CHAPTER 5 • VASCULAR FUNCTION
113
3. A decrease in systemic vascular resist-
ance by selective arterial dilation increases
blood flow from the arterial into the venous
compartments, thereby increasing venous
volume and CVP, while at the same time
reducing arterial volume and pressure (dis-
cussed later in this chapter).
4. Constriction of peripheral veins (reduced
venous compliance) elicited by sympa-
thetic activation or circulating vasocon-
strictor substances (e.g., catecholamines,
angiotensin II) causes blood volume to
be
translocated
from
peripheral
veins
into the thoracic compartment, thereby
increasing CVP.
5. Postural changes such as moving from a
standing to a reclining or squatting posi-
tion diminishes venous pooling in the legs
caused by gravity, which increases thoracic
volume and CVP.
6. A forceful expiration against a high resist-
ance (Valsalva maneuver) causes external
compression of the thoracic vena cava
(decrease in functional compliance), which
increases CVP.
7. Increased respiratory activity (abdomino-
thoracic pump) facilitates venous return
into the thorax, thereby helping to main-
tain CVP when cardiac output is elevated
during exercise.
8. Rhythmic muscular contraction (muscle
pump), particularly of the limbs during
exercise, compresses the veins and facili-
tates
venous
return
into
the
thoracic
compartment, which increases CVP.
VENOUS RETURN AND
CARDIAC OUTPUT
The Balance between Venous
Return and Cardiac Output
Venous return is the flow of blood back to
the heart. It was previously described how the
venous return to the right atrium from the
abdominal vena cava is determined by the pres-
sure gradient between the abdominal vena cava
and the right atrium, divided by the resistance
of the vena cava. However, that analysis looks
at only a short segment of the venous system
and does not show what factors determine
venous return from the capillaries. Venous
return from capillaries is determined by the dif-
ference between the mean capillary and right
atrial pressures divided by the resistance of all
the postcapillary vessels. If we consider venous
return as being all the systemic flow return-
ing to the heart, venous return is determined
by the difference between the mean aortic and
right atrial pressures divided by the systemic
vascular resistance. Therefore, the pressures
and resistances that are used as the hemody-
namic variables for determining venous return
depend on whether one is defining venous
return from specific locations in the systemic
vasculature, or if one is viewing venous return
as the flow of blood throughout all the systemic
circulation as it travels back to the heart.
An important concept to note is the follow-
ing:
under steady-state conditions, venous return
equals cardiac output when averaged over time
.
The reason for this is that the cardiovascular
system is essentially a closed system. Strictly
speaking, the cardiovascular system is not a
closed system because fluid is lost through the
kidneys and by evaporation through the skin,
and fluid enters the circulation through the
gastrointestinal tract. Nevertheless, a balance is
maintained between fluid entering and leaving
the circulation during steady—
state conditions.
Therefore, it is appropriate to view the system
as closed, and therefore cardiac output and
venous return as being equal. There may occur
transient imbalances, such as when a person
suddenly starts to run and venous return is
augmented by the muscle and abdominotho-
racic pumps; however, this augmentation leads
to an increase in cardiac output by the Frank—
Starling mechanism and cardiac stimulation
so that shortly after starting to run the cardiac
output once again equals the venous return,
although at a higher level of cardiac output.
Systemic Vascular Function Curves
Blood
flow
through
the
entire
systemic
circulation,
whether
viewed
as
the
flow
leaving the heart (cardiac output) or return-
ing to the heart (venous return), depends on
both cardiac and systemic vascular function.
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