minute, with the typical rate at about 70 beats per minute.
We will look at the regulation of the cardiovascular system in
more detail later in the chapter.
The coordinated action potentials among various heart
muscle cells create electrical currents that can be detected by
electrodes attached to the surface of the chest. The record of
the electrical changes accompany-
ing the heartbeat is called an
elec-
trocardiogram
(
EKG
or
ECG
).
Cardiologists
analyze the EKG pat-
tern to diagnose problems with the
heart (see Figure 11.6).
When the normal rhythm established by the SA node
(the normal sinus rhythm) is disrupted by a defect in the
conduction
system,
a
condition
called
arrhythmia
occurs. There
may be several causes of arrhyth-
mias, including disease (such as
coronary artery disease, hypertension, myocardial infarc-
tion, hyperthyroidism, or defective heart valves), stress,
and drugs and other chemicals (such as caffeine, cocaine,
nicotine, or alcohol). Some examples of arrhythmias in-
clude the following:
S u p ra ve n tricu la r tachycardia
—A rapid but regular heart
rate (160-200 beats/min) that originates in the atria.
Episodes begin and end suddenly and may last minutes
to hours.
H e a rt block
—A blockage of the electrical pathways
between atria and ventricles, often at the AV node.
Prem ature ventricular contraction
—An early heartbeat in
the ventricular myocardium that briefly interrupts the
normal rhythm. A similar arrhythmia, called atrial pre-
mature contraction, can occur in the atrial myocardium.
A tr ia l flu tte r
—Rapid but regular atrial contractions
(240-360 beats/min) accompanied by AV block.
V entricular tachycardia
—A rapid heart rhythm (at least
120 beats/min) that originates in one of the ventricles.
A tr ia lfib r illa tio n
—Asynchronous contractions (300-600
beats/min)
in which atrial contraction stops alto-
gether. A similar, but more deadly, condition called
ventricular fibrillation occurs in the ventricles. Ven-
tricular fibrillation is treated by passing an electrical
current through the heart to stop the contractions
(defibrillation).
Now that we have seen how the heart is structured, the
path that blood flows through it, and the way in which the
electrical activity coordinates contractions, let’s put it all
together and examine the events of the cardiac cycle.
The Cardiac Cycle Alternates
Between Systole and Diastole
With each heartbeat, there is a
cycle of events called the car-
diac cycle (see Figure 11.7).
During each cardiac cycle, the
cardiac chambers alternate be-
tween a phase of contraction,
known as
systole
, and a phase
of relaxation, called
diastole
.
1.
During diastole, or the relax-
ation period, the chamber of
the heart fills with blood.
2.
At systole, the chamber empties. The systole phase
begins with the atria contracting (atrial systole) and
forcing blood into the ventricles.
3.
Then the ventricles contact (ventricular systole),
pumping blood
into
the
pulmonary and
systemic
circulations.
Each chamber returns to diastole following contrac-
tion and begins filling again. Each phase of the cardiac
cycle corresponds to one wave of the EKG.
The entire cardiac cycle lasts about 0.8 seconds. The
atria contract for 0.1 s and then rest for 0.7 s. In con-
trast, the ventricles contract for about 0.3 s and then rest
for 0.5 s. For 0.4 s of the cycle, both the atria and ven-
tricles are relaxed. When the heart rate increases, this
relaxation time becomes shorter. Because most of the fill-
ing occurs during the first 0.1 s, as long as the chamber
spends at least that long in diastole, there will be plenty
of blood to pump out at systole.
The sound of the heartbeat comes primarily from
the turbulence in blood flow created by the closure of
the valves, not from the contraction of the heart muscle.
The first sound,
lu b b ,
is a long, booming sound from the
AV valves closing after ventricular systole begins. Next,
a short, sharp sound,
d u b b ,
comes from the semilunar
valves closing at the end of the ventricular systole. There
is a pause during the relaxation period. Thus you can
hear the cardiac cycle with a stethoscope as
lu b b , dubb,
pause;
lu b b , d u b b ,
pause.
So, how do we measure the work done by the heart?
Read on to find out.
cardiologists
(kar-
de-OL-o-jists) Physi-
cians who study the
heart and diseases
associated with it.
arrhythmia
(a-RlTH-
me-a) An irregular
heart rhythm.
systole
(SIS-to-le)
The contraction phase
of the cardiac cycle in
which blood moves out
of the heart chamber.
diastole
(dT-AS-to-le)
The relaxation phase
of the cardiac cycle
in which the heart
chamber is resting
and filling with blood.
316 CHAPTER 11
The Cardiovascular System: Heart, Blood Vessels, and Circulation
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