Bma Illustrated Medical Dictionary

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
tinnitusor 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.
heartThe muscular pump in the centre
of the chest that beats continuously
and rhythmically to send bloodto 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, artificialAn implantable mech-

anical device that takes over the action

of the heartor 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.

HEART HEART, ARTIFICIAL

Aorta

LOCATION

HEART

Descending aorta

Septum

Inferior vena cava

Superior

vena cava

Right ventricle

Tricuspid valve

Right atrium

Pulmonary valve

Pulmonary veins

Mitral

valve

Left

ventricle

Pulmonary

artery

Left

atrium

H