Page 659 - Atlas of Histology with Functional Correlations

the olfactory cilia and serves as a solvent to dissolve the odor molecules for

stimulation of the olfactory cells for odor detection.

RESPIRATORY SYSTEM—CONDUCTING

PORTION

The conducting portion of the respiratory system consists of the nasal cavities,

the pharynx, the larynx, the trachea, the extrapulmonary bronchi, and a series of
solid intrapulmonary bronchi and bronchioles with decreasing diameters that end
as terminal bronchioles. Hyaline cartilage supports and keeps the larger air
passageways always patent (open). Starting with the trachea, incomplete C-

shaped hyaline cartilage rings encircle the tube. Elastic and smooth muscle
fibers, the trachealis muscle, bridge the space between the ends of the hyaline
cartilage. The ends of the C-shaped cartilage rings face posteriorly and are
adjacent to the esophagus.

As the trachea divides into bronchi and the bronchi enter the lungs, the C-

shaped hyaline cartilage rings are replaced by irregular hyaline cartilage plates
that encircle the lumen of the intrapulmonary bronchi. As the bronchi continue to
divide and decrease in size, the cartilage plates also decrease in size and number.

When the diameters of bronchioles decrease to about 1 mm, cartilage plates
disappear from conducting passageways. Terminal bronchioles represent the
final solid conducting passageways with the diameters ranging from 0.5 to 1.0
mm. There are between 20 and 25 generations of branching of intrapulmonary
bronchi before the passageways reach the size of terminal bronchioles.

The larger bronchioles are lined with a tall, ciliated pseudostratified
epithelium similar to trachea and bronchi. As the tubular size decreases, the
epithelial height is gradually reduced, and the epithelium becomes a simple
ciliated epithelium. The epithelium of larger bronchioles also contains

numerous goblet cells. The number of goblet cells, however, decreases with the
decreasing tubule size; the goblet cells are absent from the epithelium of terminal
bronchioles.

Small terminal bronchioles are lined only with a simple cuboidal
epithelium. In place of the goblet cells, Clara cells are found mixed with the

ciliated cells in the terminal and respiratory bronchioles. Clara cells are
nonciliated, secretory cuboidal cells with dome-shaped apices that protrude into

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