Surviving the Extremes: A Doctor's Journey to the Limits of Human Endurance (21 page)

Not being too furry, we humans lose most of our heat through our skin, which acts like a car radiator. Blood, the liquid coolant, passes through all the organs, picking up heat and carrying it to the skin surface, where it is cooled by proximity to the outside temperature. Blood is distributed from the heart to the organs via a system of tubes. The big conduits are called arteries, the finer ones, arterioles. These tubes have muscles within their walls that regulate the amount and
direction of blood flow, distributing some under the body’s insulating fat layer and some to the skin surface, where it is cooled. The ring-shaped muscles can contract to narrow the tubes or relax to dilate them. They respond to signals from the hypothalamus to divert more blood to the skin whenever the body starts to overheat. The system is fine-tuned and elegant, but by itself not enough to keep us out of the danger zone. For the blood to be cooled by air, the outside temperature must be significantly below that of the skin, whose temperature is 95°F. Exposure to higher environmental temperatures, such as routinely occurs in the desert, actually heats the blood. Moreover, muscular activity greatly increases the internal heat load that must be dissipated through the skin. Another system, no matter how inelegant, is needed for thermal regulation. That system is sweating.

Extruding water from the body may be disagreeable in social situations, but it works. Sweat glands spread a layer of water onto the surface of the skin. As the water evaporates—a cooling process—it draws heat from the body, vaporizing it into the air. Sweating can dissipate heat twenty times faster than blood-cooling. When tinkering with blood vessels isn’t getting the job done and the skin is heating up, the hypothalamus turns on the sprinkler system.

About 3 million sweat glands are distributed unevenly on the skin. Most are located on the forehead, face and neck, chest and back, and in the armpits and groin—areas that benefit the most from cooling because of the large volume of blood circulating just beneath the skin. For sweating to be effective, however, the water has to evaporate. High humidity is so uncomfortable because the air, already filled with water, has little capacity to add more, so sweat simply sits on the skin’s surface.

When air temperature approaches body temperature, you need to sweat to stay alive. The higher temperature speeds up evaporation and actually improves cooling efficiency, which would otherwise steadily diminish and then reverse as the outside temperature rose. Sweating may be essential for survival in the heat, but in the desert, unless you have enough water, sweating will kill you.

Two-thirds of the human body is composed of water. The average person contains about 50 liters of fluid and loses a minimum of 2 liters in daily body maintenance. The kidneys use water to flush out waste
as urine; the lungs use it to moisturize inhaled air so that it won’t irritate the sensitive pulmonary linings. Some also seeps out passively through the skin—the body’s not-quite-watertight container. When the body turns on its sweat glands, however, water losses mount rapidly. The hypothalamus controls both the number of glands activated and the rate at which the sweat pours out. Sweating is a profligate waste of water, but the body knows no limits when it’s fighting an internal fire; at rest on a hot day, it can easily use up 5 liters. When the body is down even 1 liter, its function becomes impaired. Once it is down 5 liters, fatigue and dizziness set in. A loss of 10 liters disturbs vision and hearing and sets off convulsions. A deficit of 15 to 20 liters, roughly a third of the body’s total amount of water, is fatal.

To make matters worse, exercise and anxiety greatly accelerate sweating. Walking can use up an additional liter an hour, and sweating is also the automatic response to being nervous. So in the desert, the body stresses of heat, exercise, and anxiety all contribute to create maximum water loss. Surviving in the desert resembles trying to control a fire without an external water supply. Should the body’s core temperature rise by 2°F, the speed of metabolic reactions increases 15 percent. At 105°F, the speed will be about 50 percent above normal. Beyond that temperature, reactions accelerate even faster, and at 110°F, the brain is cooked.

To sustain the increased blood flow and the production of sweat, the body needs to resupply itself with water. Finding it requires conscious action. The hypothalamus sends a water-seeking signal to the cerebral cortex—which we experience as the sensation of thirst. The thirst signal does not get triggered immediately. Sensors in blood vessels are continually “tasting” the salt concentration of the blood. It takes a loss of about 3 percent of body water before blood becomes salty enough to set off the alarm (the alarm will also go off if excess salt is ingested through foods). There is practical value to the delay, since it means that humans are not slaves to small changes in salt concentration. We are free to perform other activities if we are not driven to drink constantly.

The body can tolerate a fluid loss of about 5 percent before developing
any obvious symptoms of dehydration, such as dizziness or fatigue, but even a 1 percent loss can impair normal functioning. Given the prevalence in our diets of salty foods and dehydrating drinks such as coffee and tea, most of us are chronically underhydrated—at a level that triggers no alarms but nonetheless subtly affects our performance. When we drink enough to bring the deficit to within 3 percent of body water, our thirst is quenched. The problem is easily solved—except when there’s no water to be found. Lacking an adequate supply of water, the blood remains too concentrated and the body must therefore draw down its own reserves to dilute it. Water is distributed in the blood, where it flows freely, and in the organs, where it is held like a wet sponge. To manage thirst, the body shifts water to the blood in the process of osmosis, wringing it out of the organs until the salt concentration in the blood returns to normal. Osmosis has its own limits. It will not allow so much to be taken from the organs that they become even saltier than the blood, but will keep the two concentrations at equal levels. This means that both blood and tissues will dry out and become salty at the same rate, with water continuing to seep out of the organs. The body will conserve water where it can. The kidneys clamp down on outflow, flushing out waste using less and less water. A progressively darkening color and a decreasing amount of urine are far more accurate indicators of dehydration than is thirst.

With no water coming in, the body will make do with what it has. It will set priorities. In the desert, the body must dissipate heat, continue oxygen intake, and develop a survival strategy. Therefore it will shunt blood to the skin, the lungs, and the brain at the expense of other less immediately essential organs, such as the stomach, liver, intestines, and muscles. Faced with the need to supply more blood to the network of heat-dissipating vessels that have opened up under the skin, while still maintaining flow to the brain and lungs, the heart pumps harder and faster. The chest pounds and the pulse rises. To aggravate matters, the drier blood is thicker and therefore harder to propel through the vessels. Increased work generates more body heat, which requires even more blood flow to the skin, creating a vicious cycle. Blood flow can’t keep up. Eventually even the most vital organs
will become deprived and the body will collapse into heat exhaustion.

Should he happen to fall in direct sunlight, an exhausted desert wanderer will die. Should he fall in the shade or at night, his condition might well correct itself. He will no longer have to endure the external heat load from the sun. His skeletal muscles will be at rest, sharply reducing his internal heat load. In an upright human, 70 percent of the blood lies below heart level; in a collapsed human, all of it is horizontal and much easier for the heart to push around, especially into the head. With blood flow restored to the brain, the victim may revive. Collapsing might actually save his life, as long as he collapses in the right place or at the right time.

Andrew Hughes was looking for that elusive road to Duse, while Jane, Matt, and Sam headed back to the car to await his return. Hughes pressed on through the day, absorbing a high solar heat load and sweating profusely. He wasn’t yet very thirsty. Responding to the heat, his surface blood vessels were fully dilated, turning his skin bright red, even in those places protected from the sun. He felt nauseated, suffered from abdominal cramps, and was growing weaker with each step as his hypothalamus diverted more and more blood away from his stomach and muscles. By nightfall, he was confused, disoriented, and hopelessly lost. He collapsed in the sand.

The cool night air and his recumbent position brought him back to life. Though he was still exhausted when he awoke in the morning, his first thoughts were of his wife and children waiting for him, depending on him. He summoned enough strength to go on, and staggered onto a road just as a Tunisian farmer was driving by. The farmer took him inside his truck and gave him some water. Once his body and mind were reconstituted, Hughes, along with the farmer, set off to look for the car. They found it two and a half hours later, but Jane, Matthew, and Sam were nowhere to be seen—nor was there any sign that they had ever returned there.

The farmer pulled the car out of the sand, and Hughes climbed into it, started it up, and began to follow the farmer’s truck. After a few miles, a tire blew out and the car again ground to a halt. For some
reason, the farmer never turned back to look for the car behind him. Hughes frantically sounded his horn to no avail, as the truck gradually disappeared. The rented car had no jack. For the second time, Hughes was alone, with no water, no idea where he was, and no family. Once again, he set out on foot in the blazing afternoon sun with no clue where to find refuge or help. That night he curled up under a bush and slept. At dawn he rose to continue walking. Burning with thirst, he stumbled across an old stone well, but the water was too far down to reach and the bucket rope had rotted away. Hughes collapsed again, from dehydration, exhaustion, and despair over the plight of his family.

Hughes was probably only few hours from death when a patrol of Tunisian soldiers happened upon him. They revived him, then informed him that they had already found the rest of his family—too late to save them from the desert. Hughes’s wife and sons had not been able to find the water tanks that were “just down the road,” nor had they, as Hughes already knew, made it back to the car.

Exactly what happened to the three of them in their final hours will never be known, but we can reconstruct what happened inside their bodies as they slipped into heat exhaustion, then into heat stroke, and finally died. Subjected to relentless solar bombardment, they endured a steadily rising heat load. Whether they were seeking the shelter of their car or resting on the desert floor, they perspired profusely, for their bodies’ primary task was to maintain internal temperature. All three endured enormous water loss; they had a woefully inadequate 1.5 liters and no way to replenish it. Their internal water supply dropped below the critical level; the volume of their blood decreased, becoming a salty sludge too thick and slow-moving to make it into the tiny end vessels that nourish individual cells, or to flow through the fine blood-cooling network just beneath the skin. Soon they lacked the water to sweat. Their skin became hot and dry—the telltale sign of heat stroke.

A human in the desert sun without thermoregulatory defenses is no different from a piece of meat roasting in an oven. Surrounded by air temperatures that routinely reach 120°F, he will collapse, only to then be baked by sand temperatures that are 20°F to 40°F higher. The result is intense heating. In a desperate attempt to the keep the body
temperature from rising that fatal 10°F, the hypothalamus relinquishes muscular control of surface blood flow, allowing vessels to dilate fully. The skin flushes from the increased blood that pools there, further diminishing the supply to the vital organs. The body is breaking down.

Chaos reigns as organs overheat. The stomach and intestines stop digesting, and the liver stops neutralizing their toxic by-products. The blood cannot filter through the kidneys to prevent the buildup of toxins. Muscles convulse in large, violent spasms. As the brain’s delicate and intricately paced chemical reactions are speeded up by the relentlessly rising temperature, the mind and body grow confused. The hypothalamus loses control of itself and of the entire thermoregulatory system. The steadily rising internal heat alters proteins; cell membranes become distorted and porous; salt leaks in. The cells swell and burst, their contents exploding into surrounding tissues, where they cause more damage and inflammation, leading to more swelling and bursting in an accelerating and now unstoppable fatal chain reaction.

Jane, Matthew, and Sam Hughes had been unable to survive what should have been a one-hour walk back to their car. Traveling over terrain with no landmarks, no water, and no shelter, they were easy marks for the pitiless sun, which killed them within two days. In the same desert, five years later and under similar conditions, Mauro Prosperi survived for nine days before being rescued, drinking by his account almost no water, and covering a distance of 130 miles. Can his story possibly be true?

The morning after that fierce sandstorm, Prosperi climbed to the top of a high dune. He saw no trace of a trail, nor could he see a support truck or a camp. The race manual was clear about what to do if you get lost: stay put and wait for rescue. Prosperi played by the rules. Toward the end of the day, a search helicopter passed overhead. He waved at it frantically, but it didn’t see him frying in the sun. He drank his last bit of water, and later, to recycle his fluids, he urinated into the empty bottle. Then he fell asleep in the cooling night at the top of the dune.

In the stillness the next morning, Prosperi gazed out at nothing
but sun and sand. There was no movement in the sky or on the horizon. Lacking water or shade, he knew the sun would most likely kill him within a day or two and decided he could no longer stay put. After wandering aimlessly for a few hours in the sun, he spotted a small structure off in the distance and made his way toward it. It turned out to be an empty Muslim shrine. The same imperative to get out of the heat of the day had inspired other species as well: he discovered he was sharing his shelter with a colony of bats.