The bronchi further divide into secondary bronchi

lead to the right and left lungs. The bronchi further divide into secondary bronchi, with three on
the right side and two on the left side, that enter the lobes of the lungs. The amount of cartilage
within these structures decreases from the trachea to bronchi to secondary bronchi. Thereafter,
each secondary bronchus further divides into tertiary bronchi (as these bronchi divide, they have
successively smaller radii). Once these tubes reach ~1 mm in radius, they are called bronchioles.
The bronchioles no longer contain cartilage but are still resistant to collapse due to their elastic
fiber composition. These bronchioles divide further into terminal bronchioles, which are the
smallest component of the conducting zone. The conducting zone functions primarily as a
passageway for the movement of air to the respiratory zone where gases can be exchanged. No
gas exchange occurs within the conducting zone. The rigid structure of the trachea, and
ultimately the thickness of the bronchioles, prevents gas transfer. Air is heated and humidified as
it moves through the conducting zone. This zone is lined by epithelial cells that include the
goblet cells and the ciliated cells. The goblet cells produce the mucus that is moved up and out of
the lungs by cilia on the ciliated cells. This mucus escalator moves foreign particles trapped in
the mucus toward the glottis where it enters the esophagus. At the same time, the smooth muscle
cells, which are absent from the trachea, progressively increase in density further into the
conducting zone. The smooth muscle cells provide the airway with the ability to change radius
and thereby resistance.
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2) One of the basic tenets of physiology is that function of a system follows its structure.
Describe the anatomy of the respiratory zone of the respiratory tract, including how that structure
contributes to function.