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A MATTER OF DEGREES
What Temperature Reveals About the Past and Future of Our Species, Planet, and Universe

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By Gino Segrè

VIKING

Copyright © 2002 Gino Segrè.
All rights reserved.
ISBN: 0670031011

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Chapter One

98.6

Ninety-eight point six. It is extraordinary how alike we are; a thermometer under the tongue of an Inuit on an Arctic ice floe, a pygmy in the Ituri forest, or a stockbroker on the floor of the New York Stock Exchange gives the same reading. Yellow, black, brown, or white, tall or short, fat or thin, old or young, male or female, it's still 98.6. For a one-month-old baby, a twenty-year-old athlete, and a centenarian, body temperature is still the same. Muscles bulge or atrophy, teeth erupt or fall out, vision is acute or clouded by cataracts, heart rates double under stress, breathing fluctuates wildly, you shiver uncontrollably or sweat buckets, but temperature stays the same. And you feel sick if it varies by the merest 2 percent. If it rises or falls by much more than 5 percent, you should consider heading for the emergency room. The resemblance of one human to another in this respect is truly remarkable. Respiration, perspiration, excretion, and other bodily functions have huge swings, keyed to maintaining a constant body temperature.

    Strictly speaking, 98.6 is only a convenient shorthand because measurements vary, albeit in a predictable way, over the range of our bodies. Our skin temperature is usually some 6 degrees lower than our internal temperature, as you can easily verify by placing a thermometer between your fingers rather than under your tongue. Oral and rectal readings differ as well, the latter commonly being about one degree higher than the former. Internal temperature varies between organs depending on metabolism and blood flow. Even before temperature was measured, our ancestors thought the hottest part of the body was the heart and in particular the heart of "hot-blooded types." Now, more prosaically, we have discovered that the distinctly less passionate liver, usually hovering at close to 105 degrees, has the honor of being the hottest.

    The idea that all humans have the same body temperature would have seemed very curious before the seventeenth century. Only rough thermometers existed and none were used to make careful readings, comparing one human to another. The assumption was that body temperature, measured only externally and roughly at that, reflected the local climate and therefore was higher in the tropics than in temperate regions. The first problem posed in Johannes Hasler's influential De logistica medica, published in 1578, is "to find the natural degree of temperature of each man, as determined by his age, the time of the year, the elevation of the pole (i.e., latitude) and other influences." Hasler's elaborate tables indicated to doctors how they should mix their medicines according to degrees of hot and cold, the needs of the patient, and what his or her surroundings were like. Of course, the notion that fever was associated with sickness was well known to him. That's why he urged practitioners to be alert to temperature shifts.

    We now know our body temperature doesn't vary with location. It does change slightly according to the time of day, gradually rising to a midafternoon peak typically 1.5 degrees higher than the nighttime minimum; 98.6 is simply a daily average. Yet even that statement needs to be qualified, as Harrison's Principles of Internal Medicine tells us:

Whereas the "normal" temperature in humans has been said to be 98.6°F. on the basis of Wunderlich's original observations more than 120 years ago, the overall mean oral temperature for healthy individuals aged 18 to 40 years is actually 98.2°F.

    A temperature reading above normal is called pyrexia, or, more commonly, fever, and one below normal is known as hypothermia. The regulatory mechanisms that keep us for the most part in the normal range are directed by a master control system embedded deep in the brain. The hypothalamus, the tiny organ that sets temperature, also controls the secretions that dictate many of the key metabolic functions, adjusts water, sugar, and fat levels in our body, and guides the release of hormones that both inhibit and enhance our activities. Harvey Cushing, the great early-twentieth-century American physician who studied the behavior of the hypothalamus and the pituitary glands, described our hypothalamus as follows:

Here in this well concealed spot, almost to be covered with a thumbnail, lies the very mainspring of primitive existence—vegetative, emotional, reproductive—on which with more or less success, man has come to superimpose a cortex of inhibitions.

    Our bodies generate heat by a variety of metabolic mechanisms fueled by food and drink. The body dissipates approximately 85 percent of the heat through the skin, the rest exiting by respiration and excretion. Since skin is the principal point of entry and exit for heat, that's where we should look for a connection to the hypothalamus. There are in fact two important pathways. One is the peripheral nervous system and the other is the rich underlying lacework of small blood vessels known as capillaries.

    Signals from these two sources are integrated in the thermoregulatory section of the hypothalamus. If the reading says the body is too cold, the capillaries constrict, thereby conserving heat; if it is too warm, the capillaries dilate. Simultaneously, hormonal messages are sent to the sweat glands, ordering them to secrete moisture—sweat—through pores in the skin. When this happens, signals to the brain strongly suggest proceeding to behavioral changes, such as the donning or shedding of clothes, maintaining all the while, of course, Cushing's "inhibitions." The blood supply entering the hypothalamus provides an ongoing check of the adjustments made and, if necessary, indicates to the hypothalamus the necessary secretions for resetting temperature. Once again we can only marvel at the efficient workings of a system that has evolved over millions of years.

Constant Temperature

We share the characteristic of keeping a constant body temperature with mammals and birds, the other so-called warm-blooded animals. Of course, it's not just blood that's warm, nor is the distinction quite so clear-cut. The separation in biology is made between homeotherms and poikilotherms, from the Greek homos, meaning "the same," as opposed to poikilos, meaning "variable," with therme being heat or temperature. The homeotherms—birds and mammals—have high metabolisms, generate heat from within, and have elaborate cooling mechanisms to assure a constant body temperature, while poikilotherms—all other animals—do not. There are exceptions to this rule; for instance, some hot-blooded animals lower their body temperature considerably, as in the well-known case of hibernation. Nevertheless, the classification is a very good approximation, certainly good enough for us to ask why homeothermy has evolved. Requiring a more complicated brain with more sophisticated controls, it has been adopted by a very small percentage of known species. There is no single answer to why they have done it, only a set of hypotheses.

    The beginnings of constant body temperature in some animals seem to coincide with the transition to living on land from an earlier aquatic existence. Life underwater is sheltered to a large extent from changes in the weather. In particular, the ambient temperature in deep water remains pretty constant. By contrast, creatures living on the Earth's surface encounter temperature variations in the course of twenty-four hours, experiencing night and day, rain and shine, wind and storm. In addition, life on the surface has evolved to the point that many animals have to rapidly make complex decisions.

    Imagine an early human ancestor being chased by a lion across the savannah. In running, all the limbs have to be moved in a coordinated manner while the brain is evaluating the best strategy for survival. Run, or turn and fight with one's primitive club? How far away is that tree, and what chances do I have for climbing it before the lion reaches me? What about my family's chances of survival, and will another member of my clan come to my assistance? Should I dive into the river or do I risk being caught by a crocodile if I do that? These calculations are all going on while the limbs are moving, the body is sweating, and the lungs are straining. Both the lion and the human's thoughts and actions need to proceed in parallel and the decisions integrated into the chosen route for survival.

    The master control that directs human thought and action is the brain, a fantastically elaborate circuitry of a hundred billion interconnected nerve cells; a comparable number is housed in the lion's cranium. The complex chemical reactions that activate the transmission and reception of signals are temperature-dependent, as are the various hormonal messages sent to the specialized organs. Since all the circuitry is temperature-dependent, having a constant body temperature—one with a little leeway for special circumstances—is simply the best evolutionary choice for animals as complex as we are. A fluctuating brain temperature would lead to unpredictable reactions, ones that might not occur in the same sequence if the learning had taken place at a different brain temperature. The human brain and that of other mammals and of birds—these extraordinary tools—work as well as they do because of the protected, controlled environment they are housed in. Simpler animals, with far less complex brains, have optimized their survival possibilities in other ways, but constant temperature is best for us.

    Going back to my example of our ancestor trying to make decisions while being chased by a lion, he (or perhaps she) wouldn't want his arms to try climbing the tree while his legs endeavor to continue running, nor his eyes to see a rock while his nose thinks it's a lion. This is also not the time to decide it would be good to have a snack. Conversely, the lion is making exactly that decision, straining to ensure, however, that he or she is chasing an early human rather than something indigestible. Chances of survival and passing your genes on to the next generation increase as the mechanisms for multiple, simultaneous decisions and actions become flexible and rapid, yet reliable. A constant-temperature brain seems to facilitate this process.

    I don't mean to suggest by this example that the elemental predator/prey interaction requires an intricate constant-temperature brain, That's clearly not true. The brains of organisms such as humans and lions operate best at constant temperature, but the reasons for such complex brains lie in the extraordinarily intricate actions, many of them social and organizational, that these creatures carry out during their lifetimes.

    Nor is constant brain temperature the only reason for homeothermy. Chemical reactions generally proceed faster as temperature rises, so a higher body internal thermostat setting affords greater activity—up to a point. When excess heat cannot be shed and information is coming too quickly, the system breaks down. Over the past few million years, we, as well as other mammals and, of course, birds, seem to have found that we function most effectively in the vicinity of 100 degrees Fahrenheit.

    A physiologist friend of mine told me to think about sexual behavior when faced with a puzzle about animal behavior. The hormonal reactions that control mating, procreation, and countless other commands work best at a high constant temperature in warm-blooded animals. We can even look to temperature for the answer to such fine-tuning questions as why do males have testicles in external scrotal sacs rather than in the more protected abdominal cavity. Presumably a somewhat lower temperature than 98.6 degrees is favored for sperm production.

    Given that the human body works most reliably if kept at a constant temperature, why is that temperature 98.6 degrees? The rough answer is a mixture of evolutionary arguments combined with a simple understanding of how our metabolism works. Most machines are quite inefficient and mammalian bodies are no exception. Typically, more than 70 percent of the energy input into the body is converted into heat. This heat then needs to be dissipated into the environment, or else the body, like any overcharged engine, becomes overheated and stops functioning properly. We feel most comfortable in external temperatures some 20 to 30 degrees below our skin's temperature, because that differential produces a comfortable rate of heat loss; any colder and we lose heat too rapidly, any warmer and we retain too much. We correct for the former by adding clothes, blankets, and muscular activity like shivering; we correct for the latter by sweating, fanning, and, when circumstances allow, just taking it easy.

    The mechanisms of heat production are intricate, another complication for the brain to oversee. While at rest, the brain and the internal organs, such as the heart, lungs, and kidneys, produce more than two-thirds of the body's heat even though they constitute well under 10 percent of body mass. In motion, the heat output by muscles can increase by a factor of ten, coming to dominate all other sources. Yet despite these dramatic changes in heat output, body temperature remains fairly constant and basic instinctive responses remain predictable. This is achieved by the body's ability to rapidly increase its transfer of heat to its surroundings while its own internal heat production is rising.

    The mechanisms for heat transfer are elaborate in detail, but the basic physics principle is that heat always flows from hotter to colder objects. All objects radiate and absorb heat, more efficiently if they are dark and less if they are light: you are cooled in a large room with stone walls that are below body temperature and warmed if those same walls are above body temperature.

    Conduction of heat is simply a variation of the same statement as applied to two objects touching one another; heat always flows from the hotter to the colder object. In both radiation and conduction, the rate of the flow, at least for temperature differences that are not too large, is roughly proportional to that difference—a rule that is referred to in older books as Newton's Law of Heat Flow. You lose heat to a metal bar in your hand twice as rapidly if the bar is at 78 degrees as you do if the bar is at 88 degrees because one is 20 degrees away from 98.6 while the other differs by only 10 degrees. A room with stone walls at 58 degrees feels colder than the same room with walls at 78 degrees.

    It is sometimes argued that our body temperature is set at 98.6 degrees for the same reason that we feel comfortable in a room at 70 degrees. A little over two million years ago, humans emerged in Africa in sites where the median daily temperature is in the low 70s. Thus, a body temperature in the high 90s optimizes the necessary dissipation of heat generated by metabolic processes during the kind of activities hunter-gatherers pursued in those climates. You can calculate the rate at which the body produces heat during normal activities, and you can also calculate the rate at which the body transfers heat to an environment in the low 70s. Both rates depend on body temperature: a simple estimate shows the two roughly coincide when that temperature is 98.6, the point where heat in equals heat out. Later humans extended their range of thermal tolerance to cold weather by wearing furs and by the unique skill they acquired—making fires.

    This adaptation to climate, however, can be at best only a small part of the reason for our 98.6-degree body temperature. Almost all other birds and mammals, with presumably very different evolutionary histories, have stabilized at more or less the same temperature. The main reasons for homeothermy are lodged in the optimization of the complicated set of chemical reactions that allow us and these other animals to carry out the complicated activities of our lives.

(Continues...)

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Excerpted from A MATTER OF DEGREES by Gino Segrè. Copyright © 2002 by Gino Segrè. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

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