at the end of the systemic circulation). Similarly, the right
ventricle will pump the blood into the pulmonary circula-
tion, where the blood will flow from the pulmonary artery
(higher pressure) to the left atrium (lower pressure).
The contractions of atria and ventricles, which gen-
erate pressure and force blood to flow through the sys-
tem, are coordinated: Both atria contract together, and
later, both ventricles contract together. At the “heart” of
this coordination is an elaborate electrical system that
stimulates muscle cells to contract. Let’s take a look.
Electrical Signals Control Heart Rate
About 1%
of cardiac muscle cells differ from the rest in
that they can generate action potentials over and over
again in a rhythmic pattern. These cells, the heart’s natu-
ral pacemaker, control the rate at which the heart beats
and form an electrical conduction system called the car-
diac conduction system. The electrical activity begins
in the sinoatrial node (SA node), passes over the atria to
the atrioventricular node (AV node), and spreads via the
Purkinje fibers throughout the ventricles, using the inter-
calated discs to pass the electrical signals (Figure 11.6).
This conduction system coordinates the contraction of the
atria and the later contraction of the ventricles in a series
of events referred to as the
cardiac cycle.
The cardiac conduction system sets the heart rate but
can be influenced by the sympathetic and parasympathetic
divisions of the autonomic nervous system. Left alone, the
heart would beat at a rate between 60 to 100 times per
The h e a rt's electrical activity • Figure 11.6
Coordination of the heartbeat is done by waves of depolarization (contraction of
cardiac muscle fibers) and repolarization (relaxation of cardiac muscle fibers) that
spread through the cardiac conduction system. This system generates action poten-
tials and transmits them to the heart muscle cells, creating
currents that can be used for
diagnostic purposes.
Sinoatrial
(SA)
node
The contraction impulse
begins in the SA node.
Atrioventricular
(AV)
node
The contraction passes in a
wavelike fashion through the atria
and is collected at the AV node.
Atrioventricular
(AV)
bundle
From the AV node, the impulse
is sent down the AV bundle.
Right and left bundle branches
The impulse is then passed to
the right and left bundle branches.
i Purkinje fibers
Finally, the impulse passes
through the Purkinje fibers, and
then on to the cells of the ventricles
Left atrium
Anterior view of
frontal section
Electrocardiogram (EKG
or
ECG)
• P wave-atrial depolarization (SA node ^ both atria)
• QRS complex—onset of ventricular contraction (AV node ^ both ventricles)
• T wave—ventricular repolarization
Note—Atrial repolarization is masked by QRS complex.
Right
ventricle
Left
ventricle
1.0
0.5
=
0.
-0.5
Conduction of all action potentials through
the heart generates electrical currents that
can be picked up by surface electrodes to
produce an EKG.
Atrial
Ventricular
contraction
contraction
R
C D
1
—1
segment
hf— H
1
1
T
P
1
1
•>
/
I____ U»
s
s
\
/
P -
1
c
-
Q
-► ! 1
1
interval 1
S
1
| l
_ n _
j
1
........
Seconds
The Heart Pumps Blood Through Blood Vessels to All Tissues
315
PROCESS DIAGRAM
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