Ganong's Review of Medical Physiology, 23rd Edition

590
SECTION VII
Respiratory Physiology

the primary lining cells of the alveoli, covering approximately
95% of the alveolar epithelial surface area.
Type II cells (gran-
ular pneumocytes)
are thicker and contain numerous lamel-
lar inclusion bodies. A primary function of these cells is to
secrete surfactant; however, they are also important in alveo-
lar repair as well as other cellular physiology. Although these
cells make up approximately 5% of the surface area, they rep-
resent approximately 60% of the epithelial cells in the alveoli.
The alveoli also contain other specialized cells, including pul-
monary 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

The trachea and bronchi have cartilage in their walls but rela-
tively little smooth muscle. They are lined by a ciliated epithe-
lium 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 bronchi-
oles, and their walls do not contain cartilage. However, their
walls contain more smooth muscle, of which the largest

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 bronchoconstric-

tion. 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 broncho-

dilation. They increase bronchial secretion, while

α

1

adrener-

gic receptors inhibit secretion. There is, in addition, a

noncholinergic, nonadrenergic innervation

of the bronchi-

oles 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

Both the

pulmonary circulation

and the

bronchial circula-

tion

contribute to blood flow in the lung. In the pulmonary

circulation, almost all the blood in the body passes via the pul-

monary artery to the pulmonary capillary bed, where it is

oxygenated and returned to the left atrium via the pulmonary

FIGURE 35–1
(
Continued
)
C)
The branching patterns of the air-
way during the transition form conducting to respiratory airway are
drawn (not all divisions are drawn, and drawings are not to scale).

Name of branches

Number

of tubes

in branch

1

4

2

8

16

32

6 x 10^4

5 x 10^5

8 x 10^6

Trachea

Bronchi

Bronchioles

Terminal bronchioles

Respiratory bronchioles

Alveolar ducts

Alveolar sacs

Respiratory zone

Conducting zone

C

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 veloc-

ity 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.)

500

400

300

200

100

Total cross section area (cm

2 )

0 5 10 15 20 23

Airway generation

Terminal

bronchioles

Conducting zone

Respiratory

zone