Maintenance of the Body
Two lobar arteries
to left lung
Three lobar arteries
to right lung
(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.
Pulmonary and Systemic Circulations
Te pulmonary circulation
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
(Figure 19.19b), which runs diagonally upward for
about 8 cm and then divides abruptly to form the
. In the lungs, the pulmonary arteries subdi-
vide into the
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
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
exiting from each lung. Te four pulmonary veins
complete the circuit by unloading their precious cargo into the
le± atrium of the heart.
right atrium, RV
right ventricle, LA
left atrium, LV
R. pulmonary veins
(a) Schematic flowchart.