744
UNIT FIVE
in the respiratory tree. It is a little like trafF
c backing up at
an exit from a highway.
In severe cases of the inherited illness
cystic f
brosis,
airways become
clogged with thick, sticky mucus, which attracts bacteria. As dam-
aged white blood cells accumulate at the infection site, their DNA
may leak out and further clog the area. A treatment that moderately
eases breathing is deoxyribonuclease (DNase), an enzyme that nor-
mally degrades accumulating extracellular DNA.
Structure of the Respiratory Tubes
The structure of a bronchus is similar to that of the trachea,
but the C-shaped cartilaginous rings are replaced with carti-
laginous plates where the bronchus enters the lung. These
plates are irregularly shaped and completely surround the
tube. However, as the branching tubes become thinner, the
amount of cartilage decreases. It F nally disappears in the
bronchioles, which have diameters of about 1 millimeter.
Also as the tubes thin, layers of surrounding smooth muscle
just beneath the mucosa become more prominent. This mus-
cular layer persists in the walls to the ends of the respiratory
bronchioles, and only a few muscle F
bers are in the walls of
the alveolar ducts.
Larynx
Trachea
Left superior
(upper) lobe
Left inferior
(lower) lobe
Right middle lobe
Right superior (upper) lobe
Right primary bronchus
Secondary bronchus
Tertiary bronchus
Right inferior (lower) lobe
Alveolar duct
Alveolus
Terminal bronchiole
Respiratory bronchiole
FIGURE 19.12
The bronchial tree consists of the
passageways that connect the trachea and the alveoli. The
alveolar ducts and alveoli are enlarged to show their locations.
FIGURE 19.13
A plastic cast of the bronchial tree.
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