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Hind Leys Biology
F211
Transport in animals 5.4
Control of the cardiac cycle
Objectives
 Describe how heart action is coordinated with reference to the sinoatrial node,
the atrioventricular node and the Purkyne tissue.
 Interpret and explain the electrocardiogram (ECG) traces with reference to
normal and abnormal heart activity.
Coordination of contraction
Cardiac muscle is unique in that it can initiate its own contraction. Heart muscle is
described as myogenic. This means that the muscle will contract and relax rhythmically
even if it is not connected to the body. The muscles from the atria and ventricles each
have their own natural frequency of contraction. The atrial muscle tends to contract at
a higher frequency than the ventricular muscle.
The property of the muscle could cause inefficient pumping (a condition known as
fibrillation) if the contractions of the chambers are not synchronised. So the heart
needs a mechanism that can coordinate the contractions of all four chambers.
Initiation of heartbeat
At the top of the right atrium, near the vena cava, is the sinoatrial node (SAN) (1).
This is a small patch of tissue that generates electrical activity, and is known as the
heart’s pacemaker. The SAN initiates a wave of excitation at regular intervals. In a
human, this occurs between 55-80 times a minute.
Contraction of the atria
The wave of contraction quickly spreads over the walls of
both atria, travelling along the membranes of the muscle
tissue. As the wave of excitation passes, it causes the
cardiac muscle cells to contract. This is atrial systole.
At the base of the atria is a disc of non-conducting tissue
which prevents the wave of excitation passing directly to
the ventricles. At the top of the septum is another node,
the atrioventricular node (AVN) (2). This is the only route
through the non-conducting tissue. The wave of excitation
is delayed in the node. This allows time for the atria to
finish contracting and for blood to flow into the ventricles
before they begin to contract.
Figure 1 The wave of
contraction over the heart.
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Hind Leys Biology
F211
Transport in animals 5.4
http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000001&ptid=57 cardiac
conduction system
Contraction of the ventricles
Following this delay, the wave excitation is carried away from the AVN and down
specialised conducting tissue called the Purkyne/Purkinje tissue. These form the bundle
of His, (3) which runs down the interventricular septum. At the base of the septum the
wave of excitation spreads outwards and upwards causing the ventricles to contract.
This means that the ventricles contract from the base upwards, pushing blood up to the
major arteries at the top of the heart.
It is the pattern of electrical conduction or electrical wave that is picked up on the
electrocardiogram or the ECG; the tracing of the heart's electrical activity.
Electrocardiograms
During an ECG a number of sensors are attached to the skin of the thorax. Some of the
electrical activity generated by the heart spreads through the tissues and onwards to
the skin. The sensors pick up the electrical excitation and convert this to a trace.
The trace of a healthy person has a particular shape. It consists of a series of waves,
labelled P, Q, R, S and T. The events these represent are shown in Figure 2.
Figure 2 The events
represented by an ECG
trace, together with a
normal trace and some
abnormal traces.
The shape of an ECG trace can sometimes indicate when part of the heart muscle is not
healthy. It can show if the heart is beating irregularly (arrhythmia), if it is in fibrillation
(the heartbeat is not coordinated), or if it has suffered a heart attack (myocardial
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Hind Leys Biology
F211
Transport in animals 5.4
infarction). ECG traces can also show if the heart has enlarged or if the Purkinje fibres
are not conducting properly.
Regular evenly spaced trace
Irregular distances between waves.
No obvious P-wave.
Ventricular hypertrophy
(increase in muscle thickness)
Deep S-wave
Figure 3 Comparison of normal and
abnormal ECG traces
1.
2.
3.
4.
5.
a)
Explain why the SAN is called the pacemaker.
Explain why atrial fibrillation decreases the efficiency of the heart.
Explain why the ventricles contract from the apex upwards.
Explain why the QRS complex has a larger peak than the P-wave.
Each one of the figures represents an ECG pattern displaying three types of
abnormal rhythms: tachycardia, bradycardia, and arrhymthmia. Identify each.
b)
c)
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Hind Leys Biology
F211
Transport in animals 5.4
6. The features of the heart ensure that blood is pumped efficiently and in the right
direction. Explain how each of the following is important, referring to the
advantages of each.
a) The spread of electrical activity across the atria from the SAN. The
muscles of the atria contract rom the top downwards. This pushes blood
down through the AV valves in to the ventricles.
b) The insulating fibrous tissue between the atria and the ventricles. This
stops the wave of atrial muscle contraction continuing through to the
ventricles. If contractions started at the top of the ventricles blood would
just be forced towards the bottom of the heart.
c) The short delay in the transmission of electrical activity through the AVN.
The delay allows the atria to complete contraction and empty blood into the
ventricles before they then contract.
d) Passing impulses down the bundle of His to the lower end of the heart. This
makes the muscles contract from the bottom of the ventricles so that eh
blood is forced upwards towards the arteries.
e) The rapid spread of the impulses through the fibres to the muscles of the
ventricles. The contraction of the muscles spreads quickly up the ventricles.
Pressure builds up rapidly and the blood is forced out at speed
Artificial pacemakers
The natural conducting systems of
the heart (SAN, AVN and bundle of
His) can become damaged by
disease or ageing. As a result the
coordination of the heartbeat can be
disrupted- known as a heart block.
An artificial pacemaker overcomes
this
by
generating
electrical
impulses and conducting them to
the heart muscle. This stimulates
contraction of the heart chambers.
Pacemakers consist of a pulse
generator and one
or two
electrodes. It is powered by a
lithium battery.
Modern pacemakers can sense
changes in the body, e.g. exercise,
and
adjust
the
heart
rate
accordingly.
http://www.pennmedicine.org/health_info/animationplayer/ loads of animations
This work can be reinforced using pages 70-72 of your textbook.
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