Page 660 - Atlas of Histology with Functional Correlations

the lumen and increase in number as ciliated cells decrease.

RESPIRATORY SYSTEM—

RESPIRATORY PORTION

The respiratory portion is the distal continuation of the conducting portion that
starts with the air passageways where respiration or gaseous exchange can occur.

Terminal bronchioles branch to give rise to respiratory bronchioles, which are
characterized by thin-walled outpockets called alveoli. This is the first region of
the respiratory tube where gaseous exchange can take place. The respiratory
bronchioles represent the transitional zone where air conduction and gaseous

exchange or respiration can take place.

Respiration occurs only in alveoli because the barrier between inspired air in
the alveoli and the capillaries is extremely thin. Each alveolus is surrounded by
capillary plexuses that bring venous blood close to the inspired air inside the
alveoli for gaseous exchange (Fig. 17.2). Other intrapulmonary structures where

respiration occurs are the alveolar ducts and alveolar sacs.

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