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KH4119_Unit 02 ES_E218-E241 03/16/05 2:22 PM Page 236
at which it beats. The increased heart rate
causes an increase in blood pressure. The brain
also signals arteries to constrict, or narrow.
Constriction of blood vessels also increases
blood pressure. If the blood pressure becomes
too high, sensors detect this and signal the
heart to beat more slowly and for the blood
vessels to relax.
Figure E5.8b shows an example of positive
feedback. Positive feedback adjusts internal
conditions toward the initial condition. In the
example shown, the initial condition is a small
clot that begins to develop in response to a
bleeding wound. Positive feedback triggers a
regulatory response in which still more clotting
fibers accumulate at the injury site. This has the
effect of increasing the size of the clot, which
helps to reduce blood loss. In this case, the
initial condition was a high occurrence of
clotting fibers at the wound. The response was
to send even more clotting fibers.
Positive and negative adjustments are
possible because most body systems are directed
by the nervous and endocrine systems. These
two organ systems reach every part of the
human body. The nervous system is known for
directing rapid, short-term, and very specific
responses in the body. A reflex, such as jerking
your hand back when you accidentally touch
something hot, is an example of a rapid nervous
system response. This reflex is a homeostatic
response to a potentially dangerous rise in skin
temperature. A reflex illustrates the interaction
of sensory, nerve, and muscle systems. In this
case, receptors in the skin send signals to nerve
cells in the spinal cord. These, in turn, stimulate
nerve cells leading to muscles in the arm. The
muscle contracts and the hand withdraws from
the hot surface.
Thus, the nervous system helps maintain
homeostasis by regulating involuntary
physiological activities. These include
stimulating the hypothalamus and triggering
automatic sensations such as thirst, cold, and
pain. In addition, the nervous system assists in
maintaining homeostasis by enabling the body
to respond voluntarily to these sensations.
In contrast, the endocrine system usually
directs slower and longer-lasting changes. For
example, the secretion of vasopressin and the
change in urine production that follows take
longer to occur than the sensation of thirst
brought about by the nervous system.
The combinations of fast and slow,
automatic and voluntary, and physiological
and behavioral responses are important for
maintaining homeostasis. Coordinated by the
nervous and endocrine systems, all of the
body’s systems work together to maintain
balance within healthy limits. By delicately
balancing positive and negative feedback
mechanisms, the body can regulate the
changing internal conditions that humans
typically experience.
The Breath of Life
Take a nice, deep breath. Let it out slowly.
What do you think happens in your body with
each breath that you take?
Each time you breathe in, you draw air
into your lungs. This action is important for
your survival because air contains oxygen.
Oxygen, as you know, is a substance that every
cell of your body needs to maintain normal
conditions. Each time you breathe out, you
expel air out of your lungs. This action is also
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Unit 2 ESSAY: The Breath of Life
important because it helps rid your body of
carbon dioxide. Carbon dioxide is a substance
that is produced in cells as a by-product of
energy metabolism.
The process of breathing requires a finely
regulated interaction of a number of organ
systems. The organ system most directly
involved in regulating your body’s interaction
with the atmosphere is the gas exchange
system. The central organs of the gas exchange
KH4119_Unit 02 ES_E218-E241 03/16/05 2:22 PM Page 237
cluster of alveoli
capillaries in
which O2 and CO2
exchanges occur
nasal
cavity
larynx
trachea
bronchus
electron
micrograph
of alveolar
capillaries
lung
diaphragm
Figure E5.9 The human gas exchange system. This lung has been cut away to
expose the branching system of bronchial tubes. Part of the lung has been enlarged to
show the air sacs and their relation to capillaries. Millions of air sacs in each lung give the
tissue a spongelike appearance. If the surface of the human alveoli were spread flat, it
would cover an area of 60 square meters (646 square feet). What is the advantage of such
a large surface area?
system are the lungs. The lungs form two
compartments that connect to the outside
environment through your trachea (windpipe)
and your nose. The air inside these lung
compartments is not actually inside the
internal environment of your body. Instead,
the tissues of the lungs themselves separate
this air from the rest of the cells of your body.
How does oxygen move from your lungs
into the internal environment of your body?
And how does carbon dioxide move from this
internal environment back into your lungs
and back into the external environment? The
answers involve a combination of simple
chemical processes and complex homeostatic
regulation.
As you draw another deep breath, think
about the path that the air must travel. The
air passes through the nose where it is
warmed, moistened, and cleaned. Sometimes
the air passes through the mouth instead.
Then it enters the trachea, passes the vocal
cords, and enters a branching system of
bronchial tubes in each lung compartment.
The surfaces of these breathing tubes are
lined with mucus and cilia. Cilia are tiny
hairlike structures that move in a wavelike
manner. They sweep debris out of the
passages. When the air finally reaches the
ends of the passages in the lungs, it enters
smaller compartments. These smaller
compartments are made up of many tiny air
sacs called alveoli. This pathway of air
entering the lungs is shown in Figure E5.9.
Once the oxygen is in the alveoli, it is in
the smallest lung compartment. However, it
has not yet passed into the body’s internal
environment. To enter the internal
environment of the body, the oxygen must
diffuse across the alveoli’s thin walls. These
ESSAY: The Breath of Life
Unit 2
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walls are called the alveolar membranes. The
large number of alveoli increases the surface
area of lung tissue. In fact, the surface area of
these alveoli is 40 times greater than the entire
outer surface of the human body. This very high
surface area increases the amount of oxygen that
can move into the body’s internal environment.
It also increases the amount of carbon dioxide
that can enter the lungs to be exhaled.
The movement of oxygen across the
alveolar membranes involves the interaction of
the gas exchange system and the circulatory
system. As shown in Figure E5.10, a system
of capillaries filled with blood surrounds each
small group of alveoli. This blood comes into
such close contact with the thin membranes
of the alveoli that simple diffusion allows
oxygen to enter the body. The diffusion of
oxygen depends on its concentration in the air
sacs and in the blood inside the capillaries
that surround them. If the concentration of
oxygen is lower in the blood than in the air
sacs, the oxygen diffuses from the air sacs into
the blood. In the blood, the oxygen binds to
the protein hemoglobin. Hemoglobin is found
in the red blood cells. Through the flow of
blood, oxygen is then carried to all parts of the
body. In this way, these two systems work
together to deliver oxygen to cells deep inside
the body that have no direct contact with the
outside environment.
capillary
oglobin
hem
CO2
CO2
alveolus
At the same time that oxygen is diffusing
into the blood, carbon dioxide is diffusing out
of the blood and into the alveoli. Remember
that carbon dioxide is a by-product of processes
that take place in cells providing energy. The
blood carries carbon dioxide away from cells all
over the body. When carbon dioxide arrives at
the lungs in the capillaries surrounding the
alveoli, it diffuses across the alveolar membranes
into the air inside the lungs. The concentration
of carbon dioxide in the blood and in the air
inside the alveoli determines the direction of
diffusion. Because the concentration of carbon
dioxide is usually higher in the blood, carbon
dioxide usually diffuses out of the blood and
into the air inside the lungs. The enormous
surface area in the lungs speeds up the release of
carbon dioxide from the blood into the lungs.
When you exhale, you release this carbon
dioxide from your lungs into the external
environment around you.
Like many other homeostatic processes,
breathing involves precise feedback systems.
These feedback systems involve the gas
exchange system, circulatory system, and
nervous system. Consider, for example, what
happens to your breathing rate during rapid
exercise. As processes in the body speed up, the
production of carbon dioxide also increases.
Carbon dioxide causes the blood to become
more acidic. Nerve cells in the aorta, brain,
CO2
red
blood
cells
capillary
O2
hemoglobin-O2
body
cells
O2
Figure E5.10 The gas exchange and circulatory systems work together. Carbon
dioxide produced in body cells is transported by red blood cells from body cells to the
lungs. Oxygen from the lungs is transported by red blood cells to all body cells.
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Unit 2 ESSAY: The Breath of Life
KH4119_Unit 02 ES_E218-E241 03/16/05 2:22 PM Page 239
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
ESSAY: Behavior and Homeostasis
Unit 2
239