Trace electrical coordination of the heart
Explain the roles of the sinoatrial node, atrioventricular node and Purkyne tissue in coordinating the cardiac cycle.
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which structure initiates the electrical impulse for each heartbeat?
What you will learn
- Describe how the SAN initiates atrial contraction
- Explain the AVN delay and Purkyne tissue conduction through ventricles
Build the explanation
The sinoatrial node (SAN) initiates an electrical impulse that spreads across the atria and causes atrial contraction. The impulse reaches the atrioventricular node (AVN), where a short delay allows the ventricles time to fill after atrial systole. The impulse then travels down conducting tissue in the septum and through Purkyne tissue in the ventricular walls. This spreads excitation from the base towards the apex and helps produce coordinated ventricular contraction from the bottom upwards.
Apply it carefully
The syllabus asks for these structures' roles in the cardiac cycle; detailed nervous or hormonal control is not required here. Electrical excitation precedes muscle contraction, and pressure changes then control the valves. Do not say the AVN starts the heartbeat or that Purkyne tissue carries blood. The delay is functional because it separates atrial and ventricular contraction.
Pause and explain
What is the role of Purkyne tissue?
Worked example
Why is there a delay at the AVN?
Show the worked solution
- Consider the order of atrial and ventricular contraction.
- The delay lets the atria finish moving blood into the ventricles before ventricular systole.
Answer The AVN delay allows the ventricles to fill after atrial contraction and before ventricular contraction begins.
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Describe the route of electrical excitation from the SAN to the ventricular walls.
Give me a hint
Follow the impulse in the order of the named structures.
Compare my reasoning
- SAN initiates and excitation spreads across atria to AVN.
- After a delay it travels down the septum and through Purkyne tissue in ventricular walls.
The SAN initiates an impulse that spreads across the atria to the AVN. After a short delay at the AVN, excitation travels down conducting tissue in the septum and through Purkyne tissue in the ventricular walls.
Look for these in your work
- SAN and atria first
- AVN delay included
- Septum and Purkyne route included
Explain why ventricular contraction follows atrial contraction rather than occurring at the same time.
Give me a hint
Include the AVN delay and filling.
Compare my reasoning
- The impulse is delayed at the AVN.
- The delay lets atrial systole move blood into ventricles before ventricular systole.
The AVN delays the impulse briefly, allowing atria to complete contraction and fill the ventricles before ventricular contraction begins.
Look for these in your work
- AVN delay stated
- Filling benefit explained
A student says: 'Purkyne tissue pumps blood from the apex.' Correct the explanation of how it supports ventricular ejection.
Give me a hint
Distinguish electrical conduction from mechanical pumping.
Compare my reasoning
- Purkyne tissue conducts electrical excitation through the ventricular walls.
- Coordinated contraction from the base towards the apex helps eject blood; the tissue itself does not pump blood.
Purkyne tissue conducts excitation through the ventricular walls. This helps the ventricles contract in a coordinated sequence from the base towards the apex, driving blood towards the artery exits; Purkyne tissue is not a blood pump.
Look for these in your work
- Electrical role distinguished
- Contraction direction linked to ejection
- Not described as pumping tissue
Common mistakes
- Putting AVN before SAN
- Ignoring the delay's role in ventricular filling
- Confusing conducting tissue with blood vessels
What can you explain now?
Which structure initiates the electrical impulse for each heartbeat?
Compare with the explanation
Sinoatrial node
The SAN acts as the heart's pacemaker in the syllabus model.
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Make it stick
What is the sequence SAN → AVN → Purkyne tissue, and why is the AVN delay useful?
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