516
U N I T 6
Respiratory Function
the same basic structure as the trachea. Each primary
bronchus, accompanied by the pulmonary arteries,
veins, and lymph vessels, enters the lung through a slit
called the
hilus
.
On entering the lungs, each primary bronchus divides
into secondary or lobular bronchi that supply each of
the lobes of the lung—three in the right lung and two
in the left. The right middle lobe bronchus, which is
particularly subject to obstruction, is relatively small in
diameter and length and sometimes bends sharply near
its bifurcation, making it particularly subject to obstruc-
tion. The secondary bronchi, in turn, divide to form the
segmental bronchi that supply the bronchopulmonary
segments of the lung. These segments are identified
according to their location in the lung (e.g., the apical
segment of the right upper lobe) and are the smallest
named units in the lung. Lung lesions such as atelectasis
and pneumonia often are localized to a particular bron-
chopulmonary segment.
Initially, the walls of the bronchial airways have the
same general structure as the trachea. At the point where
the bronchi enter the the lungs to become the intrapul-
monary bronchi, the structure changes (Fig. 21-6). The
cartilage rings are replaced by cartilage plates of irregu-
lar shape that encircle the entire circumference of the
airway. A second change that is observed in the wall of
Segmental
bronchi
Terminal
bronchioles
Trachea
Left primary
bronchus
Secondary
bronchi
A
B
Conducting zone
Transit and
respiratory zone
Trachea
BR
BL
Z
0
1
2
3
4
17
18
19
20
21
22
23
TBL
RBL
AD
AS
FIGURE 21-3.
(A)
Conducting and respiratory air pathways inferior to the larynx. (From Anatomic
Chart Company. Atlas of Human Anatomy. Springhouse, PA: Springhouse; 2001:175.)
(B)
Idealization of the human airways.The first 16 generations of branching (Z) make up the conducting
airways, and the last seven constitute the respiratory zone (or transitional and respiratory
zone). AD, alveolar ducts; AS, alveolar sacs; BL, bronchiole; BR, bronchus; RBL, respiratory
bronchiole;TBL, terminal bronchiole. (FromWei bei ER. Morphometry of the Human Lung. Berlin:
Springer-Verlag;1962:111.)
Posterior
Anterior
Esophagus
Hyaline
cartilage
Trachealis
muscle
Ciliated
epithelium
Lumen of
trachea
FIGURE 21-4.
Cross-section of the trachea, illustrating its
relationship to the esophagus, the position of the supporting
hyaline cartilage rings in its wall, and the trachealis muscle
connecting the free ends of the cartilage rings.
