C h a p t e r 2 1
Control of Respiratory Function
519
of urban dwellers, as well as smokers, usually show
many alveolar macrophages filled with carbon and
other polluting particles from the environment. The
alveolar macrophages also phagocytose insoluble infec-
tious agents such as
Mycobacterium tuberculosis.
The
activated macrophages then aggregate to form a fibrin-
encapsulated granuloma, called a
tubercle,
which serves
to contain the infection (see Chapter 23).
Pulmonary and Bronchial Circulations
The lungs are provided with a dual blood supply: the
pulmonary and bronchial circulations. The pulmonary
circulation arises from the pulmonary artery and pro-
vides for the gas exchange function of the lungs (see
Fig. 21-7). Deoxygenated blood leaves the right heart
through the pulmonary artery, which divides into a left
pulmonary artery that enters the left lung and a right
pulmonary artery that enters the right lung. Return of
oxygenated blood to the heart occurs by way of the
pulmonary veins, which empty into the left atrium. It is
important to note that this is the only part of the circu-
lation in which arteries carry deoxygenated blood and
veins carry oxygenated blood.
The pulmonary circulation serves several important
functions in addition to gas exchange. It removes throm-
boemboli (blood clots) from the circulation, functions
as a metabolic organ, and serves as a blood reservoir for
the left side of the heart. Small pulmonary vessels trap
thromboemboli, and endothelial cells lining the vessels
release fibrolytic substances that help dissolve them.
Vasoactive hormones are metabolized in the pulmonary
circulation. Angiotensin I is activated and converted to
angiotensin II by the angiotensin-converting enzyme
(ACE) located on the surface of the pulmonary capil-
lary endothelial cells. Pulmonary endothelial cells also
inactivate bradykinin, serotonin, and some of the pros-
taglandins. As a blood reservoir, the pulmonary circula-
tion contains approximately 500 mL of the total blood
volume. During a hemorrhagic event, some of this blood
can be mobilized to improve cardiac output.
The bronchial circulation provides the blood supply
for the conducting airways and the supporting struc-
tures of the lung. It also has a secondary function of
warming and humidifying incoming air as it moves
through the conducting airways. The bronchial arter-
ies arise from the thoracic aorta and enter the lungs
with the major bronchi, dividing and subdividing along
with the bronchi as they move out into the lung, sup-
plying them and other lung structures with oxygen. The
blood from the capillaries in the bronchial circulation
drains into the bronchial veins, with the blood from
the larger bronchial veins emptying into the vena cava
and blood from the smaller bronchial veins draining
into the pulmonary veins. Because the bronchial circu-
lation does not participate in gas exchange, this blood
is deoxygenated. As a result, it dilutes the oxygenated
blood returning to the left side of the heart by way of
the pulmonary veins.
The bronchial blood vessels are the only ones that
can undergo angiogenesis (formation of new vessels)
and develop collateral circulation when vessels in the
pulmonary circulation are obstructed, as in pulmonary
embolism. The development of new blood vessels helps
to keep lung tissue alive until the pulmonary circulation
can be restored.
Type I
alveolar cell
Type II
alveolar cell
Alveolar
lumen
Surfactant
Basal
lamina
Macrophage
Inhaled
particle
Endothelial
cells
Erythrocyte
Capillary
lumen
Nucleus
Lamellar
inclusion body
Mitochondria
FIGURE 21-8.
Schematic illustration of type I and
type II alveolar cells and their relationship to the
alveoli and pulmonary capillaries.Type I alveolar cells
comprise most of the alveolar surface.The type II
alveolar cells, which produce surfactant, are located
at the corners between adjacent alveoli. Also shown
are the endothelial cells, which line the pulmonary
capillaries, and an alveolar macrophage.
