Ganong's Review of Medical Physiology, 23rd Edition

590 SECTION VII Respiratory Physiology
C
Respiratory zone Conducting zone
Trachea
Bronchi
Bronchioles
Terminal bronchioles
Name of branches
Respiratory bronchioles
Alveolar ducts
Alveolar sacs
FIGURE 35–1 (Continued) C) The branching patterns of the airway
during the transition form conducting to respiratory airway are
drawn (not all divisions are drawn, and drawings are not to scale).
the primary lining cells of the alveoli, covering approximately
95% of the alveolar epithelial surface area. Type II cells (granular
pneumocytes) are thicker and contain numerous lamellar
inclusion bodies. A primary function of these cells is to
secrete surfactant; however, they are also important in alveolar
repair as well as other cellular physiology. Although these
cells make up approximately 5% of the surface area, they represent
approximately 60% of the epithelial cells in the alveoli.
The alveoli also contain other specialized cells, including pulmonary
alveolar macrophages (PAMs, or AMs), lymphocytes,
plasma cells, neuroendocrine cells, and mast cells. The mast
cells contain heparin, various lipids, histamine, and various
proteases that participate in allergic reactions (see Chapter 3).
THE BRONCHI & THEIR INNERVATION
Number
of tubes
in branch
The trachea and bronchi have cartilage in their walls but relatively
little smooth muscle. They are lined by a ciliated epithelium
that contains mucous and serous glands. Cilia are present
as far as the respiratory bronchioles, but glands are absent
from the epithelium of the bronchioles and terminal bronchioles,
and their walls do not contain cartilage. However, their
walls contain more smooth muscle, of which the largest
1
2
4
8
16
32
6 x 10 4
5 x 10 5
8 x 10 6
Total cross section area (cm 2 )
500
400
300
200
100
0
FIGURE 35–2 Total airway cross-sectional area as a function
of airway generation. Note the extremely rapid increase in total
cross-sectional area in the respiratory zone. As a result, forward velocity
of gas during inspiration falls to a very low level in this zone.
(Reproduced with permission from West JB: Respiratory Physiology: The Essentials, 4th
ed. Williams & Wilkins, 1991.)
Conducting zone
amount relative to the thickness of the wall is present in the
terminal bronchioles.
The walls of the bronchi and bronchioles are innervated by
the autonomic nervous system. Muscarinic receptors are
abundant, and cholinergic discharge causes bronchoconstriction.
The bronchial epithelium and smooth muscle contain
β 2 -adrenergic receptors. Many of these are not innervated.
Some may be located on cholinergic endings, where they
inhibit acetylcholine release. The β 2 receptors mediate bronchodilation.
They increase bronchial secretion, while α 1 adrenergic
receptors inhibit secretion. There is, in addition, a
noncholinergic, nonadrenergic innervation of the bronchioles
that produces bronchodilation, and evidence suggests
that vasoactive intestinal polypeptide (VIP) is the mediator
responsible for the dilation.
ANATOMY OF BLOOD
FLOW IN THE LUNG
5 10 15 20
Airway generation
Terminal
bronchioles
Respiratory
zone
Both the pulmonary circulation and the bronchial circulation
contribute to blood flow in the lung. In the pulmonary
circulation, almost all the blood in the body passes via the pulmonary
artery to the pulmonary capillary bed, where it is
oxygenated and returned to the left atrium via the pulmonary
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