Body Temperature

Body Temperature

The human body maintains a relatively constant temperature of about 98.2°F (36.8°C). This temperature fluctuates during the course of day because of natural biological rhythms and other influences, such as the level of activity, but unless a person is sick, the body’s temperature stays within a degree or so of nor- mal. The normal temperature is not quite the same for all people, and although this temperature is often given as 98.6°F (37°C)—a number determined in the 19th century from a sample of thou- sands of patients—more recent measurements show that many people, particularly older adults, have a slightly lower average temperature.

Temperature is a measure of the motion of atoms and mole- cules, and this motion influences the speed at which most chemical reactions proceed. A chemical reaction usually involves the inter- action of two or more molecules, which often must meet or come together in order for the reaction to occur. Higher temperatures mean greater motion, which favors chemical reactions because it brings atoms and molecules together more often. Chemistry is crucial to all living organisms, because life involves a tremendous number of chemical reactions: breaking down food molecules, maintaining and repairing tissue, signaling and communication among cells, and many others.

Another reason body temperature is strictly maintained involves the nature of proteins. Proteins are large molecules that perform many different functions in the body. In most cases, a protein must assume a certain shape or form before it can function properly. But at high temperatures, proteins begin to lose their shape—they “unfold” because their constituent atoms and molecules are jig- gling around too much to stay in place. This is similar to the melt- ing of ice; by analogy, it can be said that at high temperatures

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proteins “melt” into a different state, and in this state, they are unable to do their job.

The zeroth law of thermodynamics says that thermal equilib- rium should eventually be reached between objects that are able to transfer heat by conduction, convection, or radiation. Room temperature is usually around 68–75°F (20–23.9°C), and the air temperature outside varies but is not often much warmer than 90°F (32.2°C). Human beings are able to maintain a body tem- perature higher than that, which requires a heat source—otherwise the human body would fall to the temperature of its environment. This heat source will be discussed in a later section.

But the whole body need not be maintained at “body tempera- ture.” A person’s body temperature generally refers to the core temperature, the temperature inside the body. The skin tempera- ture is usually about 10°F (5.5°C) less than 98.2°F (36.8°C) and on cool days can get much colder. The reason has to do with ther- modynamics. Since at least a small portion of skin is almost always exposed to air, the important heat transfer mechanism of convec- tion is operating on it (along with radiation, and possibly also some conduction). Some heat loss from the skin naturally occurs. This is not disastrous, as the skin need not be maintained at 98.2°F in order to function. If the skin had to be 98.2°F, this would be costly in terms of the body’s heat supply, considering the constant losses from convection and radiation. The skin and the fat underneath act like a layer of insulation, trapping the heat of the interior, but they do not need to be as warm as the internal organs.

The relative warmth of the body’s core temperature means that people can warm their hands by blowing on them with air expelled from the mouth. But this only works when the mouth is held open widely. Air blown from compressed lips expands and loses heat rapidly, so it feels cool.

Since the skin is normally cooler than the interior of the body, nurses and physicians must be careful when they measure

a patient’s temperature. The important temperature is the core body temperature, not the skin temperature. Health-care providers place the thermometer in the patient’s mouth or in other locations that are not continuously exposed to air and room temperature.

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Devices other than thermometers can detect the body’s warmth. Thermal imagers can see warm objects, such as people, by the amount of emitted infrared radiation. Sensitive imagers can detect the presence of people even if they are concealed by cooler objects, such as behind walls or underneath a pile of rubble. The imagers benefit rescuers by helping them find survivors at night or in col- lapsed buildings and mining disasters.

Although the body makes every effort to maintain its tempera- ture within a narrow range, occasionally it fails to do so. Because temperature is so important, it does not take a change of many degrees to have a severe, even life-threatening impact.

A drop in body temperature is called hypothermia (hypo means under or less). Hypothermia does not often occur in people who are adequately bundled up, unless they are outside in winter for long periods of time. But the polar regions are so cold that sur- vival becomes a challenge, as the temperature can drop to –70°F (–56.7°C) or lower. (The record low is –126°F, or approximately –87.8°C.)

Hypothermia is also a big problem for people who become immersed for a period of time in cold water. Water has a high heat capacity, which means it can act like a heat “sponge.” A person floating in a cold ocean will become chilled very quickly as the body rapidly loses heat to the water. In cold water such as the north Atlantic Ocean, which is often only a few degrees above freezing, unprotected shipwreck survivors cannot last more than a few hours. The RMS Titanic disaster claimed a lot of lives in this way.

At the other extreme, higher temperatures (or hyperthermia) are not welcome either. Unlike hypothermia, however, high tem- peratures in the form of fevers occur quite often.

Fevers are typically part of the body’s way of fighting an infec- tion, which is caused by the invasion of some sort of microor- ganism. Certain substances flowing in the bloodstream produce a fever by their activity on a small part of the brain called the hypo- thalamus. The hypothalamus is important in many physiological processes, including the regulation of body temperature. During

a fever, the hypothalamus causes the body’s core temperature to gain a few degrees.

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Although a fever is uncomfortable, it seems to serve a purpose— it is the body’s attempt to create a less hospitable environment for the invading microorganism. A fever is also associated with increased activity of the immune system, the body’s main defense against invaders. But a fever of more than a few degrees above normal can be dangerous. High fevers can produce abnormal brain activity called seizures; one of the reasons these seizures occur is because the high temperature stimulates the chemical reactions in the brain to reach a rate that is too great for proper functioning.

Another type of unusually high body temperature is heatstroke. Heatstroke is particularly dangerous because it is a result of high temperatures that the body cannot control, such as becoming over- heated from strenuous activity or being trapped in a hot area. Small children or pets stuck in a car with rolled-up windows on

a sunny day will quickly overheat, because the car’s “greenhouse effect” sends the temperature inside the car soaring.