CHAPTER 5 • VASCULAR FUNCTION
111
lowering
right
atrial
pressure
facilitates
venous
return.
These
changes in
venous
return significantly influence stroke volume
through the Frank—Starling mechanism.
Pressures and volumes in the right atrium
and thoracic vena cava depend on the sur-
rounding intrapleural pressure. This pres-
sure is measured in the space between the
thoracic wall and the lungs and is generally
negative
(subatmospheric).
During
inspi-
ration, the chest wall expands and the dia-
phragm descends (red arrows on chest wall
and diaphragm in Fig. 5.16). This causes
the intrapleural pressure (Ppl)
to become
more negative, causing expansion
of the
lungs, atrial and ventricular chambers, and
vena cava (smaller red arrows). This expan-
sion decreases the pressures within the ves-
sels and cardiac chambers. As right atrial
pressure falls during inspiration, the pres-
sure gradient for venous return to the heart
is increased. During expiration the opposite
occurs, although the net effect of respiration
is that the increased rate and depth of ventila-
tion facilitates venous return and ventricular
stroke volume.
Although it may appear paradoxical, the
fall in right atrial pressure during inspiration
is associated with an
increase
in right atrial
and ventricular preloads and right ventricular
stroke volume. This occurs because the fall in
intrapleural pressure causes the transmural
pressure to increase across the chamber walls.
The
transmural pressure is
the difference
between the pressure within the chamber and
the pressure outside the chamber (Ppl). When
transmural pressure increases, the chamber
volume increases, which increases sarcomere
length and myocyte preload. For example, if
intrapleural pressure is normally -4 mm Hg
at end—expiration and right atrial pressure is
0 mm Hg, the transmural pressure (the pres-
sure that distends the atrial chamber) is 4 mm
Hg. During inspiration, if intrapleural pres-
sure decreases to -8 mm Hg and atrial pressure
decreases to -2 mm Hg, the transmural pres-
sure across the atrial chamber increases from
4 to 6 mm Hg, thereby expanding the cham-
ber. At the same time, because blood pressure
within the atrium is diminished, this leads
to an increase in venous return to the right
atrium from the abdominal vena cava. Similar
increases in right ventricular transmural pres-
sure and preload occur during inspiration. The
increase in sarcomere length during inspira-
tion augments right ventricular stroke volume
by the Frank—Starling mechanism. In addition,
changes in intrapleural pressure during inspi-
ration influence the left atrium and ventricle;
however, the expanding lungs and pulmonary
vasculature
act
as
a capacitance reservoir
(pulmonary blood volume increases) so that
Air
SVC
Venous
Return
Chest Wall
_
_
Diaphram
Lungs
_
_
^ R V
_
p
_
RA
pl
IVC
—►
— Venous
_
_
Return
■ FIGURE 5.16 Effects of respiration on venous return. Left panel. During inspiration, intrapleural pressure
(Pp|) decreases as the chest wall expands and the diaphragm descends (large red arrows). This increases
the transmural pressure across the superior and inferior vena cava
(SVC
and
IVC),
right atrium
(RA),
and
right ventricle (RV), which causes them to expand. This facilitates venous return and leads to an increase
in atrial and ventricular preloads. Right panel. During inspiration, Ppl and right atrial pressure (PRA) become
more negative, which increases venous return. During expiration, Ppl and PRA become less negative and
venous return falls. Numeric values for
P
and PRA are expressed as mm Hg.
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