114
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
As described in more detail below, cardiac
output under normal physiologic conditions
depends on systemic vascular function. Cardiac
output is limited to a large extent by the prevail-
ing state of systemic vascular function. There-
fore, it is important to understand how changes
in systemic vascular function affect cardiac out-
put and venous return (or total systemic blood
flow because cardiac output and venous return
are equal under steady—state conditions).
The best way to
show how systemic
vascular function affects systemic blood flow
is by use of systemic vascular and cardiac
function curves. Credit for the conceptual
understanding of the relationship between
cardiac output and systemic vascular func-
tion goes to Arthur Guyton and colleagues,
who conducted extensive experiments in the
1950s and 1960s. To develop the concept of
systemic vascular function curves, we must
understand the relationship between car-
diac output, mean aortic pressure, and right
atrial pressure. Figure 5.18 shows that at a
cardiac output of 5 L/min, the right atrial
pressure is near zero and mean aortic pres-
sure is about 95 mm Hg. If cardiac output
is reduced experimentally, right atrial pres-
sure
increases
and
mean
aortic pressure
decreases. The fall in aortic pressure reflects
the relationship between mean aortic pres-
Cardiac Output (L/min)
■ FIGURE 5.18 Effects of cardiac output on mean
aortic and right atrial pressures. Decreasing car-
diac output results in a rise in right atrial pressure
and a fall in aortic pressure. When cardiac output
is zero, both pressures equilibrate at the mean
circulatory filling pressure (Pmc).
sure, cardiac output, and systemic vascular
resistance (see Equation 5—3). As cardiac
output is reduced to zero, right atrial pres-
sure
continues
to
rise
and
mean
aortic
pressure continues to fall, until both pres-
sures are equivalent, which occurs when
systemic blood flow ceases. When all flow
ceases, pressures throughout all
the sys-
temic circulation are equal. The pressure at
zero systemic flow, which is called the mean
circulatory filling pressure, is about 7 mm
Hg. This value is found experimentally when
baroreceptor reflexes are blocked; otherwise
the value for mean circulatory filling pres-
sure is higher because of vascular smooth
muscle contraction and decreased vascular
compliance owing to sympathetic activation.
The reason right atrial pressure increases
in response to a decrease in cardiac output is
that less blood per unit time is translocated by
the heart from the venous to the arterial vas-
cular compartment. This leads to a reduction
in arterial blood volume and pressure, and to
an increase in venous blood volume and pres-
sure, which increases right atrial pressure.
When the heart is completely stopped and
there is no flow in the systemic circulation,
the intravascular pressure found throughout
the entire vasculature is a function of total
blood volume and vascular compliance.
Finally, it is important to note in Figure 5.18
that if one attempts to increase cardiac out-
put above 5 L/min by increasing heart rate,
for example, cardiac output will not increase
much above 5 L/min. The reason is that right
atrial pressure falls below zero, which collapses
the vena cava at the level of the diaphragm
where it enters the thorax from the abdomen.
This increases the resistance of the vena cava,
thereby limiting venous return into the thorax,
which limits the cardiac output.
The magnitude of the relative changes in
aortic and right atrial pressures from a normal
cardiac output to zero cardiac output is deter-
mined by the ratio of venous to arterial com-
pliances. If venous compliance (CV) equals
the change in venous volume (AVV) divided
by the change in venous pressure (APV), and
arterial compliance (CA) equals the change in
arterial volume (AVa) divided by the change in
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