Control of the Cardiac Cycle
• describe the cardiac cycle, with reference to
the action of the valves in the heart;
• describe how heart action is coordinated with
reference to the sinoatrial node (SAN), the
atrioventricular node (AVN) and the Purkyne
tissue;
Control of the Cardiac Cycle
• describe the cardiac cycle, with reference to
the action of the valves in the heart;
• describe how heart action is coordinated with
reference to the sinoatrial node (SAN), the
atrioventricular node (AVN) and the Purkyne
tissue;
The Need for Co-ordination
• Heart (cardiac) muscle is unusual as it can initiate it’s own
contraction
• This is known as myogenic
• The muscles can contract and relax rythmically even if it’s
not connected to the body
• The muscles of the atria and ventricles have their own
natural frequency of contraction- the atrial muscle has a
higher frequency (number of contractions) than the
ventricular muscle
• The property of the muscle could cause inefficient pumping
(fibrillation) if the contractions are not synchronised- so the
heart needs a mechanism for control
The Need for Co-ordination
• At the top of the right atrium,
near the point where the vena
cava empties blood into the
atrium, is the sino-atrial node
(SAN) also known as the
pacemaker
• This is a small patch of tissue
that generates electrical activity
• The SAN initiates an excitation
wave about 55-80 times a
minute
Use p70 to summarise the role of the SAN and AVN
Contraction of the Atria
• The wave of excitation quickly spreads over
the walls of both atria
• It travels along the membranes of the muscle
tissue and causes the cardiac muscles to
contract
• This is Atrial Systole
Contraction of the Atria
• At the base of the atria is a disc of
tissue that cannot conduct the
excitation wave (electrical wave)
• This means the wave cannot spread
directly to the ventricles
• At the top of the inter ventricular
septum (separating the two ventricles)
is another node- the atrio-ventricular
node (or AVN) position 2 on the
diagram
• This is the only route through the disc
of non-conducting tissue
• The wave of excitation is delayed in the
node, this allows time for the atria to
finish contracting and for the blood to
flow down into the ventricles before
they contract
1 = SAN (pacemaker)
2 = AVN
Contraction of the Ventricles
• After the delay the wave of excitation
is carried away from the AVN and
down specialised conducting tissue
called the Purkyne tissue which runs
down the inter ventricular septum
(position 3 on diagram)
• At the base of the septum the
excitation wave spreads out over the
walls of the ventricles
• As it spreads upwards from the base
of the ventricles, it causes the muscles
to contract
• This means that that ventricles
contract from the base upwards,
pushing blood up to the major arteries
– the aorta and the pulmonary artery
3 = Bundle of His
4 & 5 = Purkyne Tissue
ElectroCardiograms
• We can monitor the
electrical activity of the
heart using an
electrocardiogram or ECG
• Sensors must be attached
to the skin which pick up
the electrical signals from
the heart
• The trace of a healthy
person has a particular
shape consisting of waves
labelled P, Q, R, S and T
• P shows the excitation of the atria
• QRS indicates excitation of the ventricles
• T shows diastole (relaxing)
The Shape of an ECG
• The shape of the ECG trace can sometimes be used to show
which part of the heart is unhealthy
• It can show irregular heart beat (arrhythmia) if it is in
fibrillation (the beat is not co-ordinated), if it has suffered a
heart attack (myocardial infarction)
• It can also indicate if the heart is enlarged or if the Purkyne
system is not conducting electrical activity properly
A heart block is when there is a problem with the electrical signals in the heart
Now try SAQ 6 on p71
20 boxes
In 1 second, 25 boxes travel past the machine
One box travels at 1 second/25 boxes = 0.04 seconds per box
One heart beat lasts for approximately 20 boxes
20 boxes x 0.04 seconds per box = 0.8 seconds for each heart beat
How many 0.8 seconds are in 1 minute?
60 seconds / 0.8 seconds = 75 beats per minute
b (i) This is the time during which the ventricles are contracting
B (ii) In 1 second, 25 boxes travel past the machine
One box travels at 1 second/25 boxes = 0.04 seconds per box
Contraction time (Q-T) lasts for approximately 7 boxes
7 boxes x 0.04 seconds per box = 0.28 seconds for each contraction time
b (i) This is the time during which the ventricles are relaxed and filling with blood
B (ii) In 1 second, 25 boxes travel past the machine
One box travels at 1 second/25 boxes = 0.04 seconds per box
Contraction time (T-Q) lasts for approximately 13 boxes
13 boxes x 0.04 seconds per box = 0.52 seconds for each filling time