722
UNIT 4
Maintenance of the Body
19
Left pulmonary
artery
Air-filled
alveolus
of lung
Pulmonary
capillary
Two lobar arteries
to left lung
CO
2
O
2
Right pulmonary
artery
Three lobar arteries
to right lung
Pulmonary
veins
Right
atrium
Aortic arch
Pulmonary trunk
Right
ventricle
Left
ventricle
Pulmonary
veins
Left atrium
Gas exchange
(b) Illustration. The pulmonary arterial system is shown in blue to indicate that the blood it carries is oxygen-poor.
The pulmonary venous drainage is shown in red to indicate that the blood it transports is oxygen-rich.
Table 19.3
Pulmonary and Systemic Circulations
Pulmonary Circulation
Te pulmonary circulation
(Figure 19.19a)
functions only to
bring blood into close contact with the alveoli (air sacs) of the
lungs so that gases can be exchanged. It does not directly serve
the metabolic needs of body tissues.
Oxygen-poor, dark red blood enters the pulmonary circula-
tion as it is pumped from the right ventricle into the large
pulmo-
nary trunk
(Figure 19.19b), which runs diagonally upward for
about 8 cm and then divides abruptly to form the
right
and
lef
pulmonary arteries
. In the lungs, the pulmonary arteries subdi-
vide into the
lobar arteries
(lo
9
bar) (three in the right lung and
two in the le± lung), each of which serves one lung lobe. Te
lobar arteries accompany the main bronchi into the lungs and
then branch profusely, forming first arterioles and then the dense
networks of
pulmonary capillaries
that surround and cling to
the delicate air sacs. It is here that oxygen moves from the alveolar
air to the blood and carbon dioxide moves from the blood to the
alveolar air. As gases are exchanged and the oxygen content of
the blood rises, the blood becomes bright red. Te pulmonary
capillary beds drain into venules, which join to form the two
pul-
monary veins
exiting from each lung. Te four pulmonary veins
complete the circuit by unloading their precious cargo into the
le± atrium of the heart.
Figure 19.19
Pulmonary circulation.
RA
5
right atrium, RV
5
right ventricle, LA
5
left atrium, LV
5
left ventricle.
R. pulmonary veins
Pulmonary
trunk
L. pulmonary
veins
(a) Schematic flowchart.
Pulmonary
capillaries
of the
R. lung
Pulmonary
capillaries
of the
L. lung
R. pulmonary
artery
L. pulmonary
artery
From
systemic
circulation
To
systemic
circulation
RA
RV
LV
LA
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