174
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
all of the cardiac output of the right ventricle,
whereas the bronchial circulation receives about
1% of the left ventricular output. The following
focuses on the pulmonary circulation.
The
pulmonary
circulation
is
a
low-
resistance, low-pressure, high-compliance vas-
cular bed. Although the pulmonary circulation
receives the same cardiac output as the sys-
temic circulation, the pulmonary pressures are
much lower. The pulmonary artery systolic and
diastolic pressures are about 25 and 10 mm Hg,
respectively. The mean pulmonary artery pres-
sure is therefore about 15 mm Hg. If we assume
that the left atrial pressure averages 8 mm Hg,
the perfusion pressure for the pulmonary circu-
lation (mean pulmonary artery pressure minus
left atrial pressure) is only about 7 mm Hg.
This is considerably lower than the perfusion
pressure for the systemic circulation (about 90
mm Hg). Because the flow is the same, but the
perfusion pressure is much lower in the pul-
monary circulation, the pulmonary vascular
resistance must be very low. In fact, pulmonary
vascular resistance is generally 10- to 15-fold
lower than systemic vascular resistance. The
reason for the much lower pulmonary vascular
resistance is that the vessels are larger in diam-
eter, shorter in length, and have many more
parallel elements than the systemic circulation.
Pulmonary vessels are also much more com-
pliant than systemic vessels. Because of this, an
increase in right ventricular output does not
cause a proportionate increase in pulmonary
artery pressure. The reason for this is that the
pulmonary vessels passively distend as the pul-
monary artery pressure increases, which lowers
their resistance. Increased pressure also recruits
additional pulmonary capillaries, which further
reduces resistance. This high vascular compli-
ance and ability to recruit capillaries are impor-
tant mechanisms for preventing pulmonary
vascular pressures from rising too high when
cardiac output increases (e.g., during exercise).
If there were no change in pulmonary vascular
resistance, then increasing cardiac output five-
fold during exercise would cause mean pulmo-
nary artery pressure to increase from 15 to 43
mm Hg (assuming left atrial pressure remains
at 8 mm Hg), and the pulmonary artery systolic
pressure would be even higher.
Increased
pulmonary
vascular
pressure
can have two adverse consequences. First,
increased pulmonary artery pressure increases
the afterload on the right ventricle, which can
impair ejection, and with chronic pressure
elevation, cause right ventricular failure. Sec-
ond, an increase in pulmonary capillary pres-
sure increases fluid filtration (see Chapter 8),
which can lead to pulmonary edema. Pulmo-
nary capillary pressures are ordinarily about
10 mm Hg, which is less than half the value
found in most other organs, and this low pres-
sure is necessary to ensure that excessive fluid
filtration from the pulmonary capillaries does
not occur under normal circumstances.
Unlike other major organs, the concept of
blood flow autoregulation is not applicable to
the pulmonary circulation because pulmonary
artery pressure is the dependent variable instead
of flow. The reason for this is that the entire pul-
monary blood flow is determined by the right
ventricular output, and therefore, pulmonary
artery pressure changes as a function of this flow
and the pulmonary vascular resistance. Because
other organs of the body are in parallel with
each other, changes in left ventricular output
do not necessarily change blood flow in a given
organ except through changes in arterial pres-
sure that may occur. Therefore, in other organs,
blood flow is the dependent variable because
flow depends on perfusion pressure and organ
vascular resistance. Instead of autoregulating
blood flow, the pulmonary circulation autoregu-
lates pulmonary arterial pressure through pas-
sive changes in resistance of highly compliant
vessels and through vessel recruitment.
Because of their low pressures and high
compliance, pulmonary vascular diameters
are strongly influenced by gravity and by
changes in intrapleural pressure during res-
piration. W hen a person stands up, gravity
increases hydrostatic pressures within ves-
sels located in the lower regions of the lungs,
which distends these vessels, decreases resist-
ance, and increases blood flow to the lower
regions. In contrast, vessels located in the
upper regions of the lungs have reduced
intravascular pressures; this increases resist-
ance and reduces blood flow when a person
is standing. Changes in intrapleural pressure
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