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The Epic Interactive Encyclopedia 1998
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Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso
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Heart,_Human
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INFOTEXT
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1992-09-04
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The human heart is a special muscle, whose
function is to pump blood around the body.
It pumps constantly, at an average rate (the
pulse rate) of 72 beats per minute, without
getting tired, throughout life. For someone
who has lived to the age of 75 years old,
this means that their heart has beat
2,838,240,000 times, and has expended enough
energy to lift a 100-ton weight 5 miles up
into the air. The amount of blood pumped by
the heart is called the cardiac output, and
it is normally 6 to 8 litres per minute.
The total volume of the blood is about 4
litres, and this means that the heart pumps
the entire volume of blood completely round
the body in under two minutes, even while
asleep. In response to stress or fear or
exercise, the adrenal glands (which sit on
top of the kidneys) release adrenaline into
the circulation.
Adrenaline has a direct effect on the heart,
and raises the cardiac output. It does this
by increasing the heart rate, and by
increasing the amount of blood pumped by each
contraction of the heart (the stroke volume).
The heart rate may rise to 150 beats per
minute. If it gets any higher, the heart can
no longer work efficiently, and cardiac
output drops. An average person's cardiac
ouput may rise to 25 litres per minute or
more. The largest cardiac output ever
recorded was in an olympic cyclist, and was
48 litres per minute. The heart is a single
unit, which can be thought of as having two
halves. The right side of the heart receives
blood from the body and pumps it to the
lungs, where it is loaded with oxygen. The
left side heart receives freshly-oxygenated
blood from the lungs and pumps it round the
body, to deliver its oxygen. Blood from the
left side does not mix with blood from the
right side. Each half of the heart has two
chambers, one atrium and one ventricle. The
ventricles are thick-walled, and do most of
the pumping. Because the right side of the
heart has less work to do to pump blood to
the lungs, the right ventricle is smaller
and less powerful than the left. There are
valves between the chambers which prevent
the blood from flowing the wrong way when
the heart pumps. The opening and closing of
these valves is what makes the distinctive
sound of the heart beat. The heart beats as a
single unit because all of the muscle cells
are connected together electrically, which
means that the tiny electrical current which
triggers the contraction is transmitted
almost instantly throughout the heart. There
is a tiny nest of cells in the wall of the
right atrium called the pacemaker, which
produces regular electrical signals to tell
the heart when to pump. The electrical
current of the heart can be detected on an
instrument called an electrocardiogram (ECG).
Diseases of the heart alter the distinctive
electrical signal from the heart and doctors
can use the ECG to diagnose heart diseases.
The heart requires a huge continuous blood
supply to enable it to work properly. The
blood supply to the heart flows through the
coronary arteries, a network of blood vessels
which surrounds the heart like a crown (hence
the name). If the coronary arteries become
diseased, they may become blocked. If this
happens, the blood can no longer get through
the blockage, and the part of the heart
muscle which was supplied by the artery dies.
This is called a heart attack. Since the dead
portion no longer contracts properly, the
heart pumps less efficiently. If a person
suffering a heart attack is taken to hospital
quickly enough, doctors can now give a
treatment which will reopen the blocked
section and restore blood flow to the dying
portion of the heart muscle, and this may
lessen the severity of the attack.
Subject by: Dr Aidan M. O'Donnell