743
CHAPTER NINETEEN
Respiratory System
(f
gs. 19.12
and
19.13)
. When stripped of their associated
blood vessels and tissues, the airways appear as an upside
down tree. The successive divisions of these branches from
the trachea to the microscopic air sacs follow:
1.
Right
and
left primary bronchi.
2.
Secondary,
or
lobar, bronchi.
Three branch from the
right primary bronchus, and two branch from the left.
3.
Tertiary,
or
segmental, bronchi.
Each of these
branches supplies a portion of the lung called a
bronchopulmonary segment.
Usually ten such segments
are in the right lung and eight are in the left lung.
4.
Intralobular bronchioles.
These small branches of the
segmental bronchi enter the basic units of the lung—the
lobules.
5.
Terminal bronchioles.
These tubes branch from
an intralobular bronchiole. Fifty to eighty terminal
bronchioles occupy a lobule of the lung.
6.
Respiratory bronchioles.
Two or more respiratory
bronchioles branch from each terminal bronchiole.
Short and about 0.5 millimeter in diameter, these
structures are called “respiratory” because a few air sacs
bud from their sides, enabling them to take part in gas
exchange.
7.
Alveolar ducts.
Alveolar ducts branch from each
respiratory bronchiole
(f
g. 19.14)
.
8.
Alveolar sacs.
Alveolar sacs are thin-walled, closely
packed outpouchings of the alveolar ducts.
9.
Alveoli
(al-ve
o-li). Alveoli are thin-walled, microscopic
air sacs that open to an alveolar sac. Air can diffuse
freely from the alveolar ducts, through the alveolar sacs,
and into the alveoli
(f
g. 19.15)
.
Dust particles, asbestos fibers, and other pollutants
travel at speeds of 200 centimeters per second in the trachea
but slow to 1 centimeter per second when deep in the lungs.
Gravity deposits such particles, particularly at branchpoints
Bronchial Tree
The
bronchial tree
(brong
ke-al tre
¯) consists of branched air-
ways leading from the trachea to the microscopic air sacs in
the lungs. Its branches begin with the right and left
primary
bronchi,
which arise from the trachea at the level of the ±
fth
thoracic vertebrae. The openings of the primary bronchi are
separated by a ridge of cartilage called the
carina
(see ± g.
19.8). Each bronchus, accompanied by large blood vessels,
enters its respective lung.
Branches of the Bronchial Tree
A short distance from its origin, each primary bronchus
divides into
secondary,
or
lobar, bronchi
(two on the left
and three on the right) that, in turn, branch repeatedly
Hyaline cartilage
Ciliated epithelium
Smooth muscle
Lumen of trachea
Connective tissue
Connective
tissue
Ciliated
epithelium
Lumen of
trachea
Hyaline
cartilage
Thyroid gland
Incision
Trachea
Hyoid
bone
Thyroid
cartilage
Cricoid
cartilage
Jugular
notch
FIGURE 19.9
Cross section of the trachea. Note the C-shaped ring of
hyaline cartilage in the wall.
FIGURE 19.10
Light micrograph of a section of the tracheal
wall (63×).
FIGURE 19.11
A tracheostomy may be performed to allow air to
bypass an obstruction in the larynx.
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