We could reduce our energy needs considerably if we elected to be cold-blooded. A typical mammal uses about thirty times as much energy in a day as a typical reptile, which means that we must eat every day what a crocodile needs in a month. What we get from this is an ability to leap out of bed in the morning, rather than having to bask on a rock until the sun warms us, and to move about at night or in cold weather, and just to be generally more energetic and responsive than our reptilian counterparts.
We exist within extraordinarily fine tolerances. Although our body temperature varies slightly through the day (it is lowest in the morning, highest in the late afternoon or evening), it normally doesn’t stray more than a degree or so from 98.6 degrees Fahrenheit. (That’s in adults. Children tend to run about one degree higher.) To move more than a very few degrees in either direction is to invite a lot of trouble. A fall of just two degrees below normal, or a rise of four degrees above, can tip the brain into a crisis that can swiftly lead to irreversible damage or death. To avoid catastrophe, the brain has its trusty control center, the hypothalamus, which tells the body to cool itself by sweating or to warm itself by shivering and diverting blood flow away from the skin and into the more vulnerable organs.
That may not seem a terribly sophisticated way of dealing with such a critical matter, but the body does it remarkably well. In one well-known experiment cited by the British academic Steve Jones, a test subject ran a marathon on a treadmill while the room temperature was gradually raised from minus 49 degrees Fahrenheit to 131 degrees Fahrenheit—roughly the limits of human tolerance at both extremes. Despite the subject’s exertions and the great range of temperatures, his core body temperature deviated by less than one degree over the course of the exercise.
That experiment largely recalled a series of experiments conducted more than two hundred years earlier for the Royal Society in London by Charles Blagden, a physician. Blagden built a heated chamber—essentially a walk-in oven—in which he and willing associates would stand for as long as they could bear it. Blagden managed ten minutes at a temperature of 198 degrees Fahrenheit. His friend the botanist Joseph Banks, freshly returned from circling the world with Captain James Cook and soon to become president of the Royal Society, managed 210 degrees Fahrenheit, but only for three minutes. “To prove that there was no fallacy in the degree of heat shewn by the thermometer,” Blagden recorded, “we put some eggs and a beef-steak upon a tin frame, placed near the standard thermometer….In about twenty minutes the eggs were taken out, roasted quite hard; and in forty-seven minutes the steak was not only dressed, but almost dry.” The experimenters also measured the temperature of their urine immediately before and after the test and found that it was unchanged despite the heat. Blagden additionally deduced that perspiration had a central role in cooling the body—his most important insight, and indeed his only lasting contribution to scientific knowledge.
Occasionally, as we all know, our body temperature is elevated beyond normal in the condition known as a fever. Curiously, no one knows quite why this happens—whether fevers are an innate defense mechanism aimed at killing invading pathogens or simply a by-product of the body working hard to fight off infection. The question is important because if fever is a defense mechanism, then any effort to suppress or eliminate it may be counterproductive. Allowing a fever to run its course (within limits, needless to say) could be the wisest thing. An increase of only a degree or so in body temperature has been shown to slow the replication rate of viruses by a factor of two hundred—an astonishing increase in self-defense from only a very modest rise in warmth. The trouble is, we don’t entirely understand what is going on with fevers. As Professor Mark S. Blumberg of the University of Iowa has put it, “If fever is such an ancient response to infection, one would think that the mechanism by which it benefits the host would be easy to determine. In fact, it has been difficult.”
If elevating our temperature a degree or two is so helpful at fending off invading microbes, then why not raise it permanently? The answer is that it is just too costly. If we were to raise our body temperature permanently by only 3–4 degrees Fahrenheit, our energy requirements would shoot up by about 20 percent. The temperature we have is a reasonable compromise between utility and cost, as with most things, and actually even normal temperature is pretty good at keeping microbes in check. Just look at how swiftly they swarm in and devour you when you die. That’s because your lifeless body falls to a delicious come-and-get-it temperature, like a pie left to cool on a windowsill.
The idea, incidentally, that we lose most of our heat through the top of our heads is, it seems, a myth. The top of your head accounts for no more than about 2 percent of your body surface area, and is, on most of us, pretty well insulated by hair, so the top of your head will never be a good radiator. On the other hand, it you are outdoors in cold weather and your head is the only part of you that is exposed, then it will play a disproportionate part in any heat loss, so listen to your mother when she tells you to put a hat on.
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Maintaining equilibrium within the body is called homeostasis. The man who coined the term and is often referred to as the father of the discipline was the Harvard physiologist Walter Bradford Cannon (1871–1945). A stocky man whose grim and stiff gaze in photographs belied an apparently warm and genial manner in person, Cannon was undoubtedly a genius, and part of that genius seems to have been an ability to persuade others to do rash and uncomfortable things in the name of science. Curious to understand why our stomachs gurgle when we are hungry, he persuaded a student named Arthur L. Washburn to train himself to overcome the gag reflex in order to push a rubber tube down his throat and into his stomach, where a balloon on its end could be inflated to measure the contractions when he was deprived of food. Washburn would then spend the day going about his normal business—attending classes, working in the lab, running errands—while the balloon uncomfortably expanded and collapsed and people stared at him for being the source of strange noises and having a tube coming out of his mouth.
Cannon persuaded other of his students to consume food while being X-rayed so that he could watch as it proceeded from mouth to esophagus and onward into the digestive system. In so doing, he became the first person to observe the actions of peristalsis—that is, the muscular pushing of food through the digestive tract. These and other novel experiments became the basis of Cannon’s classic text, Bodily Changes in Pain, Hunger, Fear, and Rage, which was the last word on physiology for years.
Cannon’s interests seemed to know no bounds. He became the world authority on the autonomic nervous system—that is, all those things the body does automatically, like breathe, pump blood, and digest food—and on blood plasma. He did groundbreaking research on the amygdala and hypothalamus, deduced the role of adrenaline in survival response (he coined the term “fight or flight”), developed the first effective treatments for shock, and even found time to write an authoritative and respectful paper on the practice of voodoo. In his spare time, he was an enthusiastic outdoorsman. A mountain peak in Montana, in what is now Glacier National Park, was named Mount Cannon in honor of him and his wife after they were the first to scale it, on their honeymoon in 1901. At the outbreak of World War I, he enlisted as a volunteer for the Harvard Hospital Unit, even though he was forty-five years old and the father of five children. He spent two years in Europe as a field doctor. In 1932, Cannon distilled practically all of his knowledge and years of research into a popular book, The Wisdom of the Body, outlining the body’s extraordinary ability to regulate itself. A Swede named Ulf von Euler followed up on Cannon’s studies into the fight-or-flight impulse in humans and won the Nobel Prize in Physiology or Medicine in 1970; Cannon himself was long dead by the time the importance of his work was fully appreciated, though he is now widely venerated retroactively.