CHAPTER 11
Respiratory System
201
surface of the epiglottis is covered by
nonkeratinized stratiF
ed
squamous epithelium. In children, the epiglottis will occasion-
ally become infected with
Haemophilus
. In elderly individu-
als, the elastic cartilage of the epiglottis is often reduced in size
and is replaced by adipose tissue (±ig. 11-5C). The
vocal cords
(
folds
), which contain striated skeletal muscle and ligaments
(mainly elastic F bers), are lined by thin nonkeratinized stratiF
ed
squamous epithelium, which is F rmly attached to the underlying
vocal ligaments. The stratiF ed squamous epithelium protects the
vocal cords from mechanical stress. The main functions of the
vocal cords are to control airfl ow and facilitate speaking.
Lower Respiratory Airway
The
lower respiratory airway
includes the
trachea
,
bronchi
,
bronchioles
, and
terminal bronchioles
. Each portion of the
lower respiratory airway has unique tissue components, which
facilitate oxygen delivery, gas exchange, and immune defense
mechanisms. Individual airways decrease in diameter as they
continue branching.
THE TRACHEA
is a tube formed of cartilage and F bromus-
cular membrane, 10 to 12 cm long, with a diameter of 2 to
2.5 cm. It extends from the larynx, at the cricoid cartilage, to
the bifurcation of the bronchi. The trachea is lined by pseu-
dostratiF ed ciliated columnar epithelium and reinforced by 10
to 12 C-shaped hyaline cartilage rings (±ig. 11-6). A band of
smooth muscle is located between the two ends of the C-shaped
cartilage. The epithelium is composed of several cell types
including
goblet cells
,
ciliated columnar cells
,
basal cells
, and,
occasionally,
neuroendocrine cells
, which are also called
dif-
fuse neuroendocrine system
(
DNES
) cells (±ig. 11-7A). Chronic
irritation of the epithelium will lead to an increase in goblet
cells and a transformation to a stratiF ed squamous epithelium,
known as
squamous metaplasia
.
EXTRAPULMONARY BRONCHI
are the
primary bronchi
,
which begin at the bifurcation of the trachea and lead to the
right and left lungs. They are called “extrapulmonary” bronchi
because they are positioned outside the lungs. They are structur-
ally similar to the trachea, are lined by respiratory epithelium
(pseudostratiF ed columnar epithelium), and have C-shaped
hyaline cartilage. The
left primary bronchus
is narrower and
less vertical than the right one and gives rise to two second-
ary (lobar) bronchi. The
right primary bronchus
is wider and
shorter and more vertical than the left one; it gives rise to three
secondary (lobar) bronchi. That is the reason foreign body aspi-
ration occurs more often to the right lung.
INTRAPULMONARY BRONCHI
are secondary and tertiary
bronchi. As the primary (extrapulmonary) bronchi enter the
hiluses of the lungs they become the
secondary
(
lobar
)
bronchi
,
which eventually divide into the
tertiary
(
segmental
)
bronchi
(±igs. 11-1 and 11-8C). They are lined by respiratory epithe-
lium, and the
bronchial glands
(
seromucous glands
) are found
in the submucosa. A band of spiral smooth muscle separates
the lamina propria and submucosa of the intrapulmonary bron-
chi. The skeletal
support for each intrapulmonary bronchus is
provided by several hyaline cartilage plates instead of C-shaped
cartilage rings. As the bronchi continue branching, there is a
decrease in airway diameter and in the amount of cartilage in
their walls. The number of goblet cells, glands, and the height
of epithelial cells also decrease. However, the airways tend to
have increased amounts of smooth muscle and elastic tissues.
Smooth muscle in the bronchi is innervated by the sympathetic
and parasympathetic nervous systems. In patients with asthma,
this smooth muscle thickens with
hyperplasia
and
hypertrophy
and undergoes extensive and prolonged contraction causing
reduction in airway luminal diameter and difF
culty in exhaling
and inhaling. Bronchial branches are accompanied by branches
of the pulmonary arteries, pulmonary veins, nerves, and lymph
vessels. These structures usually travel in intersegmental and
interlobar layers of connective tissue.
BRONCHIOLES
are smaller airways deriving from tertiary
bronchi, which continue to branch into
terminal bronchioles
(±ig. 11-9). Bronchioles have no cartilage in their walls. Large
bronchioles are lined with ciliated columnar epithelial cells and
a gradually decreasing number of goblet cells. Small bronchioles
are covered with ciliated cuboidal epithelial cells and with
Clara
cells
. The number of Clara cells is greatly increased in the ter-
minal bronchioles (±ig. 11-11A). Terminal bronchioles are the
smallest and last of the conducting portion of the respiratory
system and they have no gas exchange function. Terminal bron-
chioles give rise to
respiratory bronchioles
, which connect to the
alveolar ducts
,
alveolar sacs
, and
alveoli
(±ig. 11-11A,B).
Respiratory Portion
The
respiratory portion
of the lungs includes the
respiratory
bronchioles
,
alveolar ducts
,
alveolar sacs
, and
alveoli
. This por-
tion of the respiratory system does not have cartilage and has
gradually increasing numbers of alveoli.
Respiratory Bronchioles
Respiratory bronchioles
are lined by cuboidal epithelium and
are interrupted by pouchlike, thin-walled structures called
alve-
oli
. Alveoli function in gas exchange. Respiratory bronchioles
continue to branch to become alveolar ducts (±igs. 11-9 and
11-11C).
Alveolar Ducts and Alveoli
Alveolar ducts
arise from respiratory bronchioles. They have
more alveoli and some cuboidal epithelium on the walls as
compared to respiratory bronchioles. They terminate as blind
pouches with clusters of
alveolar sacs
. An alveolar sac is com-
posed of two or more
alveoli
that share a common opening.
Alveolar ducts and alveoli are rich in capillaries, which make
gas exchange more efF cient. Alveoli are thin-walled pouches,
which provide the respiratory surface area for gas exchange
(±igs. 11-9 and 11-11C). The wall of the alveolus is formed by
a delicate layer of connective tissue with reticular and elastic
F bers covered by
type I
and
type II pneumocytes
. The type I
pneumocytes lie on a basal lamina, which is fused with the basal
lamina surrounding the adjacent capillaries to form a
blood-
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