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Chapter 13
no longer contain cartilage are termed
bronchioles.
Alveoli
fi rst begin to appear attached to the walls of the
respiratory
bronchioles.
The number of alveoli increases in the alveolar
ducts (see Figure 13–2), and the airways then end in grape-
like clusters consisting entirely of alveoli (
Figure 13–3
). The
airways, like blood vessels, are surrounded by smooth muscle,
which contracts or relaxes to alter airway radius.
The airways beyond the larynx can be divided into two
zones. The
conducting zone
extends from the top of the
trachea to the beginning of the respiratory bronchioles. This
zone contains no alveoli and has no gas exchange with the
blood (
Table 13–2
). The
respiratory zone
extends from the
respiratory bronchioles down. This zone contains alveoli and
is the region where gases exchange with the blood.
The oral and nasal cavities trap airborne particles in
nasal hairs and mucus. The epithelial surfaces of the airways,
to the end of the respiratory bronchioles, contain cilia that
constantly beat upward toward the pharynx. They also con-
tain glands and individual epithelial cells that secrete mucus.
Particulate matter, such as dust contained in the inspired
air, sticks to the mucus, which is continuously and slowly
moved by the cilia to the pharynx and then swallowed. This
so-called mucous escalator is important in keeping the lungs
clear of particulate matter and the many bacteria that enter
the body on dust particles. Ciliary activity can be inhibited
by many noxious agents including chronic cigarette smok-
ing. This is why smokers often cough up mucus that the cilia
would normally have cleared.
The airway epithelium also secretes a watery fl uid upon
which the mucus can ride freely. The production of this fl
uid
is impaired in the disease
cystic fi
brosis,
the most common
lethal genetic disease among Caucasians, in which the mucous
layer becomes thick and dehydrated, obstructing the airways.
The impaired secretion is due to a defect in the chloride chan-
nels involved in the secretory process.
Constriction of bronchioles in response to irritation helps
to prevent particulate matter and irritants from entering the sites
of gas exchange. Another protective mechanism against infec-
tion is provided by cells called macrophages that are present in
the airways and alveoli. These cells engulf and destroy inhaled
particles and bacteria that have reached the alveoli. Macrophages,
like cilia, are injured by cigarette smoke and air pollutants.
The pulmonary blood vessels generally accompany the
airways and also undergo numerous branchings. The smallest
of these vessels branch into networks of capillaries that richly
supply the alveoli (see Figure 13–3). The pulmonary circula-
tion has a very low resistance compared to the systemic circu-
lation, and for this reason the pressures within all pulmonary
blood vessels are low.
Site of Gas Exchange: The Alveoli
The alveoli are tiny, hollow sacs whose open ends are continu-
ous with the lumens of the airways (
Figure 13–4a
). Typically,
a single alveolar wall separates the air in two adjacent alveoli.
Most of the air-facing surfaces of the wall are lined by a con-
tinuous layer, one cell thick, of fl
at epithelial cells termed
type
I alveolar cells.
Interspersed between these cells are thicker,
specialized cells termed
type II alveolar cells
(
Figure 13–4b
)
that produce a detergent-like substance called surfactant.
The alveolar walls contain capillaries and a very small
interstitial space, which consists of interstitial fl uid and a loose
meshwork of connective tissue (see Figure 13–4b). In
many places, the interstitial space is absent altogether, and the
basement membranes of the alveolar-surface epithelium and
the capillary-wall endothelium fuse. Thus the blood within
an alveolar-wall capillary is separated from the air within the
alveolus by an extremely thin barrier (0.2 μm, compared with
the 7 μm diameter of an average red blood cell). The total
surface area of alveoli in contact with capillaries is roughly the
size of a tennis court. This extensive area and the thinness of
the barrier permit the rapid exchange of large quantities of
oxygen and carbon dioxide by diffusion.
In some of the alveolar walls, pores permit the fl ow of air
between alveoli. This route can be very important when the
airway leading to an alveolus is occluded by disease because
some air can still enter the alveolus by way of the pores
between it and adjacent alveoli.
Relation of the Lungs to the
Thoracic (Chest) Wall
The lungs, like the heart, are situated in the
thorax,
the com-
partment of the body between the neck and abdomen.
Thorax
and
chest
are synonyms. The thorax is a closed compartment
bounded at the neck by muscles and connective tissue and
completely separated from the abdomen by a large, dome-
Name of branches
Number
of tubes
in branch
1
4
2
8
16
32
6 x 10
4
5 x 10
5
8 x 10
6
Trachea
Bronchi
Bronchioles
Terminal bronchioles
Respiratory bronchioles
Alveolar ducts
Alveolar sacs
Respiratory zone
Conducting zone
Figure 13–2
Airway branching. Asymmetries in branching patterns between the
right and left bronchial trees are not depicted. The diameters of the
airways and alveoli are not drawn to scale.
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