108
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
change CVP (APy) by changing either venous
blood volume (AVV) or venous compliance
(CV) as described by Equation 5-11.
Eq. 5-11
APy - AVv
CV
Equation 5-11 is a rearrangement of the
equation used to define compliance (Equa-
tion
5-4),
in which
compliance
(in
this
case venous compliance) equals a change in
venous volume divided by a change in venous
pressure. Therefore, an increase in venous vol-
ume increases venous pressure by an amount
determined by the compliance of the veins.
Furthermore, a decrease in venous compli-
ance, as occurs during sympathetic activation
of veins, increases venous pressure.
The relationship described by Equation 5-11
can be depicted graphically as shown in
Figure 5.13, in which venous blood volume
is plotted against venous blood pressure. The
different
curves
represent
different
states
■ FIGURE 5.13 Compliance curves for a vein.
Venous compliance (the slope of line tangent to a
point on the curve) is very high at low pressures
because veins collapse. As pressure increases, the
vein assumes a more circular cross-section and its
walls become stretched; this reduces compliance
(decreases slope). Point
A
is the control pressure
and volume. Point
B
shows how pressure increases
along the compliance curves as volume increases.
Point
C
shows how pressure increases as vol-
ume decreases when venous tone is increased
(decreased compliance) by sym pathetic stim ula-
tion of the vein, for example.
of venous tone, and the slope of a tangent
line at any point on the curve represents the
compliance. Looking at a single curve, it is
evident that an increase in venous volume
will increase venous pressure (point
A
to B).
The amount by which the pressure increases
for a given change in volume depends on the
slope of the relationship between the volume
and pressure (i.e., the compliance). As with
arterial vessels (see Fig. 5.5), the relationship
between venous volume and pressure is not
linear (see Fig. 5.13). The slope of the compli-
ance curve (AV/AP) is greater at low pressures
and volumes than at higher pressures and
volumes. The reason for this is that at very
low pressures, a large vein collapses. As the
pressure increases, the collapsed vein assumes
a more cylindrical shape with a circular cross-
section. Until a cylindrical shape is attained,
the walls of the vein are not stretched appreci-
ably. Therefore, small changes in pressure can
result in a large change in volume by changes
in vessel geometry rather than by stretching
the vessel wall. At higher pressures, when the
vein is cylindrical in shape, increased pressure
can increase the volume only by stretching the
vessel wall, which is resisted by the structure
and composition of the wall (particularly by
collagen, smooth muscle, and elastin com-
ponents). Therefore, at higher volumes and
pressures, the change in volume for a given
change in pressure (i.e., compliance) is less.
The smooth muscle within veins is ordinar-
ily under some degree of tonic contraction. Like
arteries and arterioles, a major factor determin-
ing venous smooth muscle contraction is sym-
pathetic adrenergic stimulation, which occurs
under basal conditions. Changes in sympa-
thetic activity can increase or decrease the
contraction of venous smooth muscle, thereby
altering venous tone. When this occurs, a
change in the volume-pressure relationship
(or compliance curve) occurs, as depicted in
Figure 5.13. For example, increased sympa-
thetic activation shifts the compliance curve
down and to the right, decreasing its slope
(compliance) at any given volume (from point
A
to
C
in Fig. 5.13). This rightward diagonal
shift in the venous compliance curve results in
a decrease in venous volume and an increase
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