CHAPTER 5 â€¢ VASCULAR FUNCTION
115
arterial pressure (APa), the ratio of venous to
arterial compliance (CV
/CA) can be expressed
by the following equation:
Eq. 5-12
C
v
_
A
V
v
/
A
P
v
C
A
A
V
a
/
A
P
a
When the heart is stopped, the decrease
in arterial blood volume (AVa) equals the
increase
in
venous
blood
volume
(AVv).
Because AVa equals AVv Equation 5-12 can
be simplified to the following relationship:
Eq. 5-13
APa
APv
Equation 5-13 shows that the ratio of venous
to arterial compliance is proportional to the
ratio of the changes in arterial to venous pres-
sures when the heart is stopped. This ratio is
usually in the range of 10 to 20. If, for exam-
ple, the ratio of venous to arterial compliance
is 15, there is a 1 mm Hg increase in right
atrial pressure for every 15 mm Hg decrease
in mean aortic pressure.
If the right atrial pressure curve from Figure
5.18 is plotted as cardiac output versus right
atrial pressure (i.e., reversing the axis), the rela-
tionship shown in Figure 5.19 (black curve in
both panels) is observed. This curve is called
the systemic vascular function curve. This rela-
tionship can be thought of as either the effect of
cardiac output on right atrial pressure (cardiac
output being the independent variable) or the
effect of right atrial pressure on venous return
(right atrial pressure being the independent
variable).
When
viewed
from
the
latter
perspective, systemic vascular function curves
are sometimes called venous return curves.
The value of the xâ€”
intercept in Figure 5.19
is the mean circulatory filling pressure, which
is the pressure throughout the vascular sys-
tem when there is no blood flow. This value
depends on the vascular
compliance and
blood volume (Fig. 5.19, panel A). Increased
blood volume or decreased venous compli-
ance causes a parallel shift of the vascular
function curve to the right, which increases
mean circulatory filling pressure. Decreased
blood volume or increased venous compli-
ance causes a parallel shift to the left and a
decrease in the mean circulatory filling pres-
sure. Therefore, at a given cardiac output, an
increase in total blood volume (or decreased
venous compliance) is associated with an
increase in right atrial pressure.
Decreased
systemic
vascular
resistance
increases the slope without appreciably chang-
ing mean circulatory filling pressure (Fig. 5.19,
A
B
â–  FIGURE 5.19 Systemic function curves. Panel A shows the effects of changes in cardiac output on
right atrial pressure (PRA) and mean circulatory filling pressures (Pmc). Changes in blood volume
(Vol)
and
venous compliance (Cv) cause parallel shifts in the curves and changing Pmc. Panel B shows how changes
in systemic vascular resistance
(SVR)
alter the slope of the systemic function curves w ithout changing Pmc.
âˆ