Valves Keep the Blood Flowing the
Right Direction Through the Heart
The heart is enclosed in a protective sac called the
peri-
cardium
(
Figure 11.3
). Each side has a receiving cham-
ber called an
atrium
that collects the blood from the
circulation (
right atrium
from systemic circulation,
left
atrium
from the pulmonary circulation) and a pump
called a
ventricle
that delivers blood to the circula-
tion (
right ventricle
to the pulmonary circulation,
left
ventricle
to the systemic circulation). A large muscular
interventricular septum divides the two sides, which op-
erate simultaneously (
Figure 11.4
).
Each side of the heart has
valves
that help to keep
the blood flowing in the proper direction. One set of valves
(atrioventricular) separates the atrium from the ventricle
on each side of the heart, and another (semilunar) sepa-
rates each ventricle from its major artery. The atrioventric-
ular (AV) valves are the
triscuspid valve
(on the right side) and
the
bicuspid,
or mitral,
valve
(on the left side). The semilunar
valves are the
p ulm onic valve
(on the right) and the
aortic valve
(on the left). The AV valves are designed to withstand the
very high pressures produced by the pumping
ventricles. The semilunar valves deal with much
smaller volumes of blood (see Figure 11.4).
Cardiac muscle tissue of varying thick-
nesses forms the
myocardium
. The intercalat-
M icroscopic stru c tu re o f th e h e a rt •
ed discs of cardiac muscle allow the myocardium to function
as a unit by transmitting signals rapidly from cell to cell. The
myocardium is sandwiched between the inner
endocardium
and
the outer
epicardium ,
both of which are composed of thin lay-
ers of connective and epithelial tissues (see Figure 11.3; for
a review of muscle tissue and cardiac muscle contraction, see
Chapter 6).
The walls of both atria are thin because they are main-
ly collection chambers and need to pump blood only a short
distance into the ventricles. The walls of the left ventricle are
thicker than those of the right because the left ventricle must
do more work to drive blood flow through the entire body; the
right ventricle transports blood a shorter distance, to the lungs.
Several large blood vessels called
veins
and
arteries
carry blood toward
and away from the heart, respective-
ly (see Figure 11.4b). The blood sup-
ply to the heart itself comes off the
aorta
via the
coronary arteries
and
drains through the
coronary sinus
into the right atrium.
Here is a simple mnemonic for remembering the differ-
ences between arteries and veins. The “A” in
a
r-
tery stands for “away” from the heart, while the
“in” in ve
in
stands for “
in
to” the heart.
Let’s take a closer look at how blood flows
through the cardiovascular system.
myocardium
The
middle layer of the
heart wall, composed
of cardiac muscle.
Figure 11.3
veins
Blood vessels
that convey blood
from tissues back to
the heart.
arteries
Blood ves-
sels that carry blood
away from the heart.
The heart is composed mostly of cardiac muscle tissue, with connective
tissue and epithelial linings inside and out.
Pericardium
Heart wall
_L
Endocardium
Coronary blood vessels
(arteries, veins) supply
oxygen and nutrients to
the heart and remove
carbon dioxide and wastes.
Pericardial
cavity contains
pericardial fluid that reduces
friction between the
myocardium and pericardium.
Myocardium
Cardiac cells are
often branched
Cardiac cells have
numerous, large
mitochondria.
Pericardium covers and
protects the heart.
Epicardium is the thin
outer layer of the heart.
Myocardium is the heart
muscle itself.
Endocardium lines the inner
chambers of the heart.
Sarcolemma or plasma membrane
Intercalated discs are thickenings of
plasma membrane where two cardiac
cells come together. They hold the
cells together and contain gap junctions.
Opening of T-tubule
Gap junctions allow ions to pass directly
from one cell to another to propagate
action potentials.
Cardiac cells have one large nucleus.
previous page 347 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 349 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off