Maintenance of the Body
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
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
crawl freely along the internal alveolar
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.
Respiratory zone structures.
Diagrammatic view of respiratory bronchioles,
alveolar ducts, alveolar sacs, and alveoli.
Photomicrograph of a section of human lung,
showing the respiratory structures that form the ﬁnal divisions of the bronchial tree (80
Notice the thinness of the alveolar walls.