Figure 1. Variations of internal temperature based on the outside environment (according to Jürgen Aschoff; adapted from his own illustration)
The Temperature of the Human Body
Before studying the characteristics of fever, it is important to first have an understanding of how the body regulates its temperature.
HOMEOTHERMIA
Humans are homeothermic, which means that their body temperature should always remain the same with almost unvarying consistency. In other words, it does not change in accordance with the weather or with atmospheric conditions. Whether it is summer or winter, whether people live in the desert or the far north, their body temperature will always stay around 98.6°F. This is the core temperature (inside the body) and not the more superficial reading of 97.7°F provided by taking the temperature with a thermometer placed beneath the arm. In the animal kingdom the other homeotherms are mammals such as goats and cows and so forth, as well as birds.
The word homeotherm is used to describe the opposite of the poikilotherm. Poikilothermic animals’ body temperature varies in accordance with the ambient temperature of their environment. The temperature of these animals will rise when the sun is shining and the weather is hot, such as during the summer, for example, and will fall when it is cloudy, during the night, and during the cold seasons of the year.
One well known example can be used to illustrate these variations of body temperature in response to climatic condition: the lizard. When stretched over a stone, basking in the sun, its body temperature can rise higher than 104°F. During colder periods of the year, the lizard’s temperature can fall to 68°F, 50°F, or even 41°F, depending on the temperature of its environment. Given that the speed of its metabolism is dependent on its internal heat, in summer the lizard can flee with lightning speed, but when the weather turns cold it moves so slowly that it can be trapped without any difficulty.
Some animals, such as hibernating mammals, combine both systems: they are homeotherms in the spring and summer, but with the arrival of winter they fall into the slumber of hibernation and their body temperature matches that of their environment.
It was once common to make a distinction between warm-blooded and cold-blooded animals, but this terminology has been abandoned since the lizard, for example, theoretically a cold-blooded animal, can have a body temperature much higher than we humans who are technically warm-blooded beings!
The fact that they are capable of producing the heat necessary to maintain a consistent body temperature gives homeotherms—therefore human beings—a remarkable kind of freedom and independence. No matter what the conditions outside might be, homeothermic beings are able to move about, take care of their daily business, and defend themselves against danger.
While there are numerous advantages to being a homeotherm, there are also several drawbacks. First and foremost, homeotherms can tolerate only minimal variations in body temperature. At several degrees higher than the norm, up to 109.4°F to be exact, the mechanisms sustaining life in the body will be destroyed thus bringing on death. When the core temperature drops several degrees below the norm, to 86°F, in fact, vital functions will be paralyzed, also causing death.
The ideal temperature for the functioning of the human body is 98.6°F. This is the temperature required for the enzymes to diligently perform all the biochemical transformations necessary to organic life. Just like the chemist in the laboratory will heat the test tube when trying to combine two substances that are non-miscible in the ambient conditions, the body needs a certain degree of heat in order to function. Countless chemical substances need to be transformed, combined, separated, and recycled in the body for energy production to take place, as well as the building and repair of tissue, respiration, digestion, and so forth. The temperature of 98.6°F is also necessary for the organs to function at their optimum levels. Thanks to heat, our bodies are able to function, and, by functioning, our bodies produce the heat that helps them perform all their activities.
Because our body temperature is always stable, and without any effort required on our behalf to consciously maintain it, we often do not realize just how important it is. Yet heat is always associated with life and the living and cold is forever associated with death. A certain internal heat is essential for our survival.
Body heat is produced in multiple ways. Let’s begin by looking at the most obvious ways: physical activity and digestion.
The contraction of muscles releases heat; for, like all machines, whether a car or a washing machine, the rubbing of different parts against one another produces heat. Added to this heat production is that of the “combustion” of sugars in the muscles. By combustion I mean the transformation of sugar into energy that can be utilized by the muscles. This is how any physical activity—walking, carrying, working, speaking—can produce heat.
The digestive process also provides heat to the body. On the one hand this is because an entire series of organs—the stomach, liver, pancreas, and intestines—is working and therefore releasing heat. On the other hand it is because the foods that are eaten also supply it. Their constituent substances are “burned” by the body and transformed into caloric energy. Foods that are cooked and eaten while still warm will provide additional heat to the body.
Even in the absence of movement and digestion, such as at night when we are sleeping, our bodies continue to produce heat. This heat comes from the billions of cells making up our functioning organism. Vital functions such as breathing and blood circulation have to be maintained without stop in order to keep the body alive, even if they are taking place at a slower pace. This minimal activity (life could not be sustained at a lower rate) is what we call the basic metabolism.
Respiration contributes to the production of heat inasmuch as the ceaseless movements of inhalation and exhalation within the rib cage are triggered by the work of the different muscles.
Blood circulation occurs thanks to the beating of the heart, which provides an uninterrupted source of heat for the body. When one part of the heart contracts to propel blood into the vessels, the other part is working to aspirate blood toward the heart. In addition to these heat sources is the friction of the blood as it travels along the walls of the vessels and that of the air in the respiratory tract. Both of these frictions produce heat. Their level of heat production becomes even more considerable when their speed of movement is increased, for example, because of sustained physical activity.
The heat produced by the basic metabolism is added to that produced by the other organs and that generated by the muscles during physical exercise or by the digestive tract when we are eating. The allocation of this heat is then governed by the blood, which, thanks to its incessant circulatory movement, transports this heat produced in each specific organ throughout the body.
Opposing this constant production of heat is the loss of that heat. This is a permanently occurring situation by virtue of the fact that human beings live in environments that are most often cooler than the average temperature of the human body.
In the temperate zones of the earth, the average temperature outside is 77°F in the summer and 40°F during the winter, therefore varying from around 21 to 56 degrees cooler than that of our bodies. Even inside our houses the temperature is cooler, most thermostats being set between 68°F and 72°F.
It is thus inevitable for our bodies to lose heat because of environmental circumstances even though we dress more or less warmly depending on the season. This expression “warmly” is not entirely accurate since the clothing is not producing any heat, it is preventing heat from leaving the body.
In order for a naked human body to avoid losing heat to the ambient environment, the temperature should be around 89°F to 90°F. This is rarely the case and when it does occur is only temporary. So, as a rule, we are in a situation of losing heat on a constant basis.
REGULATION OF TEMPERATURE
When there is too much heat loss, the body has a variety of steps it can take for quickly correcting the problem.
One of the body’s short-term measures for producing heat involves collecting the heat created on the body’s surface through the contraction of muscles called the arrectores pilorum (the hair erector muscles). Although quite small, these muscles are large in quantity and can produce a measureable amount of heat on the skin surface. Their contraction creates the effect we know as goose bumps. Shivering, shaking limbs, and chattering teeth are all additional defense system reactions for confronting cold conditions. They contribute to the maintenance of body temperature through the heat they produce.
These measures are superficial, though, and aid from a deeper defense system must also be called upon. This consists of the body’s acceleration of its metabolism. By intensifying the cardiac rhythm and the amplitude of respiration, combustions increase and increased heat is supplied to the body. This metabolic acceleration is achieved by means of the nervous and hormonal systems, the thyroid and adrenal glands in particular. It can be set in motion at any time and its intensity regulated based on the body’s needs. The major director of these operations is the hypothalamus.
While it is easy for the body to intensify its metabolic activities to combat the cold, it is not, in contrast, so easy for it to reduce them to protect itself from excessive heat. In fact, it is impossible for the body to slow organic activity any lower than the basic metabolism rate that is essential for the body’s survival.
Because the body has no efficient means for reducing its naturally occurring production of heat, its one remaining option is to increase the amount of heat that the body naturally loses. It manages this through cutaneous vasodilation. By dilating the blood vessels at the surface of the skin and bringing more blood charged with this excess heat from the depths of the body to the periphery, the body encourages heat loss. An increase in pulmonary ventilation (breathing) will also achieve this goal. The intensification of respiratory movements makes it possible to expel a larger volume of heat-charged air and to inhale a larger quantity of fresh air, which will cool the body. This process is especially visible in dogs who will pant with their mouths wide open and their tongues hanging out when they are too hot. Dogs, in fact, hardly sweat and have to regulate their body temperature through their respiratory tracts.
Mentioning sweat leads naturally to the third means the body has for eliminating excess heat. Through evaporation, the small drops of sweat secreted by the sudoriferous (sweat) glands cool down the skin. In fact, in order to evaporate, these drops have to remove heat from the body. The more one perspires, the more this perspiration evaporates and the greater the amount of heat removed from the body.
PERIPHERAL TEMPERATURE
Saying that the body’s temperature is 98.6°F can give the impression that this temperature is uniform throughout the entire body. However, this is not possible; there are necessarily some regions that are less hot than others.
The cooler regions of the body are those that are in contact with the outside, where the loss of heat caused by the body’s contact with a colder environment takes place. The hottest region of the body is where the heat-producing organs are located, which is to say, inside the body. The temperature of this region remains constant because this part of the body is surrounded and protected by the body’s surface layers. The temperature of the outer portions of the body will change slightly depending on the conditions of the outside environment.
The temperature of 98.6°F that has been designated as that of the human body is that of the interior of the human body taken as a whole. (Some organs, the liver, for example, have a higher temperature.) For this reason, this temperature is also called the core temperature, in contrast to the peripheral temperature of the body’s outer layer.
The core of the human body consists of:
It should be emphasized that the closer one moves toward the limbs’ extremities, the smaller the core zone becomes, eventually vanishing completely in the hands and feet. The peripheral zone of the body, in which the temperature may vary, consists of the skin and the tissue lying directly under it. This zone is therefore quite thin but since it covers the entire body it still represents a large mass.
The temperature of the body’s peripheral layer varies as a result of two factors: the volume of the core zone it surrounds and the outside temperature. When the outside temperature is, for example, 68°F, the peripheral temperature will be 96.8°F at the level of the torso but will fall to 93.2°F and 89.6°F respectively at the thighs and biceps and 87.8°F and 82.4°F at the shinbones and forearms (see figure 1b on page 16).
If the outside temperature falls to 32°F, the temperature of the body’s peripheral layers will continue to fall but not beyond a certain physiological limit. Protective mechanisms in the body will work to keep the temperature up and preserve life in the tissue. In contrast, when the outside temperature climbs higher, to 104°F, for example, the core zone will increase, extending all the way out to the periphery instead of stopping a certain distance away (see figure 1c on page 16).
Figure 1. Variations of internal temperature based on the outside environment (according to Jürgen Aschoff; adapted from his own illustration)
MEASURING BODY TEMPERATURE
The measure of a patient’s body temperature with a thermometer allows the caregiver to verify if he or she is “running a high temperature,” in other words a fever, and how high it is. Fever, to put it plainly, occurs anytime the body’s temperature is higher than its normal state. By taking repeated readings over the course of the day, the caregiver can follow the fever’s evolution and determine whether it is rising or falling.
There are two kinds of thermometers commonly available. The most simple, and completely effective nonetheless, consists of a graduated tube containing liquid. Formerly this liquid was always mercury. Today, for safety reasons, as mercury is highly toxic, it has been replaced by colored alcohol. With this type of thermometer, after the temperature reading has been taken you must shake the tube to make the liquid fall back to the bottom and be ready for the next reading. The more recent kind of thermometer is electronic; it works on batteries and has a flexible mouthpiece. A digital screen indicates the body temperature to the nearest fraction.
As discussed previously, the core temperature of the human body (98.6°F) is not identical to that of the more superficial zones that can be reached by the standard thermometer. The temperature will be slightly cooler when the reading is taken close to the surface of the body. The body’s normal temperature when measured on the surface is 97.7°F and the onset of fever is marked as 98.6°F1 although this is a perfectly normal temperature for the core zone.
There are a number of natural cavities on the body where the temperature can be taken with a thermometer. The armpit, mouth, rectum, and inner ear cavities allow you to get beyond the surface of the body. These areas are also in the proximity of significant blood vessels, which transport blood of the same temperature as the core zone.
The axillary (armpit) temperature reading is the most commonly used as it is the easiest to execute. It is not as intrusive as the other options, and it is suitable for every age group except that of small children and infants. The thermometer should be placed in the base of the armpit with the arm closed around it so that the tip of the thermometer is completely enveloped, which will bring the temperature reading closer to that of the body’s core temperature. The normal temperature of this area is between 96.6°F and 98°F.
An oral temperature reading is obtained by placing the head of the thermometer in the mouth at the back of the cavity found beneath the tongue. The patient must keep his mouth closed and avoid drinking or eating anything hot for the half hour preceding the temperature reading. This method is rarely usable for infants but is perfectly suitable for older children and adults. The normal temperature for this area is between 97.8°F and 98.4°F.
A rectal temperature reading is much closer to that of the core temperature, because the measurement is taken at a deeper level in the body. The thermometer should be inserted 2 to 3 centimeters into the rectum. The use of a lubricating agent is recommended. It is essential to use an unbreakable thermometer here or one with a flexible tip, especially when this method is used on babies. The normal temperature in this area is between 98.4°F and 99.1°F. Because of the more invasive nature of this procedure, it is no longer one of the most commonly used methods.
Ear temperature is almost identical to the body’s core temperature. A large vessel from deep within the body passes in close proximity to the tympanic membrane (eardrum) carrying blood to this area directly from the center of the body. The eardrum is also close to the hypothalamus, the major regulating center of body temperature. To take a person’s temperature in the ear, you must use a special thermometer designed specifically for this purpose, and it must be placed exactly on the eardrum and not on the walls of the ear canal.
Locations for Taking Thermometer Readings
• Armpit
• Mouth (beneath the tongue)
• Rectum
• Ear (on the eardrum)
During the course of a fever, the temperature should always be taken with the same thermometer, at the same spot, and at the same times in order for useful comparisons to be made. It is a good idea to take the temperature three times a day: once in the morning upon waking, once in the afternoon around 2 p.m. (before or after an afternoon nap), and once in the evening around 7 or 8 p.m. This final measurement is often the most revealing. Furthermore, it is helpful to know that fever often climbs after 5 p.m. In order to better monitor the evolution of the illness, it is sometimes necessary to take one or two additional readings. Doing so enables the caregiver to better interpret the changes in the patient’s condition.
Summary
The ideal temperature for the human body, the one that allows biochemical transformations to occur without problem and for optimum organ function, is 98.6°F. The body works continuously to maintain this temperature.