8.3.48 · Transport in Mammals

Heart Action — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 20 of 21 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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8.3.4 Heart Action — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how the heartbeat is initiated, coordinated and controlled, and how the ECG records it.Myogenic — the beat starts within the heart itself, at an intrinsic rate of about 60 beats per minute.Si…

Sinoatrial node (SAN)
Atrial systole
Non-conducting tissue
Atrioventricular node (AVN)
Bundle of His
Purkyne fibres

Part 1 of 8Initiating the heartbeat

Myogenic

Myogenic the beat starts within the heart itself, at an intrinsic rate of about 60 beats per minute.

Sinoatrial node (SAN)

Sinoatrial node (SAN) specialised cells in the right atrial wall that act as the pacemaker.
They spontaneously and rhythmically depolarise, setting a rate of about 60 to 100 beats per minute.

Atrial systole

Atrial systole each firing sends a wave of excitation across both atria through linked cardiac fibres, so they contract together.

Part 2 of 8Delaying and distributing the signal

Delaying and distributing the signal

Non-conducting tissue

Non-conducting tissue fibrous tissue between atria and ventricles stops excitation passing directly to the ventricles.
- (annulus fibrosus)

Atrioventricular node (AVN)

Atrioventricular node (AVN) the impulse can cross only here, and the AVN delays it by about 0.1 s.
This lets the atria finish contracting and the ventricles fill before ventricular systole.

Bundle of His

Bundle of His it carries the signal down the septum and divides into left and right branches.

Purkyne fibres

Purkyne fibres fine fibres in the ventricle walls conduct the impulse rapidly to the apex.

Contraction from the apex

Contraction from the apex ventricles contract from the bottom upwards, pushing blood out through the arteries efficiently.

Part 3 of 8Sequence of excitation

The SAN sends out a wave of excitation.

The atria contract.

The AVN holds the impulse, then sends out its own wave of excitation.

The bundle of His carries the impulse down the septum.

Purkyne tissue spreads it around the ventricles from the apex.

The ventricles contract and blood leaves through the pulmonary artery and aorta.

Part 4 of 8Controlling the rate

Coordination

Coordination the medulla oblongata adjusts the SAN rate through the autonomic nervous system.

Sensors

Sensors chemoreceptors in the carotid arteries and aorta detect a fall in pH caused by raised CO₂,
- and stretch receptors detect blood pressure changes.

Exercise

Exercise more respiration raises CO₂ and lowers pH.
The cardiac centre sends impulses along the sympathetic nerve to the SAN.
Noradrenaline is released and heart rate increases.

High blood pressure or low CO₂

High blood pressure or low CO₂ impulses travel along the parasympathetic (vagus) nerve, which releases acetylcholine and slows the heart.

Adrenaline

Adrenaline the hormone from the adrenal glands in stress increases heart rate by acting on the SAN.

Feedback

Feedback this is an example of negative feedback.

Full pathway

Full pathway receptor, sensory pathway, coordinating centre, effector nerve and effector (SAN).

Part 5 of 8The electrocardiogram (ECG)

Recording

Recording an ECG records the heart's electrical activity from electrodes on the skin.
P wave atrial depolarisation, leading to atrial systole.
QRS complex ventricular depolarisation, leading to ventricular systole.
T wave ventricular repolarisation.

Heart rate from an ECG

60 ÷ time between two successive R peaks in seconds.

Part 6 of 8ECG patterns

Condition	ECG appearance
Tachycardia	Peaks closer together; rate above 100 beats per minute
Bradycardia	Peaks further apart; rate below 60 beats per minute
Atrial fibrillation	Irregular spacing and no clear P waves
Ectopic beat	Extra early beat followed by a pause

Part 7 of 8Disorders and training

Failed SAN

Failed SAN an implanted artificial pacemaker senses the rhythm and delivers pulses to trigger a beat when none occurs.

Heart block

Heart block damage to the AVN or bundle of His makes atria and ventricles beat at unrelated rates, and a pacemaker restores coordination.

Fibrillation

Fibrillation regions of muscle contract in an uncoordinated, rapid, irregular way, so the chambers quiver instead of pumping.
Ventricular fibrillation is an emergency because almost no blood leaves the heart.
A defibrillator's shock depolarises all cells at once so the SAN regains control.

Anticipation

Anticipation at the start of exercise, adrenaline raises the rate before any chemical change occurs.

Athletes

Athletes training enlarges the heart and raises stroke volume, so resting rate is lower for the same cardiac output.

Part 8 of 8More from the specification

Cardiac cycle diastole

Cardiac cycle diastole during diastole, muscular heart chambers are relaxed and atrioventricular valves open to admit flowing blood.

Atrial pressure

Atrial pressure thin-walled atria contract to generate modest pressure, sufficient to move blood into adjacent ventricles.

Ventricular pressure graph

Ventricular pressure graph ventricular pressure rises sharply during systole, closing atrioventricular valves and opening semilunar valves.

Phases of the Cardiac Cycle

The cardiac cycle consists of atrial systole, ventricular systole (ventricles contract, forcing blood into aorta and pulmonary arteries),
- (atria contract, pumping blood into ventricles)
and diastole.
- (heart muscle is relaxed)

Atrioventricular and Semilunar Valves

Pressure differences govern valve action: atrioventricular valves prevent backflow into atria during ventricular systole,
- (bicuspid/tricuspid)
while semilunar valves prevent backflow into ventricles during diastole.

Analyzing Cardiac Pressure and Volume Graphs

On a Wiggers cardiac pressure graph, ventricular pressure rises sharply above atrial pressure to close AV valves (lub sound),
and climbs above aortic pressure to open the aortic valve.

Myogenic Heart Contraction and Coordination

Cardiac muscle contraction is myogenic, initiated by the sinoatrial node (SAN) pacemaker in the right atrium and propagated through the AV node, bundle of His,
and Purkyne fibres.

Atrium

Upper chamber of the heart that receives blood.

Valve

Flap preventing backflow.

Diastole

Relaxation phase of the cardiac cycle.

Cardiac cycle

The sequence of events in one complete heartbeat.

Heart valve

Flap that keeps blood flowing one way in the heart.

Coronary arteries

Blood vessels that supply the heart muscle with oxygen and glucose.

Atrioventricular valve

Tricuspid or bicuspid valve between an atrium and ventricle.

Key terms

Term	Meaning
Sinoatrial node (SAN)	The pacemaker in the right atrial wall that starts each heartbeat.
Atrioventricular node (AVN)	Tissue that delays the impulse by about 0.1 s before it reaches the ventricles.
Bundle of His	Conducting tissue in the septum that divides into the Purkyne fibres.
Purkyne fibres	Fibres carrying excitation to the apex and around the ventricle walls (also spelt Purkinje).
Myogenic	Beating without an external stimulus.
Electrocardiogram (ECG)	A recording of the electrical activity of the heart.
Fibrillation	Uncoordinated, rapid, irregular contraction of heart muscle.

Avoid these mistakes

Saying nerves start the heartbeat — the beat is myogenic, and nerves and hormones only alter the rate.

Saying excitation passes straight from atria to ventricles — fibrous tissue blocks it and it goes through the AVN.

Saying the AVN speeds up the signal — it delays it by about 0.1 s.

Saying ventricles contract from the top downwards — they contract from the apex upwards.

Last updated: 18 October 2026