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The Cardiac Cycle
•Relate the events during the cardiac cycle to
the maintenance of blood flow through the
heart
•Describe the relationship between pressure
and volume changes in the heart and aorta
and events in the cardiac cycle
•Discuss the roles of the sinoatrial node,
atrioventricular node and bundle of His
Cardiac Muscle
• Cardiac muscle is able
to contract
spontaneously without
nervous or hormonal
stimulation (if provided
woth oxygen, nutrients
and the right mixture of
mineral salts)
• These contractions are
called myogenic.
The Pacemaker – the SAN
• The cycle starts at the
sinoatrial node (SA
node or SAN). This is a
small patch of tissue
that has its own natural
rhythm of contraction.
The rhythm is made
faster or slower by
nervous impulses and
hormones. The SAN
determines the rate of
contraction of the rest
of the cardiac muscle.
Cardiac Impulses
• The SAN generates
waves of electrical
impulses over the atria.
• When these reach the
junction between the
atria and ventricles they
are delayed by the
atrioventricular septum.
This delay gives time
for the waves of
contraction to spread
over the atria.
Atrioventricular node (AV node)
• The atrioventricular septum
is a layer of non-conducting
connective tissue. It
completely separates the
atria from the ventricles
except for a region in the
right atrium called the
atrioventricular node (AV
node)
• The AV node is the only
route of transmission for the
cardiac impulse. It acts as a
second pacemaker region,
relaying the cardiac impulse
to the ventricles
What if the SAN misfunctions?
• The AV node will then
take over but it is
slower than the
inherent rhythm
Systole (contraction)
• The cardiac cycle is
relayed from the AV
node over the
ventricles through a
bundle of specialised
muscle fibres called
the bundle of His.
• This bundle branches
into other fibres called
the Purkyne fibres
(or Purkinje fibres)
Ventricular systole (contraction)
• The cardiac impulse
passing through the
Purkyne fibres causes
a wave of contraction.
• It starts at the apex of
the heart (bottom)
and passes rapidly
over the ventricles
(ventricular systole).
• Regions close to the AV
node have thin Purkyne
fibres which carry
impulses more slowly
than the thick fibres that
supply the more distant
parts of the ventricles.
• This ensures that all parts
of the ventricles contract
more or less
simultaneously.
Diastole (relaxation)
• The whole heart
realxes after
ventricular systole,
allowing blood from
the veins to fill up the
heart. This is called
diastole.
Pressure changes in the heart
• The diagram shows the
pressure changes during
a cardiac cycle on the
left side of the heart
• Note that the maximal
pressure in the left
ventricle is far higher than
in the atrium
• This is because the
ventricle has to work hard
to pump blood to all parts
of the body (except the
lungs)
• The atrium only pumps
blood into the ventricle.
Electrical change in the heart:
the ECG
• Cardiac muscle contracts
as a result of electrical
stimulation.
• A wave of electrical
charge is initiated in the
pacemaker region and
spreads across the heart
• This generates an electric
current in the body fluids
around the heart
• The currents can be
detected ob the body
surface using recording
electrodes.
Electrocardiogram ECG
• A cathode ray
oscilloscope or chart
recorder is used to show
the ECG
• The P wave is caused by
atrial systole
• The QRS wave is caused
by ventricular systole
• The T wave coincides
with ventricular diastole
Heart Rate
• This can be
calculated from the
interval between one
P wave and the next.
Check your understanding
• 1. List the sequence of main events during
the cardiac cycle
• 2. Why does the pressure in the left
ventricle reach a higher peak than the
blood pressure in the left atrium?
• 3. Which part of the heart normally initiates
the cardiac cycle?