Maintenance of the Body
Te air passageways in the lungs branch and branch again,
about 23 times overall, in a pattern ofen called the
. At the tips oF the bronchial tree, conduct-
ing zone structures give way to respiratory zone structures
Conducting Zone Structures
Te trachea divides to Form the
lef main (primary)
ki) approximately at the level oF ±
in an erect
(standing) person. Each bronchus runs obliquely in the medi-
astinum beFore plunging into the medial depression (hilum) oF
the lung on its own side (²igure 22.7). Te right main bronchus
is wider, shorter, and more vertical than the lef. Consequently, it
is more common For an inhaled Foreign object to get stuck there.
Once inside the lungs, each main bronchus subdivides into
lobar (secondary) bronchi
—three on the right and two on the
lef—each supplying one lung lobe. Te lobar bronchi branch
segmental (tertiary) bronchi
, which divide
repeatedly into smaller and smaller bronchi (Fourth-order, ﬁfh-
order, etc.). Passages smaller than 1 mm in diameter are called
(“little bronchi”), and the tiniest oF these, the
, are less than 0.5 mm in diameter.
(a) Cross section of the trachea
(b) Photomicrograph of the tracheal
(c) Scanning electron micrograph of cilia in the trachea (2500
Tissue composition of the tracheal wall.
In the scanning electron micrograph
in (c), the cilia appear as yellow, grasslike projections. Mucus-secreting goblet cells (orange) with
short microvilli are interspersed between the ciliated cells.