The Heartbeat
Heartbeat
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Each heartbeat is called a cardiac cycle.
First the two atria contract at the same time.
Next the two ventricles contract at the same
time.
Then all the chambers relax.
Systole – contraction of the heart muscle
Diastole – relaxation of the heart muscle.
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Time
Atria
Ventricles
1.
0.15 sec
systole
diastole
2.
0.3 sec
diastole
systole
3.
0.4 sec
diastole
diastole
Each heartbeat last about 0.85 seconds so the
average person’s heart beats about 70 times a
minute.
The Sound of a Beating Heart
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When the heart beats it makes the familiar “lubdup” sound.
The “lub” is long and low pitched. This sound is
caused by the atrioventricular valves
(bicuspid and tricuspid) closing as the
ventricles contract.
The “dup” is shorter and sharper. This sound is
caused by the semilunar valves (pulmonary
and aortic) closing after the blood is pumped
out of the ventricles into either the pulmonary
artery or the aorta.
Intrinsic Control of the Heartbeat
The rhythmical contraction of the atria and
ventricles is due a conducting system
inside the heart.
 The conducting system is made up of the
following parts:
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1) two nodes (small regions of
neuromuscular tissue)
(sinoatrial) node – located in the upper,
dorsal wall of the right atrium. Initiates the
heartbeat (also known as the pacemaker)
 AV (atrioventricular) node – located at the
base of the right atrium. Transmits
impulses received from the SA node to the
ventricular walls.
 SA
2) Conducting bundle- specialized fibres that
conduct impulses from the AV node to the
ventricles. The conducting bundle is also
known as the Bundle of His.
 Divide
to form the right and left bundle
branches which extend into the right and left
ventricles.
 The terminal branches of the bundle braches
are called Purkinje fibers.
The SA node initiates the heartbeat and
automatically sends out an excitation
impulse every 0.85 seconds which
causes the atria to contract.
 When the impulses reach the AV node
there is a slight delay as the impulse
passes through the bundle of His that
allows the atria to finish their contraction
before the ventricles begin to contract.
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The signal for the ventricles to contract
travels from the AV node through the
branches of the bundle of His to the
Purkinje fibers. This pathway efficiently
causes the ventricles to contract.
 The SA node is called the pacemaker
because it usually keeps the heartbeat
regular.
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Electrocardiograms (ECG)
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A recording of the electrical changes that
occur in the myocardium during a cardiac cycle.
Electrodes are placed on the skin and are
connected by wires to an instrument that detects
and records the myocardium’s electrical
changes.
Electrical changes in the myocardium can be
detected on the skin’s surface because body
fluids contain ions that conduct electrical
currents from the heart throughout the body.
ECG
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A normal ECG contains
three distinct sections:
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P wave – atrial fibers
produce an electrical
change. This indicates that
the atria are about to
contract.
QRS complex – signals
that the ventricles are
about to contract.
T wave – occurs when the
ventricles are recovering
from contraction.
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The above diagram of an irregular ECG displays
a condition known as ventricular fibrillation.
This condition causes uncoordinated
contraction of the ventricles.
This condition can be caused by an injury or
drug overdose.
Once the ventricles are fibrillating, they have to
be defibrillated by applying a strong electrical
current for a short period of time.
 This
current allows the SA node to reestablish a
coordinated beat.
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Once the ventricles are fibrillating, they
have to be defibrillated by applying a
strong electrical current for a short period
of time. This current allows the SA node
to reestablish a coordinated beat.