. When stripped of their associated
blood vessels and tissues, the airways appear as an upside
down tree. The successive divisions of these branches from
the trachea to the microscopic air sacs follow:
left primary bronchi.
Three branch from the
right primary bronchus, and two branch from the left.
Each of these
branches supplies a portion of the lung called a
Usually ten such segments
are in the right lung and eight are in the left lung.
These small branches of the
segmental bronchi enter the basic units of the lung—the
These tubes branch from
an intralobular bronchiole. Fifty to eighty terminal
bronchioles occupy a lobule of the lung.
Two or more respiratory
bronchioles branch from each terminal bronchiole.
Short and about 0.5 millimeter in diameter, these
structures are called “respiratory” because a few air sacs
bud from their sides, enabling them to take part in gas
Alveolar ducts branch from each
Alveolar sacs are thin-walled, closely
packed outpouchings of the alveolar ducts.
o-li). Alveoli are thin-walled, microscopic
air sacs that open to an alveolar sac. Air can diffuse
freely from the alveolar ducts, through the alveolar sacs,
and into the alveoli
Dust particles, asbestos fibers, and other pollutants
travel at speeds of 200 centimeters per second in the trachea
but slow to 1 centimeter per second when deep in the lungs.
Gravity deposits such particles, particularly at branchpoints
¯) consists of branched air-
ways leading from the trachea to the microscopic air sacs in
the lungs. Its branches begin with the right and left
which arise from the trachea at the level of the ±
thoracic vertebrae. The openings of the primary bronchi are
separated by a ridge of cartilage called the
(see ± g.
19.8). Each bronchus, accompanied by large blood vessels,
enters its respective lung.
Branches of the Bronchial Tree
A short distance from its origin, each primary bronchus
(two on the left
and three on the right) that, in turn, branch repeatedly
Lumen of trachea
Cross section of the trachea. Note the C-shaped ring of
hyaline cartilage in the wall.
Light micrograph of a section of the tracheal
A tracheostomy may be performed to allow air to
bypass an obstruction in the larynx.