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Body Temperature Regulation and Fever
Normal Body Temperatures
Body Core Temperature and Skin Temperature
The temperature of the deep tissues of the body—the “core” of the body—usually remains very constant, within ±1°F (±0.6°C), except when a person has a febrile illness. Indeed, a nude person can be exposed to temperatures as low as 55°F or as high as 130°F in dry air and still maintain an almost constant core temperature. The mechanisms for regulating body temperature represent a beautifully designed control system. In this chapter we discuss this system as it operates in health and in disease.
The skin temperature, in contrast to the core temperature, rises and falls with the temperature of the surroundings. The skin temperature is important when we refer to the skin’s ability to lose heat to the surroundings.
Normal Core Temperature
No single core temperature can be considered normal because measurements in many healthy people have shown a range of normal temperatures measured orally, as shown in Figure 74-1 , from less than 97°F (36°C) to greater than 99.5°F (37.5°C). The average normal core temperature is generally considered to be between 98.0°F and 98.6°F when measured orally and about 1°F higher when measured rectally.
The body temperature increases during exercise and varies with temperature extremes of the surroundings because the temperature regulatory mechanisms are not perfect. When excessive heat is produced in the body by strenuous exercise, the temperature can rise temporarily to as high as 101°F to 104°F. Conversely, when the body is exposed to extreme cold, the temperature can fall below 96°F.
Body Temperature is Controlled by Balancing Heat Production and Heat Loss
When the rate of heat production in the body is greater than the rate at which heat is being lost, heat builds up in the body, and the body temperature rises. Conversely, when heat loss is greater, both body heat and body temperature decrease. Most of the remainder of this chapter is concerned with this balance between heat production and heat loss and the mechanisms by which the body controls this production and loss.
Heat Production
Heat production is a principal by-product of metabolism. In Chapter 73 , which summarizes body energetics, we discuss the different factors that determine the rate of heat production, called the metabolic rate of the body . The most important of these factors are listed again here: (1) basal rate of metabolism of all the cells of the body; (2) extra rate of metabolism caused by muscle activity, including muscle contractions caused by shivering; (3) extra metabolism caused by the effect of thyroxine (and, to a lesser extent, other hormones, such as growth hormone and testosterone) on the cells; (4) extra metabolism caused by the effect of epinephrine, norepinephrine, and sympathetic stimulation on the cells; (5) extra metabolism caused by increased chemical activity in the cells, especially when the cell temperature increases; and (6) extra metabolism needed for digestion, absorption, and storage of food (thermogenic effect of food).
Heat Loss
Most of the heat produced in the body is generated in the deep organs, especially the liver, brain, and heart, and in the skeletal muscles during physical activity. This heat is then transferred from the deeper organs and tissues to the skin, where it is lost to the air and other surroundings. Therefore, the rate at which heat is lost is determined almost entirely by two factors: (1) how rapidly heat can be conducted from where it is produced in the body core to the skin and (2) how rapidly heat can then be transferred from the skin to the surroundings. Let us begin by discussing the system that insulates the core from the skin surface.
Insulator System of the Body
The skin, the subcutaneous tissues, and especially the fat of the subcutaneous tissues act together as a heat insulator for the body. The fat is important because it conducts heat only one-third as readily as other tissues. When no blood is flowing from the heated internal organs to the skin, the insulating properties of the normal male body are about equal to three-quarters the insulating properties of a usual suit of clothes. In women, this insulation is even better.
The insulation beneath the skin is an effective means of maintaining normal internal core temperature, even though it allows the temperature of the skin to approach the temperature of the surroundings.
Blood Flow to the Skin From the Body Core Provides Heat Transfer
Blood vessels are distributed profusely beneath the skin. Especially important is a continuous venous plexus that is supplied by inflow of blood from the skin capillaries, shown in Figure 74-2 . In the most exposed areas of the body—the hands, feet, and ears—blood is also supplied to the plexus directly from the small arteries through highly muscular arteriovenous anastomoses .
The rate of blood flow into the skin venous plexus can vary tremendously, from barely above zero to as great as 30% of the total cardiac output. A high rate of skin flow causes heat to be conducted from the body core to the skin with great efficiency, whereas reduction in the rate of skin flow can decrease heat conduction from the core to very little.
Figure 74-3 shows quantitatively the effect of environmental air temperature on conductance of heat from the core to the skin surface and then conductance into the air, demonstrating an approximate eightfold increase in heat conductance between the fully vasoconstricted state and the fully vasodilated state.
Therefore, the skin is an effective controlled “heat radiator” system , and the flow of blood to the skin is a most effective mechanism for heat transfer from the body core to the skin.
Control of Heat Conduction to the Skin by the Sympathetic Nervous System.
Heat conduction to the skin by the blood is controlled by the degree of vasoconstriction of the arterioles and the arteriovenous anastomoses that supply blood to the venous plexus of the skin. This vasoconstriction is controlled almost entirely by the sympathetic nervous system in response to changes in body core temperature and changes in environmental temperature. This is discussed later in the chapter in connection with control of body temperature by the hypothalamus.
Basic Physics of Heat Loss From the Skin Surface
The various methods by which heat is lost from the skin to the surroundings are shown in Figure 74-4 . They include radiation, conduction , and evaporation , which are explained next.
Radiation Causes Heat Loss in the Form of Infrared Rays.
As shown in Figure 74-4 , in a nude person sitting inside at normal room temperature, about 60% of total heat loss is by radiation.
Most infrared heat rays (a type of electromagnetic ray) that radiate from the body have wavelengths of 5 to 20 micrometers, 10 to 30 times the wavelengths of light rays. All objects that are not at absolute zero temperature radiate such rays. The human body radiates heat rays in all directions. Heat rays are also being radiated from the walls of rooms and other objects toward the body. If the temperature of the body is greater than the temperature of the surroundings, a greater quantity of heat is radiated from the body than is radiated to the body.
Conductive Heat Loss Occurs by Direct Contact With an Object.
As shown in Figure 74-4 , only minute quantities of heat, about 3%, are normally lost from the body by direct conduction from the surface of the body to solid objects, such as a chair or a bed. Loss of heat by conduction to air , however, represents a sizable proportion of the body’s heat loss (≈15%), even under normal conditions.
Heat is actually the kinetic energy of molecular motion, and the molecules of the skin are continually undergoing vibratory motion. Much of the energy of this motion can be transferred to the air if the air is colder than the skin, thus increasing the velocity of the air molecules’ motion. Once the temperature of the air adjacent to the skin equals the temperature of the skin, no further loss of heat occurs in this way because now an equal amount of heat is conducted from the air to the body. Therefore, conduction of heat from the body to the air is self-limited unless the heated air moves away from the skin , so new, unheated air is continually brought in contact with the skin, a phenomenon called air convection .
Convective Heat Loss Results From Air Movement.
Heat from the skin is first conducted to the air and then carried away by the convection air currents.
A small amount of convection almost always occurs around the body because of the tendency for air adjacent to the skin to rise as it becomes heated. Therefore, in a nude person seated in a comfortable room without gross air movement, about 15% of his or her total heat loss occurs by conduction to the air and then by air convection away from the body.
Cooling Effect of Wind.
When the body is exposed to wind, the layer of air immediately adjacent to the skin is replaced by new air much more rapidly than is normal, and heat loss by convection increases accordingly. The cooling effect of wind at low velocities is about proportional to the square root of the wind velocity . For example, a wind of 4 miles per hour is about twice as effective for cooling as a wind of 1 mile per hour.
Conduction and Convection of Heat From a Person Suspended in Water.
Water has a specific heat several thousand times as great as that of air, so each unit portion of water adjacent to the skin can absorb far greater quantities of heat than can be absorbed by air. Also, heat conductivity in water is very great in comparison with that in air. Consequently, it is impossible for the body to heat a thin layer of water next to the body to form an “insulator zone” as occurs in air. Therefore, if the temperature of the water is below body temperature the rate of heat loss to water is usually many times greater than the rate of heat loss to air.
Evaporation
When water evaporates from the body surface, 0.58 Calorie (kilocalorie) of heat is lost for each gram of water that evaporates. Even when a person is not sweating, water still evaporates insensibly from the skin and lungs at a rate of about 600 to 700 ml/day. This insensible evaporation causes continual heat loss at a rate of 16 to 19 Calories per hour. Insensible evaporation through the skin and lungs cannot be controlled for purposes of temperature regulation because it results from continual diffusion of water molecules through the skin and respiratory surfaces. However, loss of heat by evaporation of sweat can be controlled by regulating the rate of sweating, which is discussed later in this chapter.
Evaporation is a Necessary Cooling Mechanism at Very High Air Temperatures.
As long as skin temperature is greater than the temperature of the surroundings, heat can be lost by radiation and conduction. However, when the temperature of the surroundings becomes greater than that of the skin, instead of losing heat, the body gains heat by both radiation and conduction. Under these conditions, the only means by which the body can rid itself of heat is by evaporation .
Therefore, anything that prevents adequate evaporation when the surrounding temperature is higher than the skin temperature will cause the internal body temperature to rise. This phenomenon occurs occasionally in human beings who are born with congenital absence of sweat glands. These people can tolerate cold temperatures as well as normal people can, but they can become severely stressed and die of heatstroke in tropical zones because, without the evaporative refrigeration system, they cannot prevent a rise in body temperature when the air temperature is greater than that of the body.
Clothing Reduces Conductive and Convective Heat Loss.
Clothing entraps air next to the skin in the weave of the cloth, thereby increasing the thickness of the so-called private zone of air adjacent to the skin and decreasing the flow of convection air currents. Consequently, the rate of heat loss from the body by conduction and convection is greatly depressed. A usual suit of clothes decreases the rate of heat loss to about half that from the nude body, but arctic-type clothing can decrease this heat loss to as little as one sixth.
About half the heat transmitted from the skin to the clothing is radiated to the clothing instead of being conducted across the small intervening space. Therefore, coating the inside of clothing with a thin layer of metal, such as silver or gold, which reflects radiant heat back to the body, makes the insulating properties of clothing far more effective than otherwise. By using this technique, clothing for use in the arctic can be decreased in weight by about half.
The effectiveness of clothing in maintaining body temperature is almost completely lost when the clothing becomes wet because the high conductivity of water increases the rate of heat transmission through cloth 20-fold or more. Therefore, one of the most important factors for protecting the body against cold in arctic regions is extreme caution against allowing the clothing to become wet. Indeed, one must be careful not to become overheated even temporarily because sweating in one’s clothes makes them much less effective thereafter as an insulator.
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