CHAPTER 9
Circulatory System
159
SYNOPSIS 9-1
Structure and Functions of the Heart
The heart is composed of two
atria
(singular:
atrium
) and two
ventricles
.
The wall of the heart consists of
endocardium
(inner layer),
myocardium
(cardiac muscle layer), and
epicardium
(outer
layer).
Endocardium
is composed of endothelium, subendothelium (a thin layer of connective tissue), and subendocardium.
Purkinje F bers are located in the subendocardium.
Myocardium
is composed of cardiac muscles, the thickest layer of the heart; cardiac muscles are branched and connected
to each other end to end by intercalated disks (junction complexes). Cardiac muscle F
bers require high oxygen supply.
Epicardium
is composed of mesothelium and a thicker layer of connective tissue, which contains coronary vessels, nerves,
and adipose tissue.
The
conductive system
of the heart consists of groups of specially modiF
ed cardiac muscle F
bers, the
SA node
,
AV node
,
AV bundle
, and
Purkinje f bers
.
Figure 9-2.
The
heart and its impulse conductive ±unction.
The contractions of the right and left
atria
and
ventricles
must be coordinated precisely in order for blood to be pumped efF
ciently to
the lungs for oxygenation and then to the rest of the body. A network of specially modiF
ed cardiac muscle F
bers generates and con-
ducts electrical impulses that provide the necessary coordination. This network includes the
SA node
, the
AV node
, the
AV bundle
,
and
Purkinje f
bers
. (1) The
SA node
is located in the right atrial wall near the opening of the
superior vena cava
. The SA node is
important for initiating the heartbeat impulse and controlling its frequency; it is also called the
pacemaker” of the heart. (2) The
AV
node
is also located in the right atrial wall, medial to the right AV valve, and along the lower part of the interatrial septum. (3) The
AV bundle
arises from the AV node and divides into two branches (right and left) to run along the sides of the interventricular sep-
tum. (4)
Purkinje f bers
are the terminal branches of the right and left branches of the AV bundle. Purkinje F
bers run along the infe-
rior and lateral wall of the ventricle and extend to the
papillary muscles
. These F bers are modiF
ed large cardiac muscle cells, which
contain numerous
gap junctions
and well-developed
intercalated disks
. The SA node generates the heartbeat signal, which quickly
spreads to adjacent cardiac muscle cells in the
myocardium
of both atria to cause the atria to contract. Impulses are also picked up
by the AV node and travel along the AV bundle. These impulses pass through the
right
and
le±t bundle branches
to Purkinje F bers
in the ventricles, where they induce contraction of the cardiac muscles of the ventricles. There is a time delay, which allows the atria
to contract F rst to empty blood into the ventricles, before the ventricles contract. This time delay ensures that blood fl
ows smoothly
from the atria to the ventricles and then to the conductive arteries.
Superior
vena cava
Inferior
vena cava
Sinoatrial (SA)
node
Atrioventricular
(AV) node
Atrioventricular
(AV) bundle
Left bundle branch
Purkinje fibers
Left pulmonary veins
Right bundle
branch
Papillary
muscle
Cardiac
valves
RV:
LV:
right ventricle
left ventricle
LV
Left
atrium
RV
Right
atrium
Myocardium
Epicardium
Endocardium
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