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Module 2
Exchange and transport
1.2.8 Control of the cardiac cycle
Learning Objectives
•
To explain the control of
the cardiac cycle
Success Criteria
• Explain the terms myogenic,
sinoatrial node, atrioventricular
node and purkyne tissue
(Grade E - D)
• Describe how heart action is
coordinated with reference to the
sinoatrial node (SA), atrioventricular
node (AVN) and Purkyne tissue
(Grade C –B)
• Interpret and explain ECG traces,
with reference to normal and
abnormal heart activity
(Grade B – A)
Lub dup
The sound heard through a stethoscope is
described as a lub and a dup.
• Lub – caused by closing of atrioventricular
valves
(bicuspid and tricuspid).
• Dup – caused by closing of semilunar valves.
Sinoatrial node
Tissue
Atrio – ventricular node
Purkyne
Heart Rate –beats/min
Control of the Heart Beat
•
•
•
•
Cardiac muscle is myogenic.
However, have to be controlled.
Sinoatrial node: specialised patch of muscle
SAN is the pacemaker: each time they contract
send a wave of electrical activity over whole of
atrial walls: both atria contract at same time.
•Explain the terms myogenic, sinoatrial
node, atrioventricular node and purkyne
tissue
(Grade E - D)
Control of Heart Beat
• There is a band of fibres between atria and
ventricles which don’t conduct the wave.
• Only route through is the atrio-ventricular node.
This picks up the excitation.
• AVN passes wave onto conducting fibres called
the Purkyne Tissues (after a delay of 0.1 sec),
causing the ventricle walls to contract from the
base upwards
The heart has a natural pacemaker
SAN
AVN
1. The SAN sends waves of
electrical activity which spread
through the muscle of the
atria, causing it to contract.
(Blood is forced into ventricles)
ATRIAL SYSTOLE
4. The atria relax (atrial
diastole), blood enters the
atria from the pulmonary veins
or vena cava
5. The ventricles relax
(ventricular diastole). Bicuspid
and tricuspid valves open and
the next pumpful of blood
enter the ventricles from the
atria
•Describe how heart action is
coordinated with reference to
the sinoatrial node (SA),
atrioventricular node (AVN)
and Purkyne tissue (Grade C
–B)
Complete cut and
stick
Page58-59 - help
2. The waves of
electrical activity reach
the AVN . Here there is
a slight delay-making
sure the atria have
emptied before
venticles contract
3. AVN passes activity
onto the bundle of His,
and these muscle fibres
conducted the waves of
excitation along the
Purkyne fibres. Once the
impulses reach the walls
of the ventricles they
cause the muscles to
contract, forcing blood out
of the heart
VENTRICULAR SYSTOLE
Cardiac Cycle
Heart relaxed
Atrio-Ventricular valves are open
Deoxygenated blood from vena cava flows into Right Atrium
Oxygenated blood from pulmonary vein flows into Left Atrium
Blood passes into ventricles passively
Sino Atrial node sends impulses through atria
Atria contract (top downwards) – forces additional blood into ventricles through Atrio
Ventricular valves
Blood from Right Atrium to Right Ventricle; blood from LA to LV
Ventricles receive impulses from AVN via Purkyne fibres – ventricles contract (bottom
upwards) – force of blood causes Atrio Ventricular valves to close (lub)– prevents blood
flowing back into atria
Blood is forced from Right Ventricle into Pulmonary Artery through pulmonary semilunar
valves and from Left Ventricle into aorta through aortic semi lunar valves
Heart relaxes – semilunar valves close due to force of blood (dub)- prevent backflow from
pulmonary artery and aorta into heart – Atrio Ventricular valves open
Atria fill with blood again to start cycle again
Electrocardiograms (ECG)
Checks heart function using an electrocardiograph, it records
the electrical activity of the heart
- The heart muscle depolarises (loses electrical charge) when it
contracts, and repolarises (regains charge) when it relaxes.
- Patches with wires are placed on the patients chest and wires
are connected to a monitor.
- Electric charges are recorded by an electrocardiograph, as a
electrocardiogram/ECG which shows the patients normal
heart rhythm.
Electrocardiograms [ECGs]
(record the electrical activity of the heart)
The P wave is caused by
contraction
(depolarisation) of atria
The QRS complex is caused
by contraction
(depolarisation) of
ventricles. Larger than P due
to more muscle.
The ST wave is caused
during ventricular
diastole. (repolarisation
of ventricles
http://anatimation.com/cardiaccycle/cardiac-cycle.html
Terms not to be confused by
• Tachycardia – fast heart rate
• Bradycardia – a slow heart rate
• Arrhythmia – abnormal heart rhythm
•Interpret and
explain ECG
traces, with
reference to
normal and
abnormal
heart activity
(Grade B – A)
• Fibrillation – rapid contraction of heart
muscles
TASK – Identify each ECG and explanation
Sinus Tachycardia
Rapid fire of the SAN, the heart’s
natural pacemaker, defined as a rate
greater than 100 beats/min in an
average adult.
It’s the normal response to exercise, excitement, anxiety and also
stimulants, fever, dehydration, hypothermia & shock, hypoxia
Bradycardia
Bradycardia occurs when the heart is beating more slowly than
fifty to sixty times a minute. The impulses that control a steady
heartbeat, are blocked, delayed, or slowed down.
It can occur as a result of age, AVN or SVN problems, metabolic
disturbances, as a result of taking certain medications, drug
abuse, or as a result of a pre-existing heart disease.
Trained athletes tend to have slow resting heart rates, and resting
bradycardia in athletes is not abnormal if no symptoms.
Sinus Arrhythmia
The normal increase in heart rate that occurs during inspiration
(when you breathe in).
Ventricular Fibrillation
Uncoordinated contraction of the cardiac muscle of the
ventricles in the heart. As a result, the heart fails to adequately
pump the blood,
During ventricular fibrillation, cardiac output drops to nil,
and, unless terminated promptly, death usually ensues
within minutes.
Plenary - Multiple-choice quiz
17 of 24
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