812
UNIT 4
Maintenance of the Body
22
Te alveoli have three other significant features: (1) Tey are
surrounded by fine elastic fibers of the same type that surround
the entire bronchial tree. (2) Open
alveolar pores
connecting
adjacent alveoli allow air pressure throughout the lung to be
equalized and provide alternate air routes to any alveoli whose
bronchi have collapsed due to disease. (3) Remarkably efficient
alveolar
macrophages
crawl freely along the internal alveolar
surfaces.
Although huge numbers of infectious microorganisms
are continuously carried into the alveoli, alveolar surfaces are
usually sterile. Because the alveoli are “dead ends,” aged and
dead macrophages must be prevented from accumulating in
them. Most macrophages simply get swept up by the ciliary cur-
rent of superior regions and carried passively to the pharynx.
In this manner, we clear and swallow over 2 million alveolar
macrophages per hour!
The Lungs and Pleurae
Describe the gross structure of the lungs and pleurae.
Alveolar duct
Alveolar duct
Alveoli
Alveolar
sac
Alveolar
pores
Respiratory bronchioles
Terminal
bronchiole
(a)
Alveolar
duct
Respiratory
bronchiole
Alveoli
Alveolar
sac
(b)
Figure 22.8
Respiratory zone structures.
(a)
Diagrammatic view of respiratory bronchioles,
alveolar ducts, alveolar sacs, and alveoli.
(b)
Photomicrograph of a section of human lung,
showing the respiratory structures that form the final divisions of the bronchial tree (80
3
).
Notice the thinness of the alveolar walls.
previous page 846 Human Anatomy and Physiology (9th ed ) 2012 read online next page 848 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off