555
CHAPTER FIFTEEN
Cardiovascular System
The inner layer, or
endocardium
(en
do-kar
de-um),
consists of epithelium and underlying connective tissue.
The endocardium also contains blood vessels and some
specialized cardiac muscle fibers called
Purkinje fibers,
described later in this chapter in the section entitled “Cardiac
Conduction System.”
The endocardium lines all of the heart chambers and
covers the structures, such as the heart valves, that pro-
ject into them. This inner lining is also continuous with the
inner linings (endothelium) of the blood vessels attached to
the heart and throughout the cardiovascular system.
Table
15.1
summarizes the characteristics of the three layers of the
heart wall.
Heart Chambers and Valves
Internally the heart is divided into four hollow chambers,
two on the left and two on the right. The upper chambers,
called
atria
(a
tre-ah) (sing.,
atrium
), have thin walls and
receive blood returning to the heart. Small, earlike projec-
tions called
auricles
(aw
ri-klz) extend anteriorly from the
atria, slightly increasing atrial volume (see F
g. 15.4). The
lower chambers, the
ventricles
(ven
tri-klz), force the blood
out of the heart into arteries.
A structure called the
interatrial septum
separates the
right from the left atrium. An
interventricular septum
separates
the two ventricles. The atrium on each side communicates
with its corresponding ventricle through an opening called
the
atrioventricular orif
ce
(a
tre-o-ven-trik
u-lar ori-F s),
guarded by an
atrioventricular valve
(AV valve).
Grooves on the surface of the heart mark the divi-
sions between its chambers, and they also contain major
blood vessels that supply the heart tissues. The deepest of
In
pericarditis,
inf
ammation oF the pericardium due to viral or bacte-
rial inFection produces adhesions that attach the layers oF the pericar-
dium to each other. This condition is painFul and interFeres with heart
movements.
PRACTICE
1
Where is the heart located?
2
Where would you listen to hear the apical heartbeat?
3
Distinguish between the visceral pericardium and the parietal
pericardium.
4
What is the Function oF the f
uid in the pericardial cavity?
Wall oF the Heart
The wall of the heart is composed of three distinct layers: an
outer epicardium, a middle myocardium, and an inner endo-
cardium
(f g. 15.5)
.
The
epicardium
(ep
ı˘-kar
de-um), which corresponds
to the visceral pericardium, protects the heart by reducing
friction. It is a serous membrane that consists of connec-
tive tissue covered by epithelium, and it includes capillaries
and nerve F
bers. The deeper portion of the epicardium often
contains fat, particularly along the paths of coronary arteries
and cardiac veins that provide blood fl ow through the myo-
cardium.
The middle layer, or
myocardium
(mi
o-kar
de-um),
is thick and largely consists of the cardiac muscle tissue that
pumps blood out of the heart chambers. The muscle F bers are
arranged in planes, separated by connective tissues richly sup-
plied with blood capillaries, lymph capillaries, and nerve F bers.
Diaphragm
Base of heart
Apex of heart
Heart
Sternum
FIGURE 15.3
The heart is posterior to the sternum, where it rests upon the diaphragm.
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