Infralaryngeal structures are shown in figure 1-4.
a. Trachea and Bronchi. The respiratory tree is the set of tubular structures
that carry the air from the larynx to the alveoli of the lungs. The respiratory tree is so
named because it has the appearance of an inverted tree with its trunk and branches.
These tubular parts are held open (made patent) by rings of cartilage. Their lining is
ciliated to remove mucus and other materials that get into the passageway.
b. Alveoli. The alveoli are tiny spherical balloon-like sacs that are connected to
the larger tubes of the lungs by alveolar ducts and bronchioles. The alveoli are so small
that there are billions in the adult lungs. This very small size produces a maximum
surface area through which external respiration takes place. External respiration is the
actual exchange of gas between the air, the alveolar spaces, and the adjacent blood
capillaries through their wall. (Note: Alveoli is the plural of alveolus.)
Figure 1-4. Infralaryngeal structures.