Medical Physiology, 3rd Edition

Heat and Temperature: Advantages of Homeothermy

Homeotherms maintain their activities 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, most physiological functions are sensitive to temperature changes. Thus, the activity levels of poikilotherms (species that do not regulate internal body temperature) generally depend on environmental temperature, whereas homeotherms (species that do regulate internal body temperature) can engage in most normal activities independent of ambient temperature. A lizard, for example, is capable of relatively less movement away from its lair on a cold, overcast day than on a hot, sunny day, whereas a prairie dog may be equally mobile on either day. An arctic fox acclimatizes to the extreme cold of winter by maintaining a thick, insulating coat that enables it to resist body cooling and minimizes the necessity to increase metabolic heat generation, which would require increased food intake.

The stable body temperature of homeotherms is the consequence of designated neural networks that incorporate both anticipatory and negative-feedback controls. This arrangement creates an internal environment in which chemical reaction rates are relatively high and optimal and avoids the pathological consequences of wide fluctuations in body temperature (Table 59-1). The fundamental thermoregulatory system includes (1) thermal sensors; (2) thermosensory afferent pathways; (3) an integration system in the central nervous system (CNS); (4) efferent pathways; and (5) thermal effectors capable of heat generation (i.e., thermogenesis), such as brown adipose tissue and skeletal muscle (shivering), or effectors that modulate heat transfer, such as the circulation to the skin (which dissipates heat) and the sweat glands (which augment heat loss).

In this chapter, we describe the physical aspects of heat transfer both within the homeotherm's body and between the body and the environment. We also provide a framework for understanding the major integrative role played by the CNS in regulating body temperature and consider the physiological mechanisms involved in altering rates of heat transfer and in producing extra heat in a cold environment or during fever. Finally, we look at the consequences of extreme challenges to the thermoregulatory mechanism, such as hypothermia, hyperthermia, and dehydration.

Body core temperature depends on time of day, physical activity, time in the menstrual cycle, and age

Temperature is a measure of heat content. The “normal” body temperature of an adult human is ~37°C (98.6°F) but it may be as low as 36°C or as high as 37.5°C in active, healthy people. Body temperature usually refers to the temperature of the internal body core, N59-1 measured under the tongue (sublingually), in the ear canal, or in the rectum. For clinical purposes, the most reliable (although the least practical) among these three is the last, because it is least influenced by ambient (air) temperature. Measurement devices range from traditional mercury-in-glass thermometers to electronic digital-read-out thermistors. Nearly all such instruments are accurate to 0.1°C. The least invasive approach uses an infrared thermometer to measure the radiant temperature (see p. 1196) over the temporal artery. N59-1

Contributed by Ethan Nadel

As noted in the text, the term body temperature usually refers to the temperature of the body core. Like effective circulating volume, body core temperature is a concept that is difficult to define with precision. The body core is generally understood to refer to internal organs, including the central blood volume that equilibrates with the core. Thus, the core includes the brain (the location of the central temperature sensors, which are in the hypothalamus), the heart, and other organs that are insulated from the environment and that produce heat at a relatively constant rate. However, the body core clearly excludes potential heat generators whose heat output varies with time, such as skeletal muscle and liver. Likewise, the body core also excludes the skin and portions of the upper airways (see p. 600), which are at the mercy of the environment.

Body core temperature (T_core) depends on many factors that alter either the activity of the CNS thermoregulatory network or the level of metabolism and heat content of the body, including the time of day, the stage of the menstrual cycle in women, and the individual's age.

All homeotherms maintain a circadian rhythm (~24-hour cycle) of body temperature, with variations of ~1°C. In humans, body temperature is usually lowest between 3:00 and 6:00 AM and peaks at 3:00 to 6:00 PM. The circadian rhythmicity in physiological variables is governed by groups of neurons in the suprachiasmatic nucleus in the anterior hypothalamus, whose activity is entrained by light-dark cues to a ~24-hour cycle but is independent of the sleep-wake cycle. The influence of these neurons on the CNS thermoregulatory network produces the circadian rhythmicity in body temperature.

Reproductive hormones, and the CNS circuits that govern their production, influence the CNS thermoregulatory network. Indeed, in many women, body temperature increases ~0.5°C during the postovulatory phase of the menstrual cycle (see pp. 1110–1111). An abrupt increase in body temperature of 0.3°C to 0.5°C accompanies ovulation and may be useful as a fertility guide.

Infants and older people are less able than other age groups to maintain a stable normal body temperature, particularly in the face of external challenges. Newborns do not readily shiver or sweat and have a high surface-to-mass ratio, which renders them more susceptible to fluctuations in core temperature when exposed to hot or cold environments. However, they have large deposits of brown adipose tissue, which the sympathetic nervous system can stimulate to generate heat for cold defense. Moreover, newborns can implement a modest degree of sympathetic vasoconstriction of the skin to reduce heat loss in the cold.

Older people are also subject to greater fluctuations in core temperature. Aging is associated with a progressive deficit in the ability to sense heat and cold (see p. 1244), as well as reduced ability to generate heat (reduced metabolic rate and metabolic potential because of lower muscle mass) and to dissipate heat (reduced cardiovascular reserve and sweat gland atrophy from disuse).

The body's rate of heat production can vary from ~70 kcal/hr at rest to 600 kcal/hr during exercise

Because the chemical reactions are inefficient, cellular functions produce heat (see p. 1173). The body's rate of heat production depends on the rate of energy consumption and thus of O₂ consumption () because nearly all energy substrates derived from food are oxidized. Minor variations occur, depending on the mixture of fuels (foods) being oxidized, a process that determines the respiratory quotient (RQ; see p. 681 and Table 58-6).

The body's metabolic rate, and thus its rate of heat production, is not constant. The resting metabolic rate (RMR; see p. 1170) is the energy consumption necessary to maintain the basal functions of resting cells, such as active solute transport across membranes as well as the activity of cardiac and respiratory muscles necessary for organismal survival. RMR is influenced by age, sex, circadian phase, season, digestive state, phylogeny, body size, and habitat. Voluntary or involuntary (e.g., postural and shivering) muscular activity adds to the overall metabolic heat production. Even digesting a meal increases the metabolic rate (see p. 1179). 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. At an RQ of 0.8 (see p. 681 and Table 58-6), the average person under sedentary (i.e., RMR) conditions has a resting  of 250 mL/min, which corresponds to an energy production of 72 kcal/hr (~85 watts). Because the body at RMR, by definition, performs no work on the environment, all of this energy production ultimately is dissipated as heat—as if the body were a zero-efficiency, 85-watt incandescent light bulb.

During physical exercise, the rate of energy consumption—and hence, heat generation—increases in proportion to the intensity of exercise. An average adult can comfortably sustain an energy-consumption rate of 400 to 600 kcal/hr (e.g., a fast walk or a modest jog) for extended periods. Nearly all increased heat generation during exercise occurs in active skeletal muscle, although a portion arises from increased activity of cardiac and respiratory muscles. A thermal load 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 not usually occur, primarily because of the effectiveness of the thermoregulatory heat-defense system. Within a relatively short period, the increase in body temperature resulting from exercise leads to an increased rate of heat dissipation from the skin and the respiratory system commensurate with the rate of heat production. Thereafter, the body maintains a new, but slightly elevated, steady temperature. When exercise ceases, body temperature gradually decreases to its pre-exercise level.