CHAPTER 5 • VASCULAR FUNCTION
99
relationship, or the change in volume (AV)
divided by the change in pressure (AP) at a
given pressure:
AV
Eq. 5-4
C = —
or,
AV = C ■
AP
AP
Therefore,
a highly compliant vessel will
display a relatively small increase in pressure
for a given increase in volume. Conversely, a
less compliant vessel (i.e., “stiffer” vessel) will
display a relatively large increase in pressure
for a given increase in volume.
The compliance of a blood vessel is deter-
mined in large part by the relative propor-
tion of elastin fibers versus smooth muscle
and collagen in the vessel wall (see Fig. 3.7).
Elastin fibers offer the least resistance to
stretch, whereas collagen offers the greatest
resistance. A vessel such as the aorta that has
a greater proportion of elastin fibers versus
smooth muscle and collagen has a relatively
low resistance to stretch and, therefore, has a
compliance that is greater than that found in a
muscular artery that has more smooth muscle
and less elastin.
The
relatively high
compliance
of the
aorta dampens the pulsatile output of the left
ventricle, thereby reducing the pulse pres-
sure. If the aorta were a rigid tube, the pulse
pressure would be very high with each ven-
tricular ejection. However, as blood is ejected
into the aorta, the walls of the aorta expand
to accommodate the increase in blood vol-
ume contained within the aorta because the
aorta is compliant. As the aorta expands, the
increase in pressure is determined by the
change in aortic volume divided by the com-
pliance of the aorta at that particular range of
volumes (Fig. 5.5, panel A). The less compli-
ant the aorta, the greater the pressure change
(i.e., pulse pressure) at any given change in
aortic volume (Fig. 5.5, panel B). Age and
arteriosclerotic disease decrease aortic com-
pliance, which increases aortic pulse pres-
sure. It is not uncommon for elderly people
to have aortic pulse pressures of 60 mm Hg
or more, whereas younger adults have aortic
pulse pressures of about 40 to 45 mm Hg at
resting heart rates.
A
change
in
compliance
affects
only
pulse pressure and not the mean pressure,
which remains unchanged as long as cardiac
output and systemic vascular resistance do
not change. In contrast, a change in stroke
volume normally changes mean aortic pres-
sure in addition to pulse pressure because
the cardiac output changes. For example,
if stroke volume and cardiac output are
increased by an increase in i notropy, both
pulse pressure and mean arterial pressure
<1)
E
o
>
o
c
o
<
Normal
Decreased
Compliance
(mmHg)
(mmHg)
Increased
Stroke Volume
(mmHg)
Increased
Mean Aortic Pressure
(mmHg)
■ FIGURE 5.5 Effects of stroke volume, aortic compliance, and mean aortic pressure on aortic pulse
pressure. Panel A. A t a given stroke volume (DV), the pulse pressure (DP) is determ ined by the aortic
compliance
(red line).
Panel B. Decreasing the aortic compliance (slope of
red line)
increases the pulse
pressure at a given stroke volume. Panel C. Increasing the stroke volume into the aorta increases the pulse
pressure. Panel D. At higher mean aortic pressures
(dotted line),
a given stroke volume produces a greater
pulse pressure because the aortic compliance is less at higher pressures and volumes. Panels A-C assume
a constant mean aortic pressure.
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