384
U N I T 5
Circulatory Function
Although cardiac muscle cells require calcium for
contraction, they have a less well-defined sarcoplasmic
reticulum for storing calcium than skeletal muscle cells.
Thus, cardiac muscle relies more heavily than skeletal
muscle on an influx of extracellular calcium ions for
contraction. The cardiac glycosides (e.g., digoxin) are
inotropic drugs that enhance cardiac contractility by
increasing the free intracellular calcium concentration
in cardiac muscle.
Endocardium
The endocardium is a thin, three-layered membrane
that lines the heart and covers the valves. The inner-
most layer, which lines the heart chambers, consists of
smooth endothelial cells supported by a thin layer of
connective tissue. The endothelial lining of the endocar-
dium is continuous with the lining of the blood vessels
that enter and leave the heart. The middle layer consists
of dense connective tissue with elastic fibers. The outer
layer, which is composed of irregularly arranged con-
nective tissue cells, contains blood vessels and branches
of the conduction system and is continuous with the
myocardium.
Heart Valves and Fibrous Skeleton
An important structural feature of the heart is its fibrous
skeleton, which consists of four interconnecting valve
rings and surrounding connective tissue (Fig. 17-7). The
fibrous skeleton separates the atria and ventricles and
forms a rigid support for attachment of the valves and
insertion of the cardiac muscle. The tops of the valve
rings are attached to the muscle tissue of the atria,
Endocardium
Myocardium
Visceral pericardium
(epicardium)
Pericardial
cavity
Parietal
pericardium
Fibrous
pericardium
FIGURE 17-5.
Layers of the heart, showing the visceral
pericardium, the pericardial cavity, and the parietal
pericardium.
Longitudinal portion
(contains large gap junctions)
Muscle fiber
Nucleus
Striations
Intercalated
disk
A
B
FIGURE 17-6.
(A)
Cardiac muscle fibers showing their
branching structure.
(B)
Area indicated where cell junctions lie
in the intercalated disks.
Mitral valve
Tricuspid valve
Aortic valve
Pulmonic valve
FIGURE 17-7.
Fibrous skeleton of the heart, which forms the
four interconnecting valve rings and support for attachment of
the valves and insertion of cardiac muscle.
