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CHAPTER FIFTEEN
Cardiovascular System
contract so they empty all their blood into the ventricles
prior to ventricular systole.
Once the cardiac impulse reaches the distal side of the
AV node, it passes into a group of large fibers that make
up the
AV bundle
(atrioventricular bundle or bundle of
His), and the impulse moves rapidly through them. The AV
bundle enters the upper part of the interventricular septum
and divides into right and left bundle branches that lie just
beneath the endocardium. About halfway down the septum,
the branches give rise to enlarged
Purkinje fibers
(pur-
kin
je F
berz). These larger F
bers carry the impulse to distant
regions of the ventricular myocardium much faster than cell-
to-cell conduction could. As a result, the massive ventricular
myocardium contracts as a functioning unit.
The base of the aorta, which includes the aortic valve, is enlarged and
protrudes somewhat into the interatrial septum close to the AV bun-
dle. Consequently, inF
ammatory conditions, such as bacterial endo-
carditis a±
ecting the aortic valve (aortic valvulitis), may also a±
ect the
AV bundle.
If a portion of the bundle is damaged, it may no longer conduct
impulses normally. As a result, cardiac impulses may reach the two
ventricles at di±
erent times so that they fail to contract together. This
condition is called a
bundle branch block.
The Purkinje F
bers spread from the interventricular sep-
tum into the papillary muscles, which project inward from
the ventricular walls, and then continue downward to the
apex of the heart. There they curve around the tips of the
ventricles and pass upward over the lateral walls of these
chambers. Along the way, the Purkinje F bers give off many
small branches, which become continuous with cardiac
muscle F bers.
Figure 15.18
shows the parts of the conduc-
tion system and
f gure 15.19
summarizes them.
The muscle F
bers in the ventricular walls form irregu-
lar whorls. When impulses on the Purkinje F bers stimulate
these muscle fibers, the ventricular walls contract with a
twisting motion
(f g. 15.20)
. This action squeezes blood out
of the ventricular chambers and forces it into the aorta and
pulmonary trunk.
Another property of the conduction system is that the
Purkinje F bers transmit the impulse to the apex of the heart
F rst. As a result, contraction begins at the apex and pushes
the blood superiorly toward the aortic and pulmonary semi-
lunar valves, rather than originating the impulse superi-
orly and pushing blood toward the apex, as it would if the
impulse traveled from cell to cell.
PRACTICE
21
What is the function of the cardiac conduction system?
22
What types of tissues make up the cardiac conduction system?
23
How is a cardiac impulse initiated?
24
How is a cardiac impulse transmitted from the right atrium to the
other heart chambers?
SA node
AV node
AV bundle
Right bundle
branch
Purkinje fibers
Interventricular
septum
Interatrial septum
Left
bundle
branch
Junctional
fibers
SA node
Atrial syncytium
Junctional fibers
AV bundle
Bundle branches
Purkinje fibers
Ventricular syncytium
AV node
FIGURE 15.19
Components of the cardiac conduction system.
FIGURE 15.18
The cardiac conduction system coordinates the cardiac
cycle.
A significant percentage of cases of heart failure in adults of African
descent may be due to an inherited condition called familial amyloido-
sis. A protein called amyloid forms deposits in the heart, causing angina
(chest pain), failure of cardiac muscle function (cardiomyopathy), block-
age of conduction of electrical impulses, and disturbed heart rhythm
(arrhythmia). Echocardiography can detect the amyloid deposits that
thicken the ventricular walls. It is important to distinguish amyloidosis
from other forms of arrhythmias, because drug treatments are di±
erent.
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