160
UNIT 3
Organ Systems
A
Myocardium
Myocardium
Myocardium
Endocardium
Endocardium
Endocardium
Myocardium
M
y
o
c
a
r
d
i
u
m
Myocardium
Figure 9-3A.
Layers of the heart wall: endocardium, ventri-
cle.
H&E,
3
68
The wall of the heart is much thicker than the wall of the large
vessels and is composed of three basic layers, as are the blood
vessels:
endocardium
(equivalent to the
tunica intima
),
myocar-
dium
(equivalent to the
tunica media
), and
epicardium
(equiva-
lent to
the
tunica adventitia
). The
endocardium
is the innermost
layer of the heart wall, which lines the lumen of the heart. This
layer consists of
endothelium
(simple squamous epithelium),
subendothelial connective tissue
, and
subendocardium
. The
subendocardium is in contact with the cardiac muscle and con-
tains small coronary blood vessels, nerves, and
Purkinje ±
bers
in certain areas (Fig. 9-4A). Some adipose cells are also pres-
ent within the connective tissue. The endocardium provides a
smooth lining for the four chambers of the heart and provides a
covering for the
AV valves
.
B
Nuclei of the cardiac muscle cells
Nuclei of the cardiac muscle cells
Nuclei of the cardiac muscle cells
Branched
Branched
cardiac muscle
cardiac muscle
Branched
cardiac muscle
Intercalated disk
Intercalated disk
Intercalated disk
Figure 9-3B.
Layers of the heart wall: myocardium, ventri-
cle.
H&E,
3
272; insets
3
786
Myocardium
is the thickest layer of the heart wall and makes up
the bulk of the heart. It consists of
cardiac muscle cells
that are
arranged in branching columns. The ends of the cardiac mus-
cle cells are connected to each other by
intercalated disks
. The
inset
shows cardiac muscles with their characteristic striations
and an intercalated disk (Fig. 9-4A). These muscles contract
to pump blood out of the ventricles of the heart and distribute
blood to the tissues and organs of the body. Myocardium of the
left ventricle wall is the thickest because of the fact that it must
pump the blood a great distance and overcome the high pressure
and resistance of the systemic circulation. In general, the atria
have thinner walls than the ventricles. Myocardium of the right
atrium is the thinnest because of the relatively low pressure and
resistance of the blood circulation.
C
Epicardium
Epicardium
(visceral layer)
(visceral layer)
Myocardium
Myocardium
Epicardium
(visceral layer)
Myocardium
Lumens of the
blood vessels
Figure 9-3C.
Layers of the heart wall: epicardium, ventricle.
H&E,
3
68
Epicardium
surrounds the heart. It is a layer of connective tissue
that contains nerves, blood vessels, and adipocytes. The inner
surface of the epicardium is connected with cardiac muscle, and
the outer surface is covered by
mesothelium
(see Fig. 3-2) that
faces the
pericardial cavity
. Mesothelium secretes a fl
uid known
as
pericardial fl uid
, which provides lubrication and reduces fric-
tion between the
epicardium
(
visceral pericardium
) and the
pari-
etal pericardium
during the movements caused by heart contrac-
tion. Epicardium covers and protects the heart and allows small
blood vessels and nerves to pass through to provide nutrients
and nerve innervation.
Pericardial effusion
refers to excess fl
uid in the pericardial
cavity due to infl
ammation of the pericardium (
pericarditis
);
hemopericardium
is a condition in which blood is trapped in
the pericardial cavity. In either case, compression of the thin-
walled atria and vena cava can result in cardiac tamponade
and failure of circulation.
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