557
CHAPTER FIFTEEN
Cardiovascular System
The left atrium receives the blood from the lungs
through four
pulmonary veins
—two from the right lung and
two from the left lung. The blood passes from the left atrium
into the left ventricle through the atrioventricular oriF ce,
which a valve guards. This valve consists of two leafl
ets and
is named the
mitral valve
(shaped like a miter, a type of
headpiece) or bicuspid valve or left atrioventricular valve.
It prevents the blood from fl
owing back into the left atrium
from the left ventricle when the ventricle contracts. As with
the tricuspid valve, the papillary muscles and the chordae
tendineae prevent the cusps of the mitral valve from swing-
ing backwards into the left atrium.
When the left ventricle contracts, the mitral valve closes
passively, and the only exit is through a large artery called the
aorta.
Its branches distribute blood to all parts of the body.
At the base of the aorta is an
aortic valve
(aortic semilu-
nar valve) that consists of three cusps (see F gs. 15.8 and 15.9).
It opens and allows blood to leave the left ventricle as it con-
tracts. When the ventricular muscles relax, this valve closes
and prevents blood from backing up into the left ventricle.
The mitral and tricuspid valves are also called atrioven-
tricular valves because they are between atria and ventricles.
The pulmonary and aortic valves are also called semilunar
because of the half-moon shapes of their cusps.
Table 15.2
summarizes the locations and functions of the heart valves.
The right atrium receives blood from two large veins: the
superior vena cava
and the
inferior vena cava.
These veins
return blood, low in oxygen, from tissues. A smaller vein,
the
coronary sinus,
also drains blood into the right atrium
from the myocardium of the heart.
A large
tricuspid valve
(right atrioventricular valve)
guards the atrioventricular oriF
ce between the right atrium
and the right ventricle. It is composed of three leafl
ets, or
cusps, as its name implies. This valve permits the blood to
move from the right atrium into the right ventricle and pre-
vents it from moving in the opposite direction. The cusps
fold passively out of the way against the ventricular wall
when the blood pressure is greater on the atrial side, and
they close passively when the pressure is greater on the ven-
tricular side
(f gs. 15.6, 15.7, 15.8,
and
15.9)
.
Strong, F brous strings, called
chordae tendineae
(kor
de
ten
dı˘-ne), attach to the cusps of the tricuspid valve on the
ventricular side. These strings originate from small mounds
of cardiac muscle tissue, the
papillary muscles
(pap
ı˘-ler
e
mus
elz), that project inward from the walls of the ventricle.
The papillary muscles contract when the right ventricle con-
tracts. As the tricuspid valve closes, these muscles pull on
the chordae tendineae and prevent the cusps from swinging
backwards into the right atrium.
The right ventricle has a thinner muscular wall than the
left ventricle (see F g. 15.6). This right chamber pumps the
blood a fairly short distance to the lungs against a relatively
low resistance to blood fl ow. The left ventricle, on the other
hand, must force the blood to all the other parts of the body
against a much greater resistance to fl
ow.
When the muscular wall of the right ventricle contracts,
the blood inside its chamber is put under increasing pres-
sure, and the tricuspid valve closes passively. As a result,
the only exit for the blood is through the
pulmonary trunk,
which divides to form the left and right
pulmonary arteries
that lead to the lungs. At the base of this trunk is a
pulmo-
nary valve
(pulmonary semilunar valve), which consists of
three cusps (see F gs. 15.8 and 15.9). This valve opens as
the right ventricle contracts. However, when the ventricular
muscles relax, the blood begins to back up in the pulmonary
trunk. This closes the pulmonary valve, preventing a return
fl ow into the right ventricle. Unlike the tricuspid valve, the
pulmonary valve does not have chordae tendineae or papil-
lary muscles attached to its cusps.
TABLE
15.1
|
Wall of the Heart
Layer
Composition
Function
Epicardium (visceral pericardium)
Serous membrane of connective tissue covered with
epithelium and including blood capillaries, lymph
capillaries, and nerve F
bers
±orms a protective outer covering; secretes serous ²
uid
Myocardium
Cardiac muscle tissue separated by connective tissues and
including blood capillaries, lymph capillaries, and nerve
F
bers
Contracts to pump blood from the heart chambers
Endocardium
Membrane of epithelium and underlying connective tissue,
including blood vessels and specialized muscle F
bers
±orms a protective inner lining of the chambers and valves
Mitral valve prolapse
(MVP) a³
ects up to 6% of the U.S. population. In
this condition, one (or both) of the cusps of the mitral valve stretches
and bulges into the left atrium during ventricular contraction. The valve
usually continues to function adequately, but sometimes, blood regur-
gitates into the left atrium. Through a stethoscope, a regurgitating MVP
sounds like a click at the end of ventricular contraction, then a murmur
as blood goes back through the valve into the left atrium. Symptoms of
MVP include chest pain, palpitations, fatigue, and anxiety.
Certain species of
Streptococcus
bacteria can damage the mitral
valve. Endocarditis, an in²
ammation of the endocardium due to an
infection, appears as a plantlike growth on the valve. People with
MVP are particularly susceptible to endocarditis. They take antibiotics
before undergoing dental work to prevent
Streptococcus
bacteria in
the mouth from migrating through the blood to the heart and caus-
ing infection.
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