Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 18

Hypothalamic Regulation of Hormonal Functions 283

range. In birds and mammals, the “warm-blooded”
(homeo-
thermic)
animals, a group of reflex responses that are primarily
integrated in the hypothalamus, operate to maintain body tem-
perature within a narrow range in spite of wide fluctuations in
environmental temperature. The hibernating mammals are a
partial exception. While awake they are homeothermic, but dur-
ing hibernation their body temperature falls.

NORMAL BODY TEMPERATURE

In homeothermic animals, the actual temperature at which the
body is maintained varies from species to species and, to a lesser
degree, from individual to individual. In humans, the traditional
normal value for the oral temperature is 37 °C (98.6 °F), but in
one large series of normal young adults, the morning oral tem-
perature averaged 36.7 °C, with a standard deviation of 0.2 °C.
Therefore, 95% of all young adults would be expected to have a
morning oral temperature of 36.3–37.1 °C (97.3–98.8 °F; mean ±
1.96 standard deviations; see Appendix). Various parts of the
body are at different temperatures, and the magnitude of the
temperature difference between the parts varies with the envi-
ronmental temperature (Figure 18–13). The extremities are
generally cooler than the rest of the body. The temperature of the
scrotum is carefully regulated at 32 °C. The rectal temperature is
representative of the temperature at the core of the body and var-
ies least with changes in environmental temperature. The oral
temperature is normally 0.5 °C lower than the rectal tempera-
ture, but it is affected by many factors, including ingestion of hot
or cold fluids, gum chewing, smoking, and mouth breathing.
The normal human core temperature undergoes a regular
circadian fluctuation of 0.5–0.7 °C. In individuals who sleep at

night and are awake during the day (even when hospitalized at

bed rest), it is lowest at about 6:00

AM

and highest in the eve-

nings (Figure 18–14). It is lowest during sleep, is slightly higher

in the awake but relaxed state, and rises with activity. In

women, an additional monthly cycle of temperature variation is

characterized by a rise in basal temperature at the time of ovu-

lation (Figure 25–38). Temperature regulation is less precise in

young children and they may normally have a temperature that

is 0.5 ° or so above the established norm for adults.

During exercise, the heat produced by muscular contraction

accumulates in the body and the rectal temperature normally

rises as high as 40 °C (104 °F). This rise is due in part to the

inability of the heat-dissipating mechanisms to handle the

greatly increased amount of heat produced, but evidence sug-

gests that in addition there is an elevation of the body tempera-

ture at which the heat-dissipating mechanisms are activated

during exercise. Body temperature also rises slightly during

emotional excitement, probably owing to unconscious tensing of

the muscles. It is chronically elevated by as much as 0.5 °C when

the metabolic rate is high, as in hyperthyroidism, and lowered

when the metabolic rate is low, as in hypothyroidism (Figure

18–14). Some apparently normal adults chronically have a tem-

perature above the normal range (constitutional hyperthermia).

HEAT PRODUCTION

Heat production and energy balance are discussed in Chapter

27. A variety of basic chemical reactions contribute to body
    heat production at all times. Ingestion of food increases heat
    production because of the specific dynamic action of the food
    (see Chapter 27), but the major source of heat is the contrac-
    tion of skeletal muscle (Table 18–3). Heat production can be
    varied by endocrine mechanisms in the absence of food intake
    or muscular exertion. Epinephrine and norepinephrine pro-
    duce a rapid but short-lived increase in heat production; thy-
    roid hormones produce a slowly developing but prolonged
    increase. Furthermore, sympathetic discharge decreases dur-
    ing fasting and is increased by feeding.
    A source of considerable heat, particularly in infants, is
    brown
    fat
    . This fat has a high rate of metabolism and its thermogenic
    function has been likened to that of an electric blanket.

FIGURE 18–13
Temperatures of various parts of the body of
a naked subject at various ambient temperatures in a calorimeter.
(Redrawn and reproduced, with permission, from Hardy JD, DuBois EF: Basal
metabolism, radiation, convection and vaporization at temperatures of 22–35 °C. J
Nutr 1938;15:477.)

37

36

35

34

33

32

31

30

29

28

27

26

25

24

23

22 23 24 25 26

Feet

Hands

27 28 29 30 31 3233 34

Calorimetric temperature (°C)

Temperature (

°C) of sub

ject Average

skin

Trunk

Head

Rectum

FIGURE 18–14

Typical temperature chart of a hospitalized

patient who does not have a febrile disease.

Note the slight rise in

temperature, due to excitement and apprehension, at the time of ad-

mission to the hospital, and the regular circadian temperature cycle.

38

37

36

1 2 3 4 5

Admitted to hospital

Days

Oral temp (

°C) Hyper-

thyroidism

Hypo-

thyroidism

Normal