CHAPTER 11
Respiratory System
211
Figure 11-9.
Overview of the bronchioles and alveoli.
This is a representation of the
bronchioles
and
alveoli
; the length of the different types of bronchioles is not drawn to scale.
Segmental
bronchi
bifurcate into
bronchioles
, which give rise to many branches as they move distally within the lung (Fig. 11-1). Bronchioles
have no cartilage and continue to divide into smaller bronchioles.
Terminal bronchioles
are the ±
nal parts of the conducting airway.
They extend into alveolar sacs to give rise to
respiratory bronchioles
, which connect to the alveolar ducts. Respiratory bronchioles
are small in diameter, are lined by cuboidal cells, and contain increased numbers of alveoli. Respiratory bronchioles mark the transi-
tion from the conducting portion to the respiratory portion in which gas exchange occurs. An
alveolar duct
is a hallway that con-
nects the respiratory bronchiole to an alveolar sac. Alveolar ducts are lined by squamous alveolar epithelium and knobs of cuboidal
epithelium lying on the smooth muscle cells. An
alveolar sac
is the blind end of an alveolar duct and includes a common opening for
two or more alveoli. Alveoli have very thin walls lined by alveolar epithelium that contains
type I
and
II pneumocytes
(
alveolar cells
).
The basement membrane of the type I pneumocytes and endothelial cells of the capillaries are fused together to form the
air-blood
barrier
(Fig 11-12). Type I pneumocytes are squamous cells that line the alveoli (Fig. 11-13). Type II pneumocytes are
pulmonary
surfactant
producing cells
that are important for reducing the surface tension of the alveoli (Fig. 11-14).
Alveolar macrophages
, also
called
dust cells
, lying free on the alveolar wall, are shown here and can also be found in the septa of the alveoli (Fig. 11-15). Dust
cells move around on the alveolar surface like vacuum cleaners to clear dust particles and other debris on the surface of the alveoli
and also help remove excess surfactant.
Alveoli
Bronchioles
(multiple branches)
Pulmonary
vein (to the left atrium)
Cartilage plate
Lymphatic vessel
Type I pneumocyte
Type II
pneumocyte
Alveolar
capillaries
Alveolar macrophage
(dust cell)
Pulmonary artery
(from the right ventricle)
Terminal bronchiole
(multiple branches)
Respiratory bronchiole
Alveolar sac
Alveolar duct
Alveolar duct
Smallest segmental
bronchus
(multiple branches)
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