Cardiovascular Physiology
365
SECTION B
The Heart
Anatomy
The heart is a muscular organ enclosed in a fi brous sac, the
pericardium,
and located in the chest (
Figure 12–6
). The
inner layer of the pericardium is closely affi xed to the heart
and is called the
epicardium.
The extremely narrow space
between the outer wall of the pericardium and the epicardium
is fi
lled with a watery fl uid that serves as a lubricant as the
heart moves within the sac.
The walls of the heart, the
myocardium,
are composed
primarily of cardiac muscle cells. The inner surface of the car-
diac chambers, as well as the inner wall of all blood vessels, is
lined by a thin layer of cells known as
endothelial cells,
or
endothelium.
As noted earlier, the human heart is divided into right
and left halves, each consisting of an atrium and a ventricle.
The two ventricles are separated by a muscular wall, the
interventricular septum.
Located between the atrium and
ventricle in each half of the heart are the
atrioventricular
(AV) valves,
which permit blood to fl ow from atrium to ven-
tricle but not from ventricle to atrium. The right AV valve is
called the
tricuspid valve
because it has three fi brous fl aps, or
cusps
(
Figure 12–7
). The left AV valve has two fl aps and is
thus called the
bicuspid valve.
(Its resemblance to a bishop’s
headgear has earned the left AV valve another commonly used
name,
mitral valve.
)
The opening and closing of the AV valves is a passive pro-
cess resulting from pressure differences across the valves. When
the blood pressure in an atrium is greater than in the ventri-
cle, the valve is pushed open and blood fl ows from atrium to
ventricle. In contrast, when a contracting ventricle achieves an
internal pressure greater than that in its connected atrium, the
AV valve between them is forced closed. Therefore, blood does
not normally move back into the atria, but is forced into the
pulmonary trunk from the right ventricle and into the aorta
from the left ventricle.
To prevent the AV valves from being pushed up into the
atria (a condition called
prolapse
), the valves are fastened to
muscular projections
(papillary muscles)
of the ventricular
walls by fi brous strands
(chordae tendineae).
The papillary
muscles do not open or close the valves. They act only to limit
the valves’ movements and prevent them from being everted.
The openings of the right ventricle into the pulmonary
trunk and of the left ventricle into the aorta also contain valves,
Figure 12–6
Diagrammatic section of the heart. The arrows indicate the direction of blood fl ow.
Right pulmonary veins
Aorta
Superior vena cava
Interatrial septum
Right atrium
Right AV
(tricuspid) valve
Inferior vena cava
Chordae tendineae
Right ventricle
Left pulmonary
artery
Aortic semilunar
valve
Pulmonary
semilunar valve
Right pulmonary artery
Left atrium
Left pulmonary
veins
Pulmonary trunk
Papillary muscle
Myocardium
Epicardium
Pericardium
Pericardial fluid/space
Left ventricle
Left (bicuspid)
AV valve
Interventricular septum
Arteries to head and arms
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