CHAPTER 9
Circulatory System
161
Figure 9-4C.
Myocardial Infarction.
H&E,
3
68
Myocardial infarction (MI)
, also known as a
heart attack
, occurs
when the blood supply to part of the heart is completely or partially
blocked because of
atherosclerosis
or the rupture of an
atheroscle-
rotic
plaque
and the formation of a blood clot. Symptoms range from
characteristic chest discomfort (
angina
) and shortness of breath to
sudden death.
Coronary artery disease (CAD)
is the most common
underlying cause of this medical emergency. Atherosclerotic plaques,
which consist of lipids, F broblasts, collagen, and white blood cells
(especially macrophages), are found in the wall of an affected cor-
onary artery. The plaques cause luminal narrowing or occlusion
(±ig. 9-12A). The histologic appearance of MI depends on the age
of the infarct. Early changes include edema and
hypereosinophilia
,
followed by inF ltration by neutrophils,
coagulation necrosis
, phago-
cytosis of dead cells by macrophages, formation of granulation tis-
sue, and, ultimately, the formation of a
dense collagenous
scar
.
Necrotic
cardiac muscle
Remaining
normal cardiac
muscle cells
C
Spongiosa
Spongiosa
Spongiosa
Endothelium
Papillary
muscle
Mitral valves
Tricuspid
valve
Chordae
tendineae
Endothelium
Fibrosa
Fibrosa
Fibrosa
Ventricularis
Ventricularis
Ventricularis
B
Figure 9-4B.
Cardiac valves, aortic valve.
H&E,
3
134
There are four
valves
in the heart: two
AV valves
(
mitral
and
tricuspid
valves
) in the chambers and two
semilunar valves
(
aortic
and
pul-
monary valves
) in the arteries leaving the heart. Heart valves consist
of connective tissues and both surfaces are covered by endothelium.
They are composed of three layers: (1)
Spongiosa
consists of loosely
arranged collagen and elastic F
bers and the surface is covered by the
endothelium
. This layer is continuous with the atrial or blood vessel
side. (2)
Fibrosa
is the core of the heart valve, which contains dense
irregular connective tissue. (3)
Ventricularis
is a dense connective tis-
sue layer with many elastic and collagen F
bers. The surface of the
ventricularis is covered by
endothelium
. The heart valves open and
close to allow the blood to fl
ow through the openings and to prevent
the backfl
ow of blood. Shown is an example of the
aortic
valve
. The
aortic valve
has three cusps and lies between the left ventricle and the
aorta. The
tricuspid
and
mitral
valves
are anchored to the ventricle
wall by
chordae tendineae
, which are attached to
papillary muscles
.
Common heart valve diseases include
calci± c aortic valve disease
,
valvitis
, and
rheumatic heart disease
. These diseases can lead to aortic
stenosis, aortic regurgitation, embolism, and mitral valve stenosis.
Figure 9-4A.
Purkinje ±
bers and intercalated disks, ventricle.
H&E,
3
136; inset (
right
)
3
198, inset (
left
)
3
229
Purkinje ± bers
(
impulse-conducting ± bers
) are large, modiF ed car-
diac muscle cells, which are part of the heart conducting system. They
are terminal branches of the AV bundle branches (±ig. 9-2), located in
subendocardial connective tissue
. Purkinje F bers often appear large in
size and cluster as groups. Each cell has only one or two nuclei and pale-
staining cytoplasm because it has fewer
myo± brils
than regular cardiac
muscle cells. Purkinje F bers work together with other impulse- conducting
structures to regulate the heartbeats by conveying impulses to neigh-
boring
cardiac muscle cells
(±ig. 9-2).
Intercalated disks
are specialized
junctional complexes
that contain
fascia adherens
,
desmosomes
(
macula
adherens
), and
gap junctions
, which provide connection and communi-
cation between the cardiac muscle cells. Intercalated disks bind cardiac
muscle cells together in an entire unit to prevent muscle cells being pulled
apart during contraction. They also provide
ion exchange
through gap
junctions, allowing electrical impulses to pass from one cell to another.
Nuclei of
Nuclei of
muscle cells
muscle cells
Intercalated
Intercalated
disk
disk
Purkinje
Purkinje
fibers
fibers
Purkinje fibers
Purkinje fibers
Purkinje fibers
Purkinje fibers
Subendocardial
Subendocardial
connective
connective
tissue
tissue
Cardiac muscle
Cardiac muscle
(cross section)
(cross section)
Purkinje
fibers
Purkinje fibers
Purkinje fibers
Subendocardial
connective
tissue
Cardiac muscle
(cross section)
Cardiac muscle
Cardiac muscle
(longitudinal section)
(longitudinal section)
Cardiac muscle
(longitudinal section)
Intercalated
disk
Nuclei of
muscle cells
Myocardium
Myocardium
Myocardium
Endocardium
Endocardium
Endocardium
A
CLINICAL CORRELATIONS
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