646 chapter 21 The Endocrine System: Regulation of Energy Metabolism and Growth

so on); it also elevates plasma glucose levels (due to increased glu- normal; this condition, called hyperthermia, can lead to loss of
coneogenesis and glycogenolysis in the liver) and plasma levels of consciousness, convulsions, respiratory failure, and death. Adverse
fatty acids and glycerol (due to increased lipolysis in adipocytes). effects begin to appear when body temperature approaches 41°C
These actions make fuel more readily available to cells, in the pro- (106°F); a temperature of 43°C (109°F) or higher is usually fatal
cess helping prepare the body for the strenuous physical activity (see Chapter 1, Clinical Connections: Heat Exhaustion and Heat
inherent in the fight-or-flight response. The increased availability Stroke, p. 40). The ability to maintain normal body temperature
of fuel also prepares the body for other activities requiring energy, depends on the adequacy of heat transfer with the environment.
such as tissue repair or fighting infections. Other components of the
body’s response to stress are described later in this chapter. Mechanisms of Heat Transfer
Clearly, whole-body metabolism is highly regulated, for impor-
tant reasons. Notably, diseases of body metabolism affect all organ
Between the Body and the External
systems, as is most apparent in diabetes mellitus (see Focus on Environment
Diabetes: Diabetes Mellitus, page 645). Under most conditions, the body loses heat to the environment
because the surrounding temperature is often lower than body tem-
perature. When the rate of heat loss equals the rate of heat genera-
Quick check 21.3 tion, body temperature does not change. Generally speaking, heat is
lost by four mechanisms: (1) radiation, (2) conduction, (3) evapora-
➊➊ The concentration of which hormone, insulin or glucagon, is
tion, and (4) convection.
increased during the absorptive period?
In radiation, thermal energy is transferred from the body to the
➋➊ Sympathetic nervous activity and epinephrine promote environment in the form of electromagnetic waves. A general law of
metabolic reactions characteristic of which state, the absorptive physics states that all objects emit and absorb these waves, albeit to
state or the postabsorptive state? varying degrees. When an object is warmer than its surroundings,
➌➊ For each of the following processes, indicate whether it it loses heat by emitting more energy than it absorbs. For example,
is promoted by insulin or by glucagon: gluconeogenesis, when you are outside in the cool fall air, you emit radiant energy to
glucose uptake by cells, glycogenolysis, glycogen synthesis, the air molecules around you, making your body colder. By contrast,
catabolism of energy stores, protein synthesis, a decrease in if an object is cooler than its surroundings, it gains heat by absorbing
blood glucose levels, triglyceride synthesis, lipolysis. more energy than it emits. For example, if you are sitting by a camp-
fire, your body absorbs radiant heat and becomes warmer.
Conduction is the transfer of thermal energy between objects
21.6 Thermoregulation that are in direct contact with each other. As in radiation, heat is
When nighttime comes to the desert, the snakes, lizards, and insects always transferred from the warmer object to the cooler object.
that were active in the noonday sun begin to sink into a state of When you touch cold metal, for example, you feel colder because
relative torpor; many are barely able to move at all. This change thermal energy is transferred directly from your skin to the metal.
occurs because the falling temperature causes these animals’ bod- In evaporation, heat is lost from an object through the evapo-
ies to cool, which slows down biochemical reactions and other ration of water from its surface. For water to evaporate from your
metabolic processes. By contrast, humans and other mammals are body, the water must be converted from liquid form to gaseous
less affected by changes in the ambient temperature because they form, a process that requires thermal energy obtained from the
have the ability to maintain their body temperatures within a fairly body. Water evaporates from the skin, the lining of the airways, and
Copyright © 2017. Pearson Education, Limited. All rights reserved.

narrow range through thermoregulation. Animals with this ability other moist surfaces such as the lining of the mouth. This kind of
are said to be homeothermic, whereas those lacking this ability are insensible water loss occurs continually without your being aware
called poikilothermic. of it (hence the name). Your body also loses water through the evap-
oration of sweat, a salt-containing solution secreted by numerous
small sweat glands in the skin. Unlike insensible water loss, which
Temperature Balance occurs continuously, sweat production is regulated according to the
All living things produce heat as a by-product of metabolism, but body’s needs. When increased heat loss is desirable, sweat produc-
humans (like all other homeothermic animals) are able to control tion increases. As a result, more water evaporates from the skin sur-
body temperature by regulating the rates at which heat is produced face, carrying thermal energy away from the body.
and lost from their bodies. To maintain a normal body temperature, When the environmental temperature is warmer than the body
the rate of heat production must be balanced against the rate of heat temperature, radiation and conduction transfer heat into the body.
loss. In negative heat balance, heat loss exceeds heat produced; as a This transfer adds to the heat generated by the body itself, boosting
result, body temperature falls below normal, a condi- the need for heat loss. During such circumstances, the body must
Health rely on evaporation to carry heat away by increasing the produc-
Fact tion called hypothermia. People who are trapped
in snowstorms or swept from a boat into icy waters, tion of sweat. Sweating cools the body under these conditions
for example, are vulnerable to hypothermia. Such misfortunes can because water continues to evaporate even when it is cooler than
quickly lead to stupor, loss of consciousness, multiple-organ fail- its surroundings, assuming that the humidity of the surroundings
ure, and ultimately death. In positive heat balance, heat produced is not too high. In a humid environment, sweating is not as effi-
exceeds heat loss and body temperature increases to levels above cient as in drier air because water cannot evaporate into already
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 chapter 21 The Endocrine System: Regulation of Energy Metabolism and Growth 647

water-saturated air. To facilitate thermoregulation, sweat requires to the thermoregulatory center from central thermoreceptors that
the process of evaporation; that is, dripping sweat does not cool the include temperature-sensitive neurons within this region of the
body. Thus humidity contributes to the heat index, when meteo- brain, in other areas of the central nervous system, and in other
rologists report that the air feels hotter than it is. internal organs. Input of changes in core temperature is necessary
Convection—the transfer of heat from one place to another by to initiate the thermoregulatory responses that return the core
a moving gas or liquid—contributes to heat loss on a windy day. On temperature to normal. Other thermoreceptors, called peripheral
a still day, the air that is closer to your skin warms up as it absorbs thermoreceptors, are located in the skin; they detect the tempera-
heat from your body’s surface. This warmer air forms a kind of ture of the skin, which is usually well below the core temperature
“blanket” around you that slows down the rate of heat loss by con- and is more variable. Our bodies do not regulate skin temperature,
duction. Because the air in this protective layer contains moisture but information about skin temperature enables us to compensate
that has evaporated from your skin, it tends to have a higher humid- for changes in environmental temperatures by making behavioral
ity than the surrounding air. The presence of this moisture near the adjustments, such as dressing appropriately or avoiding extreme
skin reduces the rate of evaporative heat loss. In contrast, when the temperatures altogether.
surrounding air is moving, as on a windy day, the thickness of the
protective “blanket” of air is reduced, and conductive and evapora- Thermoregulation in the
tive heat loss both increase. Thus convection is responsible for the Thermoneutral Zone
wind chill factor reported by meteorologists on cold, windy days.
The primary mechanism for regulating body temperature is to vary
the amount of blood flowing to the skin, where thermal energy in the
Regulation of Body Temperature blood can be exchanged with that in the environment (the heat actu-
The body’s thermoregulatory efforts maintain the core tempera- ally moves from blood to cutaneous tissue and then out of the body).
ture, the temperature within internal structures, including those When body temperature decreases, blood flow to the skin decreases,
of the central nervous system and abdominal and thoracic cavities so that the blood loses less of its heat to the environment. Likewise,
(Figure 21.9). This temperature is normally regulated at approxi- when body temperature increases, blood flow to the skin increases,
mately 37°C (98.6°F). Within the hypothalamus is the body’s ther- so that the blood can lose more of its heat to the environment.
moregulatory center, which contains both heat-losing and heat- Alterations in blood flow to the skin are sufficient to maintain body
promoting centers. Input about the core temperature is transmitted temperature when the environmental temperature is maintained
within a narrow range called the thermoneutral zone (25–30°C).
The sympathetic nervous system regulates cutaneous blood
flow based on input from the thermoregulatory center. As we
learned earlier (in Chapter 14), arteriole radius is regulated by
sympathetic activity, with increases in sympathetic activity caus-
ing vasoconstriction. The arterioles to skin follow this same rule.
Thus, when body temperature decreases, sympathetic activity to the
arterioles increases, causing vasoconstriction and thereby decreas-
37°C
Core ing cutaneous blood flow to conserve body heat. In contrast, when
temperature body temperature increases, sympathetic activity decreases, caus-
ing vasodilation and an increase in blood flow to the skin, thereby
transferring heat to the environment. When the environmental tem-
Copyright © 2017. Pearson Education, Limited. All rights reserved.

perature is outside the thermoneutral zone, other thermoregulatory

mechanisms must be activated to maintain body temperature.

Heat Generation in a Cold Environment

When the environmental temperature drops to less than 25°C,
decrease in cutaneous blood flow alone cannot prevent body tem-
perature from falling. In this circumstance, the heat-promoting
center of the hypothalamus communicates to effector organs to
stimulate shivering and decrease sweat production (Figure 21.10).
Shivering is the primary mechanism by which our bodies can
increase heat production for the purpose of increasing body tem-
perature, a process called shivering thermogenesis. Shivering
involves the rapid rhythmical contraction of skeletal muscles and is
controlled by a spinal reflex. As the muscles contract, they generate
Figure 21.9 Core body temperature. Body temperature is maintained heat. Human infants and hibernating mammals are also capable
near 37°C in core regions of the body, including the thoracic and abdominal of generating heat through non-shivering thermogenesis. Infants
cavities, cranium, and most proximal areas of the limbs. Note that temperature have a special form of adipose tissue, called brown adipose tis-
decreases from the 37°C when moving from the core to the skin. sue, that generates heat through the uncoupling of the electron
Stanfield, C. (2017). Principles of human physiology, global edition. Pearson Education, Limited.
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 648 chapter 21 The Endocrine System: Regulation of Energy Metabolism and Growth

Temperature of
external environment

Thermoreceptors Hypothalamus
Body temperature
Detect temperature Integration

Sweat glands Blood vessels in skin Skeletal muscles

Negative
feedback Sweat Constriction Shivering
production

Blood flow

Heat
Heat loss generation

Body temperature
Initial stimulus
Physiological response
Result

Figure 21.10 Events occurring in the body during thermoregulation.

transport chain from oxidative phosphorylation. Thus the energy the quantity of water and electrolytes reabsorbed depends on the
released by electrons is “lost” as heat instead of being harnessed rate of sweat production; as the rate increases, less is reabsorbed
to synthesize ATP. By comparison, adult humans have little, if any, and more is secreted outside the body. The sympathetic nervous
brown adipose tissue. system controls the rate of sweat production; as sympathetic
activity increases (whether due to a warm environment or activa-
tion of the fight-or-flight response), sweat production increases.
Heat Loss in a Warm Environment
Sympathetic innervation of sweat glands is atypical, in that the
When environmental temperature increases above the thermoneu- sympathetic postganglionic neurons secrete acetylcholine instead
tral zone, the body must respond with more than cutaneous vasodi- of norepinephrine.
lation if it is to remove enough heat to bring body temperature back Apocrine sweat glands produce sweat in a similar manner, but
Copyright © 2017. Pearson Education, Limited. All rights reserved.

toward normal. Specifically, the body produces sweat for evapora- proteins and fatty acids are also found in the sweat produced via
tive heat loss. this mechanism. Proteins and fatty acids secreted on the skin sur-
The average person has approximately 2.5 million sweat glands face provide nutrition that allows bacteria to grow. As the bacteria
located in skin throughout the body, except in the lips, nipples, and grow, they generate waste products that create the odor associated
external genitalia. Two types of sweat glands are distinguished: (1) with sweat in the axillary and genital areas.
eccrine glands (the more common), which are located all over the
body but especially in the forehead, palms of the hands, and soles
of the feet, and (2) apocrine glands, which are located primarily in
the axillary region (arm pits) and the anal-genital region. Eccrine
Alterations in the Set Point
glands empty into pores at the surface of the skin, whereas apo- for Thermoregulation: Fever
crine glands empty into hair follicles. Eccrine glands are active at During an infection, certain white blood cells produce cytokines
birth, but apocrine glands do not become active until puberty. The that function as pyrogens, chemicals that induce fever. A normal
amount of sweat produced by both types of glands depends on the response of the immune system (described in Chapter 23), this
body temperature and level of sympathetic activity. effect promotes several immune responses that fight the infection.
Eccrine sweat glands produce a primary secretion of Thus fever is actually considered beneficial because it enhances the
water, sodium, and chloride, plus a trace amount of potassium body’s ability to defend itself.
(Figure 21.11). As this fluid makes its way through the duct lead- Pyrogens induce a fever through actions on the thermoregula-
ing to the skin surface, the sodium and chloride are actively reab- tory center, adjusting the temperature to be maintained by ther-
sorbed, and water follows the same path by osmosis. However, moregulatory processes to a higher level. The body responds by
Stanfield, C. (2017). Principles of human physiology, global edition. Pearson Education, Limited.
Created from keeleuni on 2023-03-15 20:28:55.