The Cardiovascular System: The Heart
, inﬂammation of the pericardium, roughens the
serous membrane surfaces. Consequently, as the beating heart
rubs against its pericardial sac, it creates a creaking sound (
cardial friction rub
) that can be heard with a stethoscope. Peri-
carditis is characterized by pain deep to the sternum. Over time,
it may lead to adhesions in which the visceral and parietal peri-
cardia stick together and impede heart activity.
In severe cases, large amounts of inﬂammatory ﬂuid seep
into the pericardial cavity. Tis excess ﬂuid compresses the
heart and limits its ability to pump blood, a condition called
), literally, “heart plug.” Physi-
cians treat cardiac tamponade by inserting a syringe into the
pericardial cavity and draining oﬀ the excess ﬂuid.
Layers of the Heart Wall
Te heart wall, richly supplied with blood vessels, is composed
of three layers: the epicardium, myocardium, and endocardium
As we have noted, the superﬁcial
is the visceral
layer of the serous pericardium. It is o±en inﬁltrated with fat,
especially in older people.
Te middle layer, the
(“muscle heart”), is
composed mainly of cardiac muscle and forms the bulk of the
heart. Tis is the layer that contracts. In the myocardium, the
branching cardiac muscle cells are tethered to one another by
crisscrossing connective tissue ﬁbers and arranged in spiral or
. Tese interlacing bundles eﬀec-
tively link all parts of the heart together.
Te connective tissue ﬁbers form a dense network, the ﬁ-
, that reinforces the myocardium inter-
nally and anchors the cardiac muscle ﬁbers. Tis network of
collagen and elastic ﬁbers is thicker in some areas than others.
For example, it constructs ropelike rings that provide additional
support where the great vessels issue from the heart and around
Parietal layer of serous
layer of serous
The pericardial layers and layers of the heart wall.
the heart valves (see Figure 18.6a, p. 666). Without this sup-
port, the vessels and valves might eventually become stretched
because of the continuous stress of blood pulsing through them.
Additionally, because connective tissue is not electrically excit-
able, the cardiac skeleton limits the spread of action potentials
to speciﬁc pathways in the heart.
Te third layer of the heart wall, the
the heart”), is a glistening white sheet of endothelium (squamous
epithelium) resting on a thin connective tissue layer. Located on
the inner myocardial surface, it lines the heart chambers and
covers the ﬁbrous skeleton of the valves. Te endocardium is
continuous with the endothelial linings of the blood vessels
leaving and entering the heart.
The circular and spiral arrangement of cardiac
muscle bundles in the myocardium of the heart.