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and arteries that lead to the head detect this
increased acidity. These special cells send a
signal to the respiratory centers in the brain.
The respiratory centers respond by stimulating
the diaphragm and rib muscles to contract more
rapidly. Rapid contraction of these muscles
increases the breathing rate. A faster breathing
rate increases the rate at which oxygen is
brought into the body. A faster breathing rate
also increases the rate at which carbon dioxide is
released from the body. When you stop
exercising, the rate of carbon dioxide production
declines. The blood, then, becomes less acidic.
This change is detected by the sensory receptors
in the blood vessels. The information is relayed
to the respiratory centers in the brain. Finally,
signals are sent to the diaphragm and rib
muscles to contract more slowly.
This regulatory system works
automatically. You do not have to control
your breathing rate consciously. The signals
involved are very powerful. Although you
have some control over your breathing rate,
you cannot hold your breath indefinitely.
Once the carbon dioxide level in your blood
reaches a critical level, the homeostatic
signals override your efforts to hold your
breath, and you are forced to exhale and take
another breath.
Take one last deep breath. Can you
describe what is happening in your lungs as
you inhale and exhale? Can you remember
how the rate of your breathing is normally
controlled? Now consider this. Because of
several complex homeostatic systems, many
important adjustments that you never have to
think about take place in your body.
What is the evidence that this is going
on? Think of all the little breaths you took
between those two nice deep breaths.
Behavior and Homeostasis
Remember Josh, the character in A Pause That
Refreshes? (Chapter 4)? What made Josh head
to the refrigerator for a cool drink? Why does
a lizard move toward a heated rock when its
external environment cools off? What makes
you reach for a sweatshirt when you enter an
air-conditioned movie theater? Those
questions all are focused on behaviors that
seem to help maintain homeostasis. But what
are the signals that prompt an organism to
respond to changing conditions?
All those examples of behavior have a
physiological basis. In other words,
homeostasis is maintained by processes inside
the body. Sometimes these internal processes
result in behaviors we can see. But what is
happening on the inside? Your body’s internal
conditions are controlled by a variety of
monitoring and feedback systems that are
connected. All organisms receive stimuli that
prompt their monitoring and feedback
systems. These stimuli arrive in many forms:
light, temperature, sound, water, and chemicals.
Living systems vary greatly in the type of
response they have for different stimuli. The
feedback processes sometimes involve responses
that include behaviors we can observe.
Internal conditions such as the level of
carbon dioxide, body temperature, and salt
concentration are examples of conditions that
are controlled by physiological processes. You
have learned that carbon dioxide plays an
important role in regulating breathing rate. In
general, the acid-base balance of the blood
determines your breathing rate. Breathing fast
is a typical behavioral response to increased
exercise. This response restores carbon dioxide
to acceptable levels. Under unusual conditions,
such as fever, aspirin poisoning, or anxiety, the
body responds with hyperventilation. In this
potentially dangerous situation, the body
“overbreathes.” This overbreathing increases
the breathing rate above the body’s need to
blow off carbon dioxide. Consequently, carbon
dioxide is lost more rapidly than it is produced
in the tissues. Your brain then does not get the
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Unit 2
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Figure E5.11 Violent shivering can
increase the body’s heat production by as
much as 18 times normal.
feedback to signal breathing. Eventually, you
might pass out from lack of oxygen. We can
reduce the danger of this response by placing a
paper bag over the victim’s mouth and nose.
This trick increases the level of carbon dioxide
that the person breathes in. The body then
receives the minimum carbon dioxide level
that is associated with a normal breathing rate.
Scientists categorize the mechanism
animals use to regulate body temperature into
two major groups. Endothermic mechanisms
are those that generate heat internally.
Ectothermic mechanisms are mechanisms that
collect heat from outside the body. Mammals
and birds are endothermic. Animals such as
fish, reptiles, and insects are ectothermic.
Regardless of which type of animal an organism
is, temperature regulation is a critical survival
tool. Many fundamental cell processes depend
on enzymes that function best in very narrow
temperature ranges. This is why doctors are
concerned when their patients run high fevers.
A slight increase in temperature can help kill
pathogens. However, a large or sustained
increase will destroy vital cell functions. This
situation can put the patient’s life at risk.
Mammals and birds maintain relatively
constant temperatures by balancing heat
production with heat loss. For example, as you
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Unit 2 ESSAY: Behavior and Homeostasis
digest food, you generate heat. You can increase
heat production by eating more, exercising, and
shivering. You can decrease heat loss by adding
insulation. You can put on a sweater. A bird, for
example, fluffs up its feathers. On the other
hand, you lose heat through the evaporation of
water by sweating and breathing fast. You also
lose heat through heat transfer. Taking your
sweater off increases the loss of heat through
transfer. These behaviors are all responses to
changes in the external conditions. In each case,
the organism used feedback to regulate a
response that started in the body. Each response
also included a behavior that we could see.
Across time, through evolution, a variety
of interesting adaptations have arisen that help
organisms maintain a constant temperature.
Some of the most notable are the adaptations
of mammals to extreme climates. For example,
small desert mammals may live underground
or be active at night to minimize the effect of
the hot, dry days. Small mammals in very cold
environments will live in tunnels under the
snow. The temperature in these tunnels does not
drop below –5°C (–23°F), even when outside
air temperatures fall below –50°C (–58°F).
Ectotherms do not have internal processes
that help regulate their internal temperature.
Instead, they have internal receptors that
trigger specific responses when their internal
Figure E5.12 Polar bear. Polar bears
live only in the Northern Hemisphere,
nearly always in association with sea ice.
They maintain their internal temperatures
in a very cold climate by hibernating in
dens during the coldest months. What
behaviors do you use to stay warm?
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Figure E5.13 The lizard maintains a fairly constant body temperature by changing its
body position relative to the position of the sun.
temperatures rise or fall out of a safe range.
The behavioral responses of desert lizards
have been studied extensively. Biologists have
found that the lizards’ responses are so finely
tuned that they are able to maintain a body
temperature between 36°C (97°F) and 39°C
(102°F). How? Simply by moving in and out
of the sunshine and by adjusting their
orientation to the sun.
Reptiles, birds, fish, and humans show a
variety of adaptations for regulating their salt
concentration. Marine reptiles such as turtles
have special salt glands above their eyes that
excrete the excess salt they take in with their
food. Birds have a similar adaptation, except
that the salt solution drains out of their beaks.
In humans and other mammals, excess sodium
is removed by the kidneys and excreted in the
urine. Some animals, such as the spider crab
that lives in estuaries (a saltwater environment),
can sense changes in the salt level. But they do
not have a physiological mechanism for
removing the salt. Instead, the spider crab
moves to areas of lower or higher salt
concentrations. In this way, the spider crab can
maintain its internal balance.
Many animals have observable behaviors
that are related to maintaining homeostasis.
The ability of humans to think about their
behavior, make choices about behaviors, and
access technological solutions increases the
range of responses that we have. We can cool
and heat our external environment. We have
developed sports drinks to restore our
electrolyte balance after we sweat. We have
access to a wide range of foods, beverages, and
drugs that can restore or destroy a homeostatic
balance.
So, although Josh’s body signaled him to
restore the water balance in his body, his
conscious behavior determined whether that
balance would be restored. By choosing an
energy drink, not a sports drink, Josh made his
internal condition worse instead of improving it.
Remember, an energy drink contains a high
level of the diuretic caffeine. As you learned in
Chapter 1, humans are distinguished from other
animals by a set of characteristics, especially the
capacity of our brain. That powerful brain gives
us the capacity to choose to ignore certain
signals or do something about them.
Figure E5.14 Western gull (Larus
occidentalis). Western gulls have a
wingspan of 30–40 cm (12–16 inches). Note
the drop of salt water at the tip of this
bird’s beak. Salt glands help sea birds
eliminate excess sodium. How does your
body control its salt concentration?
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Unit 2
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