112
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
the left ventricular filling is not enhanced
during inspiration. During expiration, how-
ever, blood is forced from the pulmonary
vasculature into the left atrium and ventricle,
thereby increasing left ventricular filling and
stroke volume.
The
net
effect of respiration is
that increasing the rate and depth of respiration
increases venous return and cardiac output.
If a person exhales forcefully against a closed
glottis (Valsalva maneuver), intrapleural pres-
sure becomes very positive, which causes
the transmural pressure to become negative,
thereby collapsing the thoracic vena cava. This
dramatically increases resistance to venous
return and reduces venous return. Because
of the accompanying decrease in transmural
pressure across the ventricular chamber walls,
ventricular
volume
decreases
(particularly
the more compliant right ventricle) despite
the large increase in the pressure within the
chamber. Decreased chamber volume (i.e.,
decreased preload) leads to a fall in ven-
tricular stroke volume by the Frank—Starling
mechanism. Similar changes can occur when
a person strains while having a bowel move-
ment or when a person lifts a heavy weight
while holding his or her breath.
Summary of Factors Affecting
Central Venous Pressure
As previously discussed, CVP plays a very
important role in cardiac filling and ventricular
stroke volume (via the Frank—Starling mecha-
nism). Conditions that elevate CVP increase car-
diac output, whereas conditions that decrease
CVP decrease cardiac output. Furthermore, as
described in Chapter 8, elevation in CVP can
lead to peripheral edema. Therefore, it is impor-
tant to understand how the following different
conditions influence CVP (see Fig. 5.17):
1. An
increase
in
total
blood
volume
(hypervolemia), as occurs in renal failure
or with activation of the renin—angioten-
sin—aldosterone system (see Chapter 6),
increases thoracic blood volume and there-
fore CVP.
2. A decrease in cardiac output, caused by
decreased heart rate (e.g., bradycardia) or
stroke volume (e.g., ventricular failure),
causes blood to back up into the venous
circulation
(increased
venous
volume)
as less blood is pumped into the arterial
circulation.
The
resultant
increase
in
thoracic blood volume elevates CVP.
t Total Blood
Volume
Muscle
Pump
i
Cardiac
Output
i
SVR
Respiratory
Pump
Valsalva
i
Venous
Reclining or
Compliance
Squatting
■ FIGURE 5.17 Summary of conditions that alter central venous pressure
(CVP).
(+), increase CVP; (-),
decrease CVP.
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