378
Chapter 12
murmur heard throughout systole suggests a stenotic pulmo-
nary or aortic valve, an insufﬁ cient AV valve, or a hole in the
interventricular septum. In contrast, a murmur heard during
diastole suggests a stenotic AV valve or an insufﬁ cient pulmo-
nary or aortic valve.
The Cardiac Output
The volume of blood each ventricle pumps, usually expressed in
liters per minute, is called the
cardiac output (CO).
Cardiac
output is also the volume of blood ﬂ
owing through either the
systemic or the pulmonary circuit per minute.
The cardiac output is determined by multiplying the
heart rate (HR)—the number of beats per minute—and the
stroke volume (SV)—the blood volume ejected by each ven-
tricle with each beat:
CO = HR
×
SV
Thus, if each ventricle has a rate of 72 beats/min and ejects
70 ml of blood with each beat, the cardiac output is:
CO = 72 beats/min
×
0.07 L/beat = 5.0 L/min
These values are within the normal range for a resting, average-
sized adult. Coincidentally, total blood volume is also approxi-
mately 5 L, so essentially all the blood is pumped around the
circuit once each minute. During periods of strenuous exercise
in well-trained athletes, the cardiac output may reach 35 L/min;
the entire blood volume is pumped around the circuit seven
times a minute! Even sedentary, untrained individuals can
reach cardiac outputs of 20–25 L/min during exercise.
The following description of the factors that alter the
two determinants of cardiac output—heart rate and stroke
volume—applies in all respects to both the right and left sides
of the heart because stroke volume and heart rate are the same
for both under steady-state conditions. Note that heart rate
and stroke volume do not always change in the same direc-
tion. For example, stroke volume decreases following blood
loss, whereas heart rate increases. These changes produce
opposing effects on cardiac output.
Control of Heart Rate
Rhythmical beating of the heart at a rate of approximately
100 beats/min will occur in the complete absence of any ner-
vous or hormonal inﬂ uences on the SA node. This is the inherent
autonomous discharge rate of the SA node. The heart rate may
be lower or higher than this, however, because the SA node is
normally under the constant inﬂ uence of nerves and hormones.
A large number of parasympathetic and sympathetic
postganglionic ﬁ bers end on the SA node. Activity in the para-
sympathetic (vagus) nerves causes the heart rate to decrease,
whereas activity in the sympathetic nerves causes an increase.
In the resting state, there is considerably more parasympa-
thetic activity to the heart than sympathetic, so the normal
resting heart rate of about 70 beats/min is well below the
inherent rate of 100 beats/min.
Figure 12–22
illustrates how sympathetic and para-
sympathetic activity inﬂ uence SA node function. Sympathetic
Normal open valve
Laminar flow = quiet
(a)
Stenotic valve
Narrowed valve
Turbulent flow = murmur
(b)
Normal closed valve
No flow = quiet
Insufficient valve
Leaky valve
Turbulent backflow = murmur
Figure 12–21
Heart valve defects causing turbulent blood ﬂ ow and murmurs.
(a) Normal valves allow smooth, laminar ﬂ ow of blood in the
forward direction when open and prevent backward ﬂ ow of
blood when closed. No sound is heard in either state.
(b) Stenotic valves cause rapid, turbulent forward ﬂ ow of
blood, making a high-pitched, whistling murmur. Valve
insufﬁ ciency results in turbulent backward ﬂ ow when the valve
should be closed, causing a low-pitched gurgling murmur.
Figure 12–21
physiological
inquiry
What valve defect(s) would be indicated by the following
sequence of heart sounds?
“lub-whistle-dup-gurgle”
Answer can be found at end of chapter.
Figure 12–22
Effects of sympathetic and parasympathetic nerve stimulation on
the slope of the pacemaker potential of an SA-nodal cell. Note
that parasympathetic stimulation not only reduces the slope of the
pacemaker potential but also causes the membrane potential to be
more negative before the pacemaker potential begins.
Adapted from Hoffman and Craneﬁ eld.
Membrane potential (mV)
Time
Threshold
potential
b
a
c
60
0
–40
–60
a
,
b,
and
c
are pacemaker potentials:
a
= control
b
= during sympathetic stimulation
c
= during parasympathetic stimulation