98
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
output (CO), systemic vascular resistance
(SVR), and central venous pressure (CVP) as
shown in Equation 5-3.
Eq. 5-3
MAP = (CO ■
SVR) + CVP
This equation is based on Equation 5-1,
where AP = F . R. The AP in Equation 5-3 rep-
resents the pressure drop across the entire sys-
temic circulation, which is MAP - CVP; the
CO and SVR are the F and R, respectively, of
Equation 5-1. Therefore, from Equation 5-3,
changes in cardiac output, systemic vascular
resistance, or CVP affect mean arterial pres-
sure. If cardiac output and systemic vascular
resistance change reciprocally and propor-
tionately, MAP will not change. For example,
if cardiac output is reduced by one-half and
systemic vascular resistance is doubled, mean
arterial pressure will remain unchanged.
Figure 5.4, which is based upon Equation 5-3,
shows that as cardiac output is increased, a
linear increase occurs in arterial pressure
(assuming that resistance and venous pressure
remain constant). An increase in systemic vas-
cular resistance (increased slope of the line)
results in a greater arterial pressure for any
given cardiac output. Conversely, a decrease
in resistance results in a lower arterial pressure
for any given cardiac output.
Cardiac output, systemic vascular resistance,
and venous pressure are constantly changing,
and they are interdependent (i.e., changing
■ FIGURE 5.4 The relationship between cardiac
output
(CO),
systemic vascular resistance
(SVR),
mean arterial pressure (MAP), and central venous
pressure
(CVP).
Increasing SVR increases MAP at
any given cardiac output
(dotted line),
whereas
decreasing SVR decreases MAP at a given cardiac
output. This figure is based on Equation 5-3, in
which MAP = (CO . SVR) + CVP.
one variable can change each of the other
variables). For example, increasing systemic
vascular resistance increases the afterload on
the heart, which decreases cardiac output and
alters CVP, as described in more detail later in
this chapter. Furthermore, extrinsic control
mechanisms acting on the heart and circulation
can affect these variables. If, for example, car-
diac output suddenly falls by 20% (as can occur
when standing), mean arterial pressure will not
decrease by 20% because the body compen-
sates by increasing systemic vascular resistance
through baroreceptor mechanisms to maintain
constant pressure (see Chapter 6).
Aortic Pulse Pressure
As blood flows down the aorta and into dis-
tributing arteries, characteristic changes take
place in the shape of the pressure wave con-
tour. As the pressure pulse moves away from
the heart, the systolic pressure rises, and the
diastolic pressure falls. The change in the
shape of the pressure pulse is related to a num-
ber of factors including (1) decreased compli-
ance of distal arteries and (2) reflective waves,
particularly
from
arterial
branch
points,
which summate with the pulse wave trave-
ling down the aorta and arteries. In addition,
mean arterial pressure declines as the pres-
sure pulse travels down distributing arteries
owing to the resistance of the arteries; how-
ever, the reduction in mean pressure is small
(just a few mm Hg) because the distributing
arteries have a relatively low resistance. There-
fore, the values measured for arterial pressure
differ depending on the site of measurement.
When the arterial pressure is measured using a
sphygmomanometer (i.e., blood pressure cuff)
on the upper arm, the pressure measurement
represents the pressure within the brachial
artery. The measured pressures, however, are
not identical with the systolic and diastolic
pressures found in the aorta or the pressures
measured in other distributing arteries.
The
compliance
of
the
aorta
and
the
ventricular stroke volume determine pulse
pressure.
Compliance
is
defined
by
the
relationship between volume and pressure, in
which compliance (C) equals the slope of that
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