C h a p t e r 2 1
Control of Respiratory Function
517
the intrapulmonary bronchus is the addition of a cir-
cumferential ring of smooth muscle.
The segmental bronchi continue to branch, form-
ing smaller bronchi, until they become the terminal
bronchioles, the smallest of the conducting airways. As
these bronchi branch and become smaller, their wall
structure changes. The cartilage gradually decreases
and there is an increase in smooth muscle and elastic
tissue with respect to the thickness of the wall. By the
time the bronchioles are reached, there is no cartilage
present and their walls are composed mainly of smooth
muscle and elastic fibers. Bronchospasm, or contraction
of these muscles, causes narrowing of the bronchioles
and impairs air flow. The elastic fibers, which radiate
from the outer surface of the bronchial wall and con-
nect with elastic fibers arising from other parts of the
bronchial tree, exert tension on the bronchial walls; by
pulling uniformly in all directions, they help maintain
airway patency.
Lungs and Respiratory Airways
The lungs are the functional structures of the respiratory
system. In addition to their gas exchange function, they
inactivate vasoactive substances such as bradykinin,
Left mainstem
bronchus
Left main
bronchus
Carina
B
A
Left pulmonary
veins
Left pulmonary
artery
Right main
bronchus
Right
Right main bronchus
Right superior
lobe bronchus
Right middle
lobe bronchus
Right inferior
lobe bronchus
Inferior lobe
Middle lobe
Superior lobe
Left
FIGURE 21-5.
(A)
Anterior view of respiratory structures including the lobes of the lung, the larynx,
the trachea, and the main bronchi on the left and the main pulmonary artery and vein on the right.
(B)
The carina that is located at the bifurcation of right and left mainstem bronchi.
Bronchus
Bronchiole
Alveolus
Mucous blanket
Cilia
Goblet cell
Pseudostratified
epithelium
Type I
alveolar
cell
Capillary
Type II
alveolar
cell
Cuboidal
epithelium
Mucous gland
Submucosal
connective
tissue
Cartilage
Smooth muscle
cells
FIGURE 21-6.
Airway wall
structure: bronchus, bronchiole,
and alveolus.The bronchial
wall contains pseudostratified
epithelium, smooth muscle
cells, mucous glands,
connective tissue, and cartilage.
In smaller bronchioles, a simple
epithelium is found, cartilage is
absent, and the wall is thinner.
The alveolar wall is designed
for gas exchange, rather than
structural support.
