks the stiff cell walls of bacteria. When airborne bacteria land in the wet pools of your eyes, their cell walls become so weakened by lysozyme that they can no longer resist osmotic swelling, and that tempting oasis becomes a death trap as they distend and disintegrate. Penicillin and other antibiotics likewise weaken bacterial cell walls, thereby enlisting your body water as a defensive osmotic weapon.
But the diffusion of sodium across your membranes also has more complex effects than the mere distortion of cells. It also produces your thoughts and feelings as well as the countless movements of your body.
* * *
As in a computer, the hundreds of billions of threadlike cells in your nervous system generate electrical fields, but they don’t transmit electricity as wires do. Cool a copper wire, and it conducts electrons just fine. Cool a nerve cell, or neuron, too much and it stops working properly, as victims of hypothermia can attest. It is not electrons that rush through the networks of your nerves, but disturbances that are more akin to waves on the surface of an ocean than the currents within it. A primary driver of those waves is the thermal dance of sodium ions.
Wiggle your toes now, if you will. Sodium helps you to do this, and it also tells you what it feels like to do so. All such voluntary movements as well as your sensations and thoughts are manifestations of nerve impulses. And all such impulses involve the diffusion of ions across the thin membranes of neurons.
To imagine how this works, think of thousands of sports fans in the bleachers of a stadium as they do “the wave.” As soon as one person raises and drops his or her arms, the next person repeats the motion and so on down the line, thereby producing an undulation that ripples through the crowd. None of them leave their seats: It is only the disturbance that travels, and if orchestrated correctly it can move faster than a person could run.
A nerve impulse is much like a stadium wave, but instead of people flapping their arms up and down, vast numbers of tiny channels open and close. A single neuron can carry millions of these channels, which briefly open passageways through the cell membrane that are available only to sodium. Each channel pore is surrounded by proteins that can shear the shell of water molecules away from a sodium cation and open a custom-fitted tunnel just wide enough for it to squeeze through.
When it opens for a fraction of a second before snapping shut again, such a channel allows thousands of cations to diffuse into the neuron, where sodium is roughly ten times less abundant than it is on the outside of the cell. This triggers neighboring channels to do the same until the rush of diffusing sodium reaches the end of the elongated cell. From there the signal passes to other neurons or muscle cells, which can relay the message further or otherwise react to it.
In the case of your recent toe wiggle, the decision to obey my request spread through your brain at nearly a hundred feet per second. This triggered a ripple of sodium diffusion that rolled down your spinal cord at high speed: Impulses such as these can travel at more than two hundred miles per hour. Then it shot down your leg to your toe in another split second. When the requisite muscles twitched and moved your toe, sensory neurons fired sodium waves back up to your brain to report “mission accomplished.”
It is all so mechanistic, so strictly elemental in nature, that it is difficult to believe that invisible atomic waves allow my written words to speak to you right now through neurons that connect your eyes to your brain. But they do. Just as flickers of digital information allow a computer to produce a virtual reality so compelling that you might wish to live within it, the serial diffusion of atoms from a salt shaker helps you to see, feel, hear, taste, smell, and think your way into a vivid reality of your own making.
This simplified version of the process illustrates the importance of sodium to your nervous system, although there is more to the story that we need not go into here beyond some brief mentions. Chloride channels help to regulate the charge balance of your neurons, and calcium channels on the downstream ends of neurons help to pass the signal on to adjacent cells. Potassium ions await the arrival of sodium waves before diffusing out of your neurons through floodgates of their own, and the whole system must be reset by tiny molecular pumps that toss sodium back out and potassium back in before another signal can occur. Potassium is also the dominant wave ion in the neurons of your inner ear, so the next time you eat a potassium-rich banana you can reasonably imagine atomic whispers of the tropics emerging from the auditory cells inside in your head.
These events don’t take long: Some neurons can recharge and transmit new signals more than five hundred times per second. But they cost you because although osmosis and diffusion are automatic, membrane pumps run on food energy. According to some estimates about one-fifth of the energy budget of your body at rest is spent on powering your nervous system, and every fifth breath of air is drawn to provide oxygen for the production of that energy.
It is fortunate that you don’t have to think about all this in order to function. Your atoms handle most of it for you, day and night. And it is fortunate for the rest of animal life on Earth, as well, because all creatures from salt-licking deer to tear-sipping moths use sodium waves for nerve signaling just as we do.
Some animals even use the need for sodium diffusion itself as a defense. Several species of aquatic newt carry a protective nerve-targeting chemical called tetrodotoxin. This is the same deadly agent in fugu, the Japanese puffer fish that is notorious among thrill-seeking diners for its tendency to kill those who eat it when it is prepared incorrectly. Many times more powerful than cyanide, tetrodotoxin blocks the sodium channels on neurons, causing paralysis even in small doses. The toxin needs only to incapacitate a critical number of tiny sodium channels, so just the few milligrams that might fit on the head of a pin are enough to kill an adult within hours.
Some insecticides, such as permethrin, ruin the nervous systems of insects by overstimulating their sodium channels instead. Permethrin binds to the sodium channels of insects and holds them open so the neurons can’t be reset properly, thereby leaving pests writhing, paralyzed, or dead. Your own neurons are sufficiently different from those of insects that permethrin doesn’t interfere with them, but if you dislike being around neurotoxins on principle then you might also want to avoid chrysanthemums, which many people use to no obvious ill effect in salads. They produce a similar nerve-abusing insecticide, called pyrethrin.
Many other plants also make defensive neurochemicals of their own, from the nicotine in tobacco to the distinctive flavor molecules in spices. The apparent heat of a relatively mild jalapeño pepper is a sensory illusion that is triggered in your mouth by capsaicin, a molecule that tricks your sodium channels into initiating signals of heat and pain. Bite a fiery habañero and a howling chorus of deluded neurons in your mouth will fling open their sodium gates as they would if you had swallowed glowing coals. So convincing is the illusion of warmth that your sweat glands can even be fooled into drenching your skin in response.
The next time you indulge in a hot spicy meal, you might begin to think differently about the salty water that glistens on your skin and wells up in your eyes. These are not just your own sweat and tears, but also components of the earth itself.
Your sweat and your tears are salty because they come from briny lymph that seeps from your blood vessels like mineral-rich groundwater. You produce tear fluids continuously from lacrimal glands in the undersides of your eyelids, and other glands also add evaporation-resistant films of oil to the surfaces of your eyes, mucus to the basal layers of liquid, and germ-killing lysozyme throughout. These substances moisten and protect your cornea as the blinking of your lids spreads them smoothly, and then they soak back into the deeper recesses of your body through pores in the inner corners of your eyes.
The osmotic balance of your tears is similar to that of your blood and lymph, but your lacrimal glands provide them with slightly less sodium, which presumably reduces losses to the environment. By opening or closing various membrane channels and selectively pumping salt ions here and there, the glands encourage water to follow by osmosis into canals that empty like salt springs into a “lacrimal lake” in the lower sectors of your eyes. Each spring-fed lake normally holds up to thirty microliters of liquid, close to the volume of a typical raindrop, but an irritant, a deep yawn, or the neuronal signals of joy, pain, or sadness can make it overflow into tears.
Imagine the journeys that the sodium atoms in your tears have taken before rolling down your cheeks, perhaps to trigger the salt sensors of your tongue or the lacrimal glands of a sympathetic friend. Locked away in stone for long ages, these same atoms might once have wandered among great forests of coral and splashed the sands of pristine tropical beaches. Mingling with them are atoms of chlorine that once hissed in volcanic vents, floated in the same primeval seas, and met their ionic partners there before tumbling into your mouth from a tasty morsel of food.
And if you prefer to keep those well-traveled atoms with you while dining outdoors on a warm evening in Thailand, then you might want to keep an eye out for moths, as well.