202
UNIT 3
Organ Systems
air barrier
. The blood-air barrier is an important
structure for
oxygen and carbon dioxide exchange (Fig. 11-12A,B). The
neighboring alveoli are separated by
alveolar septa
, which con-
tain elastic connective tissue and may have capillaries within
them. The lumina of the neighboring alveoli may be connected
to each other by small
alveolar pores
.
TYPE I PNEUMOCYTES
are also called
type I alveolar cells
.
These cells cover 95% to 97% of the alveolar surface, whereas
type II pneumocytes cover the rest of the surface. Type I pneu-
mocytes are squamous cells with a fl at, dark, oval nucleus.
Tight junctions
between type I pneumocytes help prevent move-
ment of extracellular fl
uid into the alveolar sacs. Type I pneu-
mocytes are unable to divide; however, they can be regenerated
from type II pneumocytes (Fig. 11-13A,B).
TYPE II PNEUMOCYTES
cover about 3% to 5% of the alve-
olar surface and form tight junctions with type I
pneumocytes.
Their cytoplasm contains numerous
characteristic secretory
lamellar bodies
, which are mainly composed of
phospholipids
and
proteins
. These components can be released by exocytosis
into the alveolar lumen to form a thin ± lm of
pulmonary sur-
factant
. The function of the pulmonary surfactant is to increase
pulmonary compliance and decrease surface tension of the alve-
oli to prevent them from collapsing. Type II pneumocytes can
undergo mitosis to regenerate and also can form type I pneumo-
cytes. The pulmonary surfactant is recycled by type II pneumo-
cytes or cleared by alveolar macrophages (Fig. 11-14A,B).
ALVEOLAR MACROPHAGES
are also called
dust cells
. They
originate in bone marrow and circulate in blood as monocytes.
They become mature and migrate into the connective tissue of
the alveolar septa and into the lumina of the alveoli from blood
capillaries. They move around on the epithelial surfaces and
help to clear particles, as well as excessive surfactant, out of the
respiratory spaces (Fig. 11-15A,B).
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