of pseudostratiF ed, ciliated columnar epithelium and mucous-
secreting goblet cells. However, along the way, the number of
goblet cells and the height of the other epithelial cells decline,
and cilia become scarcer. In the F ner tubes, beginning with
the respiratory bronchioles, the lining is cuboidal epithelium;
in the alveoli, it is simple squamous epithelium closely associ-
ated with a dense network of capillaries. The mucous lining
gradually thins, until none appears in the alveoli.
exible optical instrument called a
to examine the trachea and bronchial tree. This procedure (bronchos-
copy) is used in diagnosing tumors or other pulmonary diseases and
to locate and remove aspirated Foreign bodies in the air passages.
Functions of the Respiratory Tubes and Alveoli
The branches of the bronchial tree are air passages, which
continue to F
lter incoming air and distribute it to the alve-
oli in all parts of the lungs. The alveoli, in turn, provide a
large surface area of thin epithelial cells through which gas
exchanges can occur
. If the 300 million alveoli in
the human lung were spread out, they would cover an area
of between 70 and 80 square meters—nearly half the size of
a tennis court.
Elastic F bers are scattered among the smooth muscle
cells and are abundant in the connective tissue that sur-
rounds the respiratory tubes. These F bers play an important
role in breathing, as is explained later in this chapter.
As the tubes become smaller in diameter, the type of cells
that line them changes. The lining of the larger tubes consists
Capillary network on
surface of alveolus
The respiratory tubes end in tiny
alveoli, each surrounded by a capillary network.
Light micrograph oF alveoli (250×).