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CARDIOVASCULAR PHYSIOLOGY CONCEPTS
■ FIGURE 5.22 Combined cardiac and systemic function curves: effects of chronic heart failure. The
normal operating intercept (point
A)
is shifted to point
B
when cardiac function alone is depressed by
diminished inotropy. Compensatory increases in total blood volume (Vo/) and systemic vascular resistance
(SW?), along with reduced venous compliance (C„), shift the systemic function to the right and decrease
the slope. The new combined intercept (point C) represents partial compensation in cardiac output at the
expense of a large increase in right atrial pressure (PRA).
changes during heart failure can be depicted
using cardiac and systemic function curves
as shown in Figure 5.22. In this figure, point
A
represents the operating point in a normal
heart, and point B indicates where a heart
might operate when it is in failure in the
absence of systemic compensation—cardiac
output would be greatly reduced and right
atrial pressure would be elevated. Compen-
satory increases in blood volume and sys-
temic vascular resistance, along with reduced
venous compliance, shift the systemic func-
tion curve to the right and decrease the slope.
The new, combined intercept (point C) rep-
resents a partial compensation in the cardiac
output at the expense of a large increase in
right atrial pressure. The increased atrial pres-
sure helps to support ventricular preload and
stroke volume through the Frank—Starling
mechanism.
In summary, total blood flow through the
systemic circulation depends on both car-
diac and systemic vascular function. Car-
diac stimulation in a normal heart has only
a modest effect on cardiac output; however,
if systemic function is additionally altered by
decreasing venous compliance and systemic
vascular resistance, the cardiac output is able
to increase. Without changes in systemic
function, cardiac output is limited by the
return of blood to the heart and ventricular
filling.
SUMMARY OF IMPORTANT CONCEPTS
*
Regulation of arterial pressure and
organ blood flow is primarily the
function of the small resistance
vessels—arteries and arterioles.
Capillaries are the principal site for
exchange and most of the blood
volume is found in the venous
capacitance vessels.
*
Mean arterial pressure is determined
by the product of cardiac output and
systemic vascular resistance, plus CVP.
*
Aortic pulse pressure is primarily
determined by ventricular stroke
volume and aortic compliance.
*
Vascular resistance is inversely related
to the vessel radius to the fourth
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