944
UNIT 4
Maintenance of the Body
24
muscle activity is one of the most important means of modify-
ing body temperature.
Body temperature averages 37°C
6
0.5°C (98.6°F) and is
usually maintained within the range 35.8–38.2°C (96–101°F),
despite considerable change in external (air) temperature. A
healthy individual’s body temperature fluctuates approximately
1°C (1.8°F) in 24 hours, lowest in early morning and highest in
late a±ernoon or early evening.
Te adaptive value of temperature homeostasis becomes ap-
parent when we consider how temperature affects enzymatic
activity. At normal body temperature, conditions are optimal
for enzymatic activity. Rising body temperature accelerates en-
zymatic catalysis: With each rise of 1°C, the rate of chemical
reactions increases about 10%. If temperature rises above the
homeostatic range, neurons are depressed and proteins begin
to denature. Children below the age of 5 go into convulsions
when body temperature reaches 41°C (106°F), and 43°C (about
109°F) appears to be the absolute limit for life.
In contrast, most body tissues can withstand marked reduc-
tions in temperature if other conditions are carefully controlled.
Tis fact underlies the use of body cooling during open heart
surgery when the heart must be stopped. Low body tempera-
ture reduces metabolic rate (and consequently nutrient require-
ments of body tissues and the heart), allowing more time for
surgery without incurring tissue damage.
Core and Shell Temperatures
Different body regions have different resting temperatures.
Te body’s
core
(organs within the skull and the thoracic and
abdominal cavities) has the highest temperature and its
shell
(essentially the skin) has the lowest temperature in most cir-
cumstances. Of the two body sites used routinely to obtain body
temperature clinically, the rectum typically has a temperature
about 0.4°C (0.7°F) higher than the mouth and is a better indi-
cator of core temperature.
It is core temperature that is precisely regulated. Blood serves
as the major
agent of heat exchange
between the core and shell.
Whenever the shell is warmer than the external environment,
the body loses heat as warm blood is allowed to flush into skin
capillaries. On the other hand, when heat must be conserved,
blood largely bypasses the skin. Tis reduces heat loss and allows
the shell temperature to fall toward that of the environment.
For this reason, core temperature stays relatively constant,
but the temperature of the shell may fluctuate substantially, for
example, between 20°C (68°F) and 40°C (104°F), as it adapts to
changes in body activity and external temperature. (You really
can
have cold hands and a warm heart.)
Mechanisms of Heat Exchange
Heat exchange between our skin and the external environ-
ment works in the same way as heat exchange between inani-
mate objects
(Figure 24.25)
. It helps to think of an object’s
temperature—whether that object is a radiator or your skin—
as a guide to its heat content (think “heat concentration”).
Ten just remember that heat always flows down its concen-
tration gradient from a warmer region to a cooler region. Te
activity of the liver during such periods probably accounts for
the bulk of additional energy use. In contrast, fasting or very
low caloric intake depresses ²MR and results in a slower break-
down of body reserves.
Check Your Understanding
30.
Which of the following contributes to a person’s BMR?
Kidney function, breathing, jogging, eating, fever.
31.
Samantha is tall and slim, but athletic and well toned. Her
friend Ginger is short and stocky, bordering on obese.
Which would be expected to have a greater BMR, relatively
speaking?
For answers, see Appendix H.
Regulation of Body Temperature
Distinguish between core and shell body temperature.
Describe how body temperature is regulated, and indicate
the common mechanisms regulating heat production/
retention and heat loss from the body.
As shown in
Figure 24.24
, body temperature represents the
balance between heat production and heat loss. All body tissues
produce heat, but those most active metabolically produce the
greatest amounts. When the body is at rest, most heat is gener-
ated by the liver, heart, brain, kidneys, and endocrine organs,
with the inactive skeletal muscles accounting for only 20–30%.
Tis situation changes dramatically with even slight changes
in muscle tone. When we are cold, shivering helps warm us up,
and during vigorous exercise, skeletal muscles can produce 30
to 40 times more heat than the rest of the body. A change in
• Basal metabolism
• Muscular activit
y
(shivering)
Thyr
oxine and
epinephrine
(stimulating effects
on metabolic rate)
• Radiation
C
onduction/
convection
• Evaporation
Heat production
Heat loss
T
emperature effect
(w
ar
mer cells
metabolize faster,
producing more heat)
Figure 24.24
Body temperature remains constant as long as
heat production and heat loss are balanced.
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