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58 Regulation of Body TemperatureEthan Nadel

HEAT AND TEMPERATURE: THE ADVANTAGES OF HOMEOTHERMY

Homeothermy Enables an Organism to Maintain Its Activity Over a Wide Range of Environmental

Temperatures

The ability to regulate internal body temperature has provided higher organisms independence from the

environment. Because the rates of most physical and chemical reactions depend on temperature, mostphysiological functions are sensitive to temperature changes. Thus, the activity levels of poikilotherms(species that do not regulate internal body temperature) by and large depend on environmental

temperature, whereas those of homeotherms (species that do regulate internal body temperature) arerelatively stable over a broad range of ambient conditions. A lizard, for example, is capable of relatively lessmovement away from its lair on a cold, overcast day than on a hot, sunny day, whereas a prairie dog maybe equally mobile on either day. An arctic fox acclimatizes to the extreme cold of winter by growing a thick,insulating coat that enables it to resist body cooling and minimize the necessity to increase metabolic heatgeneration, which requires increased food intake.

The thermoregulatory mechanism of homeotherms creates an internal environment in which reaction ratesare relatively high and optimal. At the same time, an effective thermoregulatory system avoids the

pathologic consequences of wide deviations in body temperature (Table 58-1). The thermoregulatorysystem incorporates both anticipatory controls and negative-feedback controls. The components of thissystem are: (1) thermal sensors, (2) afferent pathways, (3) an integration system in the central nervoussystem (CNS), (4) efferent pathways, and (5) target organs that control heat generation and transfer, suchas skeletal muscle (e.g., shivering to generate heat), circulation to the skin (to dissipate heat), and thesweat glands (to dissipate heat).

The focus of this chapter is temperature regulation in homeotherms. We examine the physical aspects ofheat transfer both within the body and between body and environment, as well as the physiologicalmechanisms that are involved in altering the rates of transfer. Lastly, we look at the consequences ofextreme challenges to the thermoregulatory mechanism, such as hypothermia and dehydration.

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Table 58-1. CONSEQUENCES OF DEVIATIONS IN BODY TEMPERATURE

TEMPERATURE (°C) CONSEQUENCE

40-44 Heat stroke, brain lesions

38-40 Fever or exercise

36-38 Normal range

34-36 Mild hypothermia

30-34 Impairment of temperature regulation

27-29 Cardiac fibrillation

Body Core Temperature Depends on Time of Day, Physical Activity, Time in the Menstrual Cycle, and Age

Temperature scales are relative scales of heat content. The centigrade scale is divided into equalincrements, referenced to the freezing (0°C) and boiling (100°C) points of water. The "normal" bodytemperature of an adult human is approximately 37°C (i.e., 98.6°F) but may be as low as 36°C or as high as37.5°C in active, healthy people. "Body temperature" usually refers to the temperature of the body core,measured under the tongue (sublingually), in the ear canal, or in the rectum. For clinical purposes, the mostreliable (although the least practical) among these three is the last, because it is least influenced by air temperature. Measurement devices range from traditional mercury-in-glass thermometers to electronic,digital readout thermistors. Nearly all such instruments are accurate to 0.1°C. Body Core Temperature

Body core temperature depends on the time of day, the stage of the menstrual cycle in women, the level of

the person's activity, and the individual's age.

 All homeotherms maintain a circadian (24-h cycle) body-temperature rhythm, with variations ofapproximately 1°C. In humans, body temperature is usually lowest between 3:00 to 6:00 a.m., and peaks at

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3:00 to 6:00 p.m. This circadian rhythmicity is inherent in the autonomic nervous system, independent of thesleep-wakefulness cycle, but entrained by light-dark cues to a 24-h cycle.

In many women, body temperature increases approximately 0.5°C during the post-ovulatory phase of themenstrual cycle (p. 1159). An abrupt increase in body temperature of 0.3°C to 0.5°C accompanies ovulationand may be useful as a fertility guide.

Physical activity generates excess heat as a byproduct of elevated metabolic rate. A portion of this excess

heat remains in the body, causing core temperature to rise and triggering appropriate heat-loss responses.Core temperature remains elevated during activity and for an extended period during recovery.

Infants and older people are less able to maintain a "normal" body temperature than are the rest of thepopulation, particularly in the face of an external challenge. Newborns do not readily shiver or sweat andthus are more like poikilotherms than like homeotherms. These properties, along with a highsurface-to-mass ratio, render infants more susceptible to fluctuations in core temperature when exposed toa hot or cold environment. Older people are also subject to greater fluctuations in core temperature. Agingis associated with a progressive deficit in the ability to sense heat and cold, and also reduced ability togenerate heat (reduced metabolic rate and metabolic potential due to lower muscle mass) and to dissipateheat (reduced cardiovascular reserve and sweat-gland atrophy from disuse).

The Body's Rate of Heat Production Can Vary From Approximately 80 kcal/h at Rest to 600 kcal/h DuringJogging

The body's rate of heat production is closely linked to the rate of metabolism; minor variations occurdepending on the mixture of fuels (foods) being oxidized. Because of their inherent inefficiency, metabolictransformations generate heat (p. 1212). Ultimately, all of the energy contained in fuels appears as eitherheat, mass storage/growth, or physical work done on the environment.

The body's metabolic rate, and thus its rate of heat production, is not constant. The resting metabolic rate(RMR; see p. 1211) is the rate necessary to maintain the functions of resting cells; these functions includeactive transport as well as cardiac and respiratory-muscle activity. Voluntary or involuntary (e.g., shivering)muscular activity adds to the overall metabolic heat production. Even digesting a meal increases themetabolic rate (p. 1215). An increase in tissue temperature itself raises the metabolic rate, according to the

van't Hoff relation (i.e., a 10°C increase in tissue temperature more than doubles the metabolic rate).Furthermore, certain hormones, notably thyroxine and epinephrine, increase the cellular metabolic rate.Because the body's heat-production rate is variable, the rate of heat loss must match it if the bodytemperature is to remain constant.

Under sedentary (i.e., RMR) conditions, when the body's energy requirements are low, the rate of heat

production is approximately 80 kcal/h (∼90 watts) in an adult of average size. During physical exercise, therate of energy production-and hence, heat generation-increases in proportion to the intensity of exercise.

 An average adult can comfortably sustain an energy-production rate of 400 to 600 kcal/h for extendedperiods (e.g., a fast walk or a modest jog). Nearly all of this increased heat generation occurs in activeskeletal muscle, although a portion arises from increased cardiac and respiratory-muscle activity. A thermalload of this magnitude would raise core temperature by 1.0°C every 8 to 10 minutes if the extra heat could

not escape the body. Physical activity would be limited to 25 to 30 minutes, at which time the effects of excessive hyperthermia (>40°C) would begin to impair body function. This impairment, of course, does notoccur, primarily because of the efficiency of the thermoregulatory system. Within a relatively short period,the body increases its rate of heat dissipation to match heat production. Thereafter, the body maintains anew, elevated steady temperature until exercise ceases.

Printed from STUDENT CONSULT: Medical Physiology (on 28 August 2006)© 2006 Elsevier