Download The Mammalian Respiratory System

S E C T I O N
8.2
The Mammalian Respiratory System
E X P E C TAT I O N S
List the steps in the path
taken by air as it moves from
the outside of the animal to
the internal gas exchange site
in the lungs.
Describe the role played by
each part of the respiratory
tract.
Explain how the mammalian
respiratory system is adapted
to reduce water loss.
Figure 8.11 All mammals,
including marine mammals
such as seals and whales,
have lungs.
What do these seals, which spend most of their
lives in the ocean, have in common with a desert
rat which might never see so much as a puddle in
its life? Despite their very different habitats, the
respiratory systems of all mammals share the same
basic features.
As you have seen, respiration refers to all parts
of the process that supplies oxygen to body cells
and rids the body of carbon dioxide. In mammals,
respiration can be subdivided into the following:
Breathing, which can be further divided into
inspiration, the act of taking air into the lungs, and
expiration, the act of breathing out;
External respiration, the exchange of oxygen and
carbon dioxide between air and blood;
Internal respiration, the exchange of oxygen and
carbon dioxide between blood and the cells of the
surrounding tissue (discussed in Chapter 9); and
Cellular respiration, the complex series of chemical
reactions that take place mainly in the mitochondria
of cells.
The Respiratory Tract
Lungs, with their many folds and fine membranes,
are delicate, fragile structures. As a result, they
must be shielded not only to prevent water loss,
but also to guard against damage. The lungs of
mammals are located deep within the body, where
they are protected by the bone and muscular
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structure of the thoracic cavity. This adaptation
means that a suitable passageway becomes
necessary to allow air to move from the external
environment to the respiratory surface deep inside
the animal. The organs of the lung system therefore
include a number of different structures, each with
an important role to play. The following paragraphs
trace the passage of air through these organs, using
the human respiratory system as an example.
The Upper Respiratory Tract
The air first enters the nostrils (in humans and
many other animals, it can also enter via the mouth).
The nostrils conduct the air into the hollow nasal
passages where several things occur. Thin bones,
called turbinates, hang suspended from the nasal
chambers. Their presence increases the surface area
of these chambers. The turbinates are covered with
a thin membrane that secretes mucus, which
moistens the air. The epithelial linings of the nasal
chambers and the turbinate bones are well
supplied with capillaries, which serve both to
warm the incoming air and also to increase its
relative humidity. This warming and moistening
helps to protect the delicate tissues of the lungs.
The air then passes successively through the
pharynx, the glottis, and the larynx (see Figure 8.12).
The pharynx is the section of alimentary canal that
connects the mouth and nasal cavity to the larynx
and esophagus. The glottis is the opening of the
trachea, the passageway that conducts air to the
lungs. This opening is protected by the epiglottis, a
flap-like structure that helps to prevent food from
entering the trachea. The pharynx is the
intersection between the trachea and the
esophagus, the passageway for food.
The larynx, or “voice box,” houses the vocal
cords, which are held securely in place by the
cartilaginous material present in the walls of the
larynx (Figure 8.13). The larynx contains the two
folded structures of the vocal cords. When you
breathe normally, there is a large gap between the
two cords. When you prepare to speak, muscles
around the larynx contract, bringing the cords
closer together. The passage of air through this
narrower space causes the cords to vibrate,
producing a sound. The pitch of the sound varies
with the length of the cords: a long cord produces a
low sound, while a shorter cord produces a higher
sound. At puberty, the vocal cords of males grow
quickly. This often produces a “breaking” quality
in the voice until the vocal cords finish growing.
PLAY
To view the pathway of air through the respiratory tract,
refer to your Electronic Learning Partner.
sinus
nasal cavity
nostril
sinus
hard palate
opening of
auditory tube
oral cavity
nasopharynx
tongue
uvula
tonsil
epiglottis
pharynx
glottis
vocal cords
esophagus
larynx
trachea
Figure 8.12 Together, the nasal passages, glottis,
pharynx, larynx, and trachea are referred to as the upper
respiratory tract.
epiglottis
base of
tongue
vocal
cord
larynx
glottis
vocal
cords
inner lining
of trachea
trachea
Figure 8.13 A cross section of the larynx, showing the
vocal cords. Air passing between the cords causes them to
vibrate, producing sound. You can change the pitch of the
sound you make by expanding or tightening the glottis; the
glottis
tighter the glottis, the higher the sound. Pitch is also
determined by the length of the cords. The cords tend to be
longer in men than in women, which is why men tend to
have deeper voices.
The Breath of Life • MHR
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Figure 8.15 The alveolar tissue of the lung. The walls of the
alveoli are only a single cell thick.
Figure 8.14 The interior of the nasal passage
After passing through the larynx, air goes down
the flexible tube of the trachea. In mammals, the
trachea is commonly called the “windpipe.” The
trachea is supported in part by semicircular
cartilage rings. These rings prevent the trachea
from collapsing and are arranged so they do not
interfere with the passage of food down the
esophagus, which is adjacent to the trachea.
The nasal and other passages of the upper
respiratory tract are lined with ciliated cells that
secrete mucus. The mucus traps foreign particles
such as dust and bacteria, while the continual
beating of the cilia helps to propel this material
back into the nose and throat where it can be
expelled by coughing or sneezing. When you catch
a cold, more mucus is secreted, which is why you
find yourself repeatedly blowing your nose.
BIO
FACT
The average healthy human adult produces about 0.9 L of
mucus every day.
The Lower Respiratory Tract
At about the level of your armpit the trachea
branches into two smaller passageways called
bronchi (singular bronchus). One bronchus enters
each lung (see Figure 8.16). Here, each bronchus
subdivides many times to produce a network of
finer and finer tubes called bronchioles. Like the
trachea and nasal passages, the bronchi and
bronchioles are lined with a ciliated mucous
membrane.
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MHR • Internal Systems and Regulation
Each bronchiole ends in a grape-like cluster of
tiny sacs called alveoli (singular alveolus). It is in
these sacs, which are always kept moist, that the
actual exchange of gases takes place. If you could
take all of the alveoli in the average human lung
and spread them out on a smooth surface, they
would cover approximately 70 to 90 m2 — an area
about the size of a tennis court. The wall of each
sac is one cell thick and is adjacent to a network of
tiny capillaries (Figure 8.15). These capillaries are
the site for the exchange of oxygen and carbon
dioxide in the body. While most of the exchange of
gases takes place through simple diffusion, a
process of facilitated diffusion accounts for some
(possibly as much as 30%) of the oxygen transfer.
This allows the blood to take up oxygen more
quickly than would otherwise be possible. The
transport of oxygen across the alveolar membrane
is facilitated by a particular protein-based molecule
in the alveolar cell wall.
The entire arrangement of bronchioles and
alveoli is kept in a relatively permanent position by
elastic connective tissue that fills the spaces between
the individual structures. In addition, the alveoli
are lined with a lipoprotein-based lubricating film
that helps to keep them from collapsing.
Wo rd
LINK
The word “lung” comes from the Old English word lungen,
meaning “light in weight.” The lungs are the lightest organs of
the body. Because of their many air sacs, they will float in
water. The lungs of a livestock animal, particularly a sheep or
cow, are sometimes called its “lights.”
alveoli
nasal cavity: filters,
warms, and moistens
incoming air
pharynx: connects
the nasal cavity
and larynx
epiglottis
medulla
oblongata
larynx: contains
the vocal cords
esophagus
trachea: carries
air to the
bronchi
bronchus:
carries air
to the lung
O2 -rich blood
capillary network
right lung
bronchiole:
carries air
to alveoli
left lung
diaphragm
alveolus: site of
external respiration
C O2 -rich blood
Figure 8.16 The path of air through the human respiratory tract. The complex
structure of the lungs serves to maintain a moist respiratory surface across which
gases can be exchanged between the external environment and the body.
Each lung is divided into lobes. The right lung
has three lobes, while the left lung has only two (to
accommodate the heart). A lobe is made up of a
number of lobules, each with its own bronchiole.
The lungs themselves are enveloped in layers of
tissue called pleura (singular pleuron). This
flexible membrane contains the lungs while still
allowing them to expand and contract during
SECTION
inspiration and expiration. Each pleuron is made
up of two layers separated by a thin film of
lubricating fluid. The condition known as pleurisy
occurs when the pleura become inflamed, typically
as a secondary infection related to pneumonia or
other thoracic diseases. Pleurisy can be extremely
painful and requires prompt medical attention.
REVIEW
1.
K/U Explain how the two basic requirements for gas
exchange are met by the structure of the mammalian
lung.
4.
MC Some cold-relief medications work by inhibiting
the production of mucus. What side effects would
you expect from these products?
2.
K/U In what ways does the respiratory tract alter
incoming air to prepare it for gas exchange in the
lung? What organs of the respiratory tract are
involved in this?
5.
I Jarvic 7 is an artificial heart. Design a device that
could act as an artificial lung. Provide detailed
reasoning to support your design.
3.
C “A lung is an inside-out gill.” Make one list of the
ways in which this statement is true, and another list
of the ways in which this statement is false. How can
you summarize your findings in a single sentence?
UNIT PROJECT PREP
If you have decided to study a respiratory disorder in
your Unit Project, make sure you can describe the
location and function of the affected respiratory structures.
The Breath of Life • MHR
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