Control of the heartbeat and ECGsEdexcel International A Level Biology: Revision notes

Section 1

Myogenic cardiac muscle

Cardiac muscle is myogenic: it can initiate its own contraction without any nervous stimulation. The heart will therefore keep beating, rhythmically, even when its nerves are cut. Nerves from the brain only change the rate (see cardiac output).

The beat is started and coordinated by specialised cardiac muscle tissue that conducts electrical impulses.

Key termsmyogeniccardiac muscle

Common mistake

Myogenic does not mean the heart ignores the nervous system. The brain still speeds it up or slows it down.

Section 2

The conduction pathway

The sinoatrial node (SAN), in the wall of the right atrium, is the pacemaker. It generates a wave of depolarisation that spreads across both atria, causing atrial systole.
A layer of fibrous (non-conducting) tissue separates the atria from the ventricles, so the wave cannot pass directly.
The wave reaches the atrioventricular node (AVN), which delays it (about 0.1 s) so the atria can finish emptying before the ventricles contract.
The impulse passes down the bundle of His in the septum to the apex.
Purkyne fibres carry it up the ventricle walls, so the ventricles contract from the apex upwards (ventricular systole), pushing blood into the arteries.

Key termsSANAVNbundle of HisPurkyne fibressystoledepolarisation

Exam tip

Remember the order SAN, atria, AVN (delay), bundle of His, Purkyne fibres, ventricles.

Section 3

Reading an ECG

An electrocardiogram (ECG) records the electrical activity of the heart using electrodes on the skin.

P wave: depolarisation of the atria (atrial systole).
QRS complex: depolarisation of the ventricles (ventricular systole).
T wave: repolarisation of the ventricles.

Heart rate (beats per minute) = $\frac{60}{\text{time for one cycle (s)}}$ , where one cycle is the time between successive R waves.

Worked example: R-R interval 0.80 s gives $\frac{60}{0.80} = 75$ beats per minute.

Key termsECGP waveQRS complexT wave

Section 4

Using ECGs to diagnose abnormal rhythms

A normal resting rate is 60 to 100 beats per minute with regular intervals and a P wave before each QRS.

Tachycardia: rate above 100 beats per minute (short R-R intervals).
Bradycardia: rate below 60 beats per minute (long R-R intervals).
Ectopic heartbeat: an extra beat, or one occurring early, followed by a pause.
Atrial fibrillation: irregular R-R intervals and no distinct P waves, as the atria quiver instead of contracting in a coordinated way, so the ventricles fill less efficiently.
Heart block: the AVN fails, so P waves and QRS complexes are unrelated and the ventricles beat slowly.

Key termstachycardiabradycardiaectopic heartbeatatrial fibrillationheart block

Must know

Cardiac muscle is myogenic; the SAN is the pacemaker
SAN, atria, AVN (delay), bundle of His, Purkyne fibres, ventricles contract from the apex
Fibrous tissue separates atria and ventricles
P wave, QRS complex, T wave; heart rate = 60 divided by cycle time in seconds
ECGs diagnose tachycardia, bradycardia, ectopic beats, fibrillation and heart block

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Control of the heartbeat and ECGs

A medical student keeps the heart of a mammal beating in warm, oxygenated saline solution after it has been removed from the body. All the nerves that normally supply the heart have been cut, yet the heart carries on beating regularly for several minutes.
Explain why the heart continues to beat after its nerves have been cut.2 marks
In a healthy heart, a wave of electrical activity spreads from the sinoatrial node (SAN) across the walls of both atria. A layer of fibrous tissue that does not conduct electricity separates the atria from the ventricles, so the wave cannot pass directly between them.
Explain the importance of the delay in the impulse at the AVN.2 marks
A cardiologist records electrocardiograms (ECGs) from two patients, A and B. A normal resting heart rate is 60 to 100 beats per minute. Patient A: the time between successive R waves is constant at 0.50 s, and every QRS complex is preceded by a P wave. Patient B: the time between successive R waves varies irregularly between 0.30 s and 0.90 s, no distinct P waves can be seen and the baseline shows rapid, irregular, small waves.
Calculate the heart rate of patient A and state what it shows.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).