Superior
vena cava
Aorta
Pulmonary
artery
Left
atrium
LOCATION
Mitral
valve
Pulmonary veins
H
HEART
HEART, ARTIFICIAL
HEART
Pulmonary valve
Right atrium
Tricuspid valve
Right ventricle
Septum
Inferior vena cava
Descending aorta
Left
ventricle
of
child development
and when hearing im-
pairment is suspected.
The tests are some-
times included in a
general medical exam-
ination. Hearing tests
may also be used to
identify the cause of
tinnitus
or dizziness.
An audiometer (an
electrical instrument)
is used to test an indi-
vidual's ability to hear
sounds at different fre-
quencies and volumes.
The
lowest
level
at
which a person can
hear and repeat words
(the speech reception
threshold) is tested, as
is the ability to hear
words clearly (speech
discrimination).
The
type of hearing loss
(see
deafness
)
is deter-
mined by holding a
tuning fork to different parts of the ear.
heart
The muscular pump in the centre
of the chest that beats continuously
and rhythmically to send
blood
to the
lungs and the rest of the body. Much of
the heart consists of myocardium, a
special type of muscle. The heart muscle
is supplied with oxygen and nutrients
by
2
coronary arteries
.
The internal surface of the heart is
lined with a smooth membrane, called
endocardium, and the entire heart is
enclosed in a tough, membranous bag,
the
pericardium
. Inside the heart there
are 4 chambers. A thick central muscu-
lar wall, the septum, divides the heart
cavity into right and left halves. Each
half consists of an upper chamber,
called an
atrium
, and a larger lower
chamber, called a
ventricle
.
The right atrium receives deoxygen-
ated blood from the entire body via
2
large veins called the
venae cavae
.
This blood is transferred to the right
ventricle and pumped to the lungs via
the pulmonary artery to be oxygenated
and to lose carbon dioxide. The left atri-
um of the heart receives oxygenated
blood from the lungs (via the pulm-
onary veins); this blood is transferred to
the left ventricle and then pumped
to all tissues in the body. One-way
valves at the exits from each chamber
ensure that blood flows in only
1
direc-
tion (see
heart valves
).
As resistance to blood flow through
the general circulation is much greater
than resistance through the lungs, the
left side of the heart must contract
more forcibly than the right, and has
greater muscular bulk.
heart, artificial
An implantable mech-
anical device that takes over the action
of the
heart
or assists the heart in main-
taining the circulation. There are 3 main
types of artificial heart.
An intra-aortic balloon pump, com-
prising a balloon in the aorta that
inflates with each heartbeat, increases
the volume of blood entering the circu-
lation. A left ventricular assist device
takes blood from the left ventricle and
pumps it electrically into the abdomi-
nal aorta. There are also mechanical
hearts, which are powered from outside
the body, usually by compressed air.
271
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