Chapter 18
The Cardiovascular System: The Heart
681
18
Te situation in the right side of the heart is essentially the
same as in the leF side
except
for pressure. Te pulmonary cir-
culation is a low-pressure circulation as evidenced by the much
thinner myocardium of its right ventricle. So, typical systolic
and diastolic pressures for the pulmonary artery are 24 and
10 mm Hg, compared to systolic and diastolic pressures of
120 and 80 mm Hg, respectively, for the aorta. However, the two
sides of the heart eject the same blood volume with each heartbeat.
Check Your Understanding
12.
The second heart sound is associated with the closing of
which valve(s)?
13.
If the mitral valve were insufficient, would you expect to hear
the murmur (of blood flowing through the valve that should
be closed) during ventricular systole or diastole?
14.
During the cardiac cycle, there are two periods when all four
valves are closed. Name these two periods.
For answers, see Appendix H.
Cardiac Output (CO)
Name and explain the effects of various factors regulating
stroke volume and heart rate.
Explain the role of the autonomic nervous system in
regulating cardiac output.
Cardiac output (CO)
is the amount of blood pumped out by
each
ventricle in 1 minute. It is the product of heart rate (HR)
and stroke volume (SV).
Stroke volume
is defined as the vol-
ume of blood pumped out by one ventricle with each beat. In
general, stroke volume correlates with the force of ventricular
contraction.
Using normal resting values for heart rate (75 beats/min) and
stroke volume (70 ml/beat), the average adult cardiac output
can be computed:
Te normal adult blood volume is about 5 L (a little more
than 1 gallon). As you can see, the entire blood supply passes
through each side of the heart once each minute.
Notice that cardiac output varies directly with SV and HR.
Tis means that CO increases when the stroke volume increases
or the heart beats faster or both, and it decreases when either or
both of these factors decrease.
Cardiac output is highly variable and increases markedly in
response to special demands, such as running to catch a bus.
Cardiac reserve
is the difference between resting and maxi-
mal CO. In nonathletic people, cardiac reserve is typically four
to five times resting CO (20–25 L/min), but CO in trained
athletes during competition may reach 35 L/min (seven times
resting CO).
How does the heart accomplish such tremendous increases in
output? ±o understand this feat, let’s look at how stroke volume
this period, and the AV valve flaps begin to driF toward the
closed position. (Te remaining 20% is delivered to the ven-
tricles when the atria contract toward the end of this phase.)
Now the stage is set for atrial systole. ²ollowing depo-
larization (P wave of ECG), the atria contract, compressing
the blood in their chambers. Tis causes a sudden slight
rise in atrial pressure, which propels residual blood out of
the atria into the ventricles. At this point the ventricles are
in the last part of their diastole and have the maximum
volume of blood they will contain in the cycle, an amount
called the
end diastolic volume
(
EDV
). Ten the atria relax
and the ventricles depolarize (QRS complex). Atrial dias-
tole persists through the rest of the cycle.
2
Ventricular systole (atria in diastole).
As the atria relax,
the ventricles begin contracting. Teir walls close in on
the blood in their chambers, and ventricular pressure
rises rapidly and sharply, closing the AV valves. Te split-
second period when the ventricles are completely closed
chambers and the blood volume in the chambers remains
constant as the ventricles contract is the
isovolumetric
contraction phase
(i
0
so-vol
0
u-met
9
rik).
Ventricular pressure continues to rise. When it finally
exceeds the pressure in the large arteries issuing from the
ventricles, the isovolumetric stage ends as the SL valves
are forced open and blood rushes from the ventricles into
the aorta and pulmonary trunk. During this ventricular
ejection phase, the pressure in the aorta normally reaches
about 120 mm Hg.
3
Isovolumetric relaxation: early diastole.
During this brief
phase following the ± wave, the ventricles relax. Because
the blood remaining in their chambers, referred to as the
end systolic volume
(
ESV
), is no longer compressed, ven-
tricular pressure drops rapidly and blood in the aorta and
pulmonary trunk flows back toward the heart, closing the
SL valves. Closure of the aortic valve raises aortic pressure
briefly as backflowing blood rebounds off the closed valve
cusps, an event beginning at the
dicrotic notch
shown on
the pressure graph. Once again the ventricles are totally
closed chambers.
All during ventricular systole, the atria have been in diastole.
Tey have been filling with blood and the intra-atrial pressure
has been rising. When blood pressure on the atrial side of the
AV valves exceeds that in the ventricles, the AV valves are forced
open and ventricular filling, phase
1
, begins again. Atrial pres-
sure drops to its lowest point and ventricular pressure begins to
rise, completing the cycle.
Assuming the average heart beats 75 times each minute, the
cardiac cycle lasts about 0.8 s, with atrial systole accounting for
0.1 s and ventricular systole 0.3 s. Te remaining 0.4 s is a period
of total heart relaxation, the
quiescent period
.
Notice two important points: (1) Blood flow through the
heart is controlled entirely by pressure changes and (2) blood
flows down a pressure gradient through any available opening.
Te pressure changes, in turn, reflect the alternating contraction
and relaxation of the myocardium and cause the heart valves to
open, which keeps blood flowing in the forward direction.
CO
5
HR
3
SV
5
75 beats
3
70 ml
min
beat
5
5250 ml
5
5.25 L
min
min
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