a. Trachea and Bronchi. The respiratory tree (figure 1-7) is the set of tubular
structures that carry air from the larynx to the alveoli of the lungs. If you were to turn
figure 1-7 UPSIDE DOWN, the trachea would become the trunk of the tree, and the
bronchi would be the branches. (NOTE: Figure 1-7 is right side up.) These tubular
parts are held open by rings of cartilage. The lining is ciliated to remove mucus and
other materials that get into the passageway.
b. Alveoli. The alveoli (alveolus, singular) are tiny round (balloon-like) sacs that
are connected to larger tubes of the lungs by tiny tubes known as alveolar ducts and
bronchioles. The alveoli are so small that there are billions in adult lungs. This very
small size produces a maximum surface area through which external respiration takes
place. External respiration is the actual exchange of gases between the air in the
alveolar spaces and the adjacent blood capillaries through their walls. The inner
surfaces of the alveoli must be kept wet in order for this transfer of gases to be possible.
Figure 1-7. Infralaryngeal structures ("respiratory tree").