AS Level PE 8386 · Area 3 · The Cardiovascular System

The Conduction System and the Cardiac Cycle

The heart beats on its own because it has its own electrical wiring. The sinoatrial node fires, the wave of excitation spreads, the atrioventricular node delays it, and the ventricles contract from the bottom up. The cardiac cycle is what that electrical activity produces: the chambers filling and emptying, valves opening and closing, and pressures changing.

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Syllabus coverage

  • Explain the conduction system of the heart, with reference to the sinoatrial node, the atrioventricular node, the wave of excitation, the bundle of His and Purkyne tissue
  • Explain the cardiac cycle, with reference to atrial systole, ventricular systole and diastole
  • Explain the link between the conduction system of the heart and the cardiac cycle

Part of the AS Level PE 8386 syllabus 8386.

The cardiac cycle0Atrial systoleVentricular systoleDiastoleOne heartbeatPressureAortic pressureVentricular pressureAtria contract,pushing blood intothe ventriclesVentricles contract,blood is forced outinto the arteriesThe heart relaxesand refillsOne cycle = one heartbeat. Stroke volume is the blood ejected in one ventricular contraction; cardiacoutput = heart rate x stroke volume.
The cardiac cycle: atrial systole, ventricular systole and diastole, showing the changes in pressure in the chambers and the opening and closing of the valves.

The heart has its own pacemaker

Cardiac muscle contracts without any instruction from the brain. This is called myogenic contraction, and it happens because the heart contains its own conduction system — a network of specialised muscle fibres that carry an electrical impulse through the chambers.

The impulse begins at the sinoatrial node (SAN), a small group of cells in the wall of the right atrium. The SAN sets the rhythm of the heartbeat, which is why it is called the pacemaker of the heart. Each time it fires, the heart beats once.

Because the SAN fires on its own, the heart continues to beat even if the nerve supply is interrupted — the nervous and hormonal systems only adjust the rate, they do not start it.

If a question asks what would happen if the SAN stopped working, the answer is about the rhythm of the heartbeat and the initiation of the impulse — not about the chambers filling or emptying.

The wave of excitation, step by step

Learn the sequence as five steps. Every conduction-system question in this topic can be answered by walking through it in order.

The impulse from the SAN spreads as a wave of excitation across the walls of both atria, causing them to contract. It cannot pass directly into the ventricles, because the ventricles are separated from the atria by a layer of non-conducting tissue.

The wave therefore reaches the atrioventricular node (AVN), a second group of cells at the base of the right atrium, which delays the impulse slightly (about 0.1 seconds). This delay is essential: it gives the atria time to finish emptying into the ventricles before the ventricles contract.

The impulse then travels down the bundle of His in the septum, and finally spreads upwards through the Purkyne tissue in the ventricle walls.

Because the Purkyne tissue carries the impulse from the base of the ventricles upwards, the ventricles contract from the apex (bottom) upwards. This squeezes blood towards the exit valves at the top of the ventricles and pushes it out into the arteries.

StructurePositionFunction
Sinoatrial node (SAN)Wall of the right atriumInitiates the electrical impulse and sets the rhythm — the pacemaker
Atrioventricular node (AVN)At the base of the right atrium, between the atria and ventriclesReceives the wave of excitation and delays it by about 0.1 seconds so the atria can empty before the ventricles contract
Bundle of HisRunning down the septumCarries the impulse from the AVN down through the septum towards the base of the ventricles
Purkyne tissueIn the walls of the ventriclesSpreads the impulse up through the ventricle walls so they contract from the apex upwards
Two facts earn the most marks here: the AVN delays the impulse, and the ventricles contract from the apex upwards. Both are explanations, not just names.

The cardiac cycle

The cardiac cycle is one complete sequence of contraction and relaxation. It has three phases, each named for what the chambers are doing.

Atrial systole — the atria contract. Pressure in the atria rises above the pressure in the ventricles, the tricuspid and bicuspid valves are pushed open, and the remaining blood is squeezed into the ventricles.

Ventricular systole — the ventricles contract. Pressure in the ventricles rises above that in the atria, so the tricuspid and bicuspid valves are forced shut. Pressure continues to rise, opening the pulmonary and aortic semilunar valves so blood is pushed into the pulmonary artery and the aorta.

Diastole — the whole heart relaxes. Pressure in the ventricles falls, the semilunar valves close to stop backflow into the ventricles, the tricuspid and bicuspid valves open again, and blood flows into the atria from the pulmonary vein and the vena cava and then passes down into the ventricles. The heart fills ready for the next beat.

Pressures, valves and blood flow in each phase

The exam question is almost always about directions of pressure change and which valves are open. The rule to remember is that valves are pushed open by a pressure difference, and they close when that difference reverses. Nothing opens a valve except pressure.

An easy way in: blood always moves from a higher-pressure chamber to a lower-pressure chamber, so if you know which chamber is contracting, you know which way the blood and the valves are going.

PhaseChamber activityPressureValvesBlood flow
Atrial systoleAtria contractAtrial pressure rises above ventricular pressureTricuspid and bicuspid valves open; semilunar valves stay closedBlood is forced from the atria into the ventricles, topping them up
Ventricular systoleVentricles contractVentricular pressure rises above atrial pressure, then above arterial pressureTricuspid and bicuspid valves snap shut; pulmonary and aortic semilunar valves are forced openBlood is ejected into the pulmonary artery (from the right ventricle) and the aorta (from the left ventricle)
DiastoleWhole heart relaxesVentricular pressure falls below atrial and arterial pressureSemilunar valves close; tricuspid and bicuspid valves openBlood flows from the vena cava and pulmonary vein into the atria and on into the ventricles, so the heart refills

How the conduction system drives the cardiac cycle

The conduction system and the cardiac cycle are two descriptions of the same event: the electrical sequence is the cause, and the mechanical sequence is the effect.

The SAN fires → the wave of excitation spreads through the atria → the atria contract (atrial systole). The AVN delays the impulse → the atria finish emptying and the ventricles fill both passively and from the atrial contraction.

The impulse passes down the bundle of His and up the Purkyne tissue → the ventricles contract from the apex upwards (ventricular systole), which closes the atrioventricular valves and opens the semilunar valves so blood is ejected.

While the ventricles are contracting, the atria are relaxing, and after the impulse has passed, the muscle fibres repolarise and the whole heart relaxes (diastole), so the cycle can begin again.

The link question is one of the most common in this topic. Answer it in both directions: name the electrical structure, then say which phase of the cycle its impulse produces.

Why this matters for the performer

The conduction system explains why the heart can beat faster the instant exercise starts. The SAN continues to set the rhythm, but the autonomic nervous system changes how quickly it fires, so the same electrical sequence is repeated more often.

The cardiac cycle explains why filling matters as much as contraction. During ventricular systole the ventricles relax and refill with blood, and the delay created by the AVN is what keeps that refilling possible at high heart rates. A short, effective delay is why a trained performer can sustain a high heart rate with a large stroke volume.

  • The AVN delay is essential at rest and during exercise — without it the atria would not empty before the ventricles contract
  • Blood is ejected when the ventricles contract from the apex upwards, pushing blood towards the exit valves at the top
  • Diastole is the filling phase, so it is the phase that determines how much blood is available to be pumped out

Key terms you must be able to define

Myogenic contraction
Contraction of cardiac muscle that is initiated from within the heart itself rather than by nervous stimulation.
Sinoatrial node (SAN)
A small group of cells in the wall of the right atrium that initiates the electrical impulse and sets the rhythm of the heartbeat — the pacemaker of the heart.
Atrioventricular node (AVN)
A group of cells at the base of the right atrium that receives the wave of excitation and delays it, so the atria empty before the ventricles contract.
Wave of excitation
The electrical impulse that spreads across the walls of the atria and ventricles, causing them to contract.
Bundle of His
A bundle of conducting fibres that carries the impulse from the AVN down through the septum towards the base of the ventricles.
Purkyne tissue
Specialised conducting tissue in the walls of the ventricles that spreads the impulse so the ventricles contract from the apex upwards.
Cardiac cycle
One complete sequence of contraction and relaxation of the heart, consisting of atrial systole, ventricular systole and diastole.
Atrial systole
Contraction of the atria, which raises atrial pressure and pushes the remaining blood through the tricuspid and bicuspid valves into the ventricles.
Ventricular systole
Contraction of the ventricles, which raises ventricular pressure, closes the tricuspid and bicuspid valves and opens the pulmonary and aortic semilunar valves so blood is ejected.
Diastole
The relaxation phase of the cardiac cycle, in which the whole heart relaxes, the semilunar valves close to prevent backflow and the heart refills with blood.

Applied to your sport

Cambridge requires you to apply this knowledge to physical activities. Use these as models, then write your own for the sport you play.

Track and field athletics

As a middle-distance runner leaves the start line, the SAN fires more often under nervous stimulation, so the same conduction sequence — SAN, wave of excitation, AVN, bundle of His, Purkyne tissue — is repeated at a faster rate and the heart beats more times each minute.

Competitive swimming

During a hard set, the AVN delay allows the atria to empty into the ventricles before ventricular systole begins, so each stroke of the arm is supported by a full ventricular output of blood.

Badminton

Between rallies, in diastole, the whole heart relaxes and refills through the vena cava and pulmonary vein, ready for the next burst of intense play.

Olympic weightlifting

During a maximal lift the ventricles contract forcefully in ventricular systole, ejecting blood into the aorta against high resistance, while the semilunar valves snap shut at the end of the contraction to stop blood flowing back into the ventricle.

Flashcards

Click a card to flip it. Mark honestly — the tracker uses this.

Q1Which structure initiates the electrical impulse in the heart?
The sinoatrial node (SAN), in the wall of the right atrium — the pacemaker.
Q2Where is the atrioventricular node and what does it do?
At the base of the right atrium. It receives the wave of excitation and delays it by about 0.1 seconds so the atria empty before the ventricles contract.
Q3What is the wave of excitation?
The electrical impulse that spreads across the walls of the atria and ventricles, causing them to contract.
Q4What is the function of the bundle of His?
It carries the impulse from the AVN down the septum towards the base of the ventricles.
Q5What is the function of the Purkyne tissue?
It spreads the impulse up through the ventricle walls so the ventricles contract from the apex upwards.
Q6Why do the ventricles contract from the bottom upwards?
So blood is squeezed towards the exit valves at the top of the ventricles and ejected into the pulmonary artery and the aorta.
Q7Name the three phases of the cardiac cycle.
Atrial systole, ventricular systole and diastole.
Q8What happens during atrial systole?
The atria contract, atrial pressure rises above ventricular pressure, the tricuspid and bicuspid valves open and blood is forced into the ventricles.
Q9What happens to the valves during ventricular systole?
The tricuspid and bicuspid valves are forced shut by rising ventricular pressure, and the pulmonary and aortic semilunar valves are forced open so blood is ejected.
Q10What happens during diastole?
The whole heart relaxes, ventricular pressure falls, the semilunar valves close, the tricuspid and bicuspid valves open and blood flows into the atria and ventricles to refill the heart.
Q11Why is the AVN delay essential?
It allows the atria to finish emptying into the ventricles before the ventricles contract, so the ventricles fill properly before ejecting blood.
Q12What is myogenic contraction?
Contraction of cardiac muscle initiated from within the heart itself, rather than by a nerve impulse from the brain.
0 of 12 marked as known

Exam-style questions

7 questions, 20 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO11 mark
State the name of the structure that initiates the heartbeat.

Mark scheme — tick each point you actually made

Examiner tip: One mark for one name. Do not describe the whole conduction system — there is no credit for it on a one-mark question.
DescribeAO12 marks
Describe the spread of the impulse through the heart from the AVN to the ventricles.

Mark scheme — tick each point you actually made

Examiner tip: Two structures, two marks, in the correct order. Mention the direction of spread — downwards then upwards — or you lose the explanation mark.
ExplainAO22 marks
Explain the function of the atrioventricular node.

Mark scheme — tick each point you actually made

Examiner tip: The second mark is for the consequence of the delay. Saying only "it slows the impulse" without the reason caps you at one mark.
DescribeAO13 marks
Describe what happens during ventricular systole.

Mark scheme — tick each point you actually made

Examiner tip: Answer in the order chamber → pressure → valve → blood flow. Three marks means three distinct events.
ExplainAO23 marks
Explain how the conduction system of the heart causes atrial systole and ventricular systole.

Mark scheme — tick each point you actually made

Examiner tip: This is the link question. Every mark needs both halves: the electrical structure and the phase of the cardiac cycle it produces.
ExplainAO24 marks
Explain the changes in pressure, valves and blood flow that occur during diastole.

Mark scheme — tick each point you actually made

Examiner tip: Four marks for the four elements: pressure, semilunar valves, atrioventricular valves and blood flow. Work through them in that order every time.
AnalyseAO25 marks
Analyse how the conduction system and the cardiac cycle work together to maintain blood flow to the working muscles of a performer during a game.

Mark scheme — tick each point you actually made

Examiner tip: An analyse question wants the whole chain, in order, with the benefit to the performer at the end. Follow one drop of blood from the SAN to the working muscles.

Exam technique for this topic

  • Learn the conduction sequence as five fixed steps (SAN → wave of excitation across the atria → AVN → bundle of His → Purkyne tissue). Questions usually award one mark per step.
  • When asked about the AVN, always include the delay and its reason: it allows the atria to empty into the ventricles before the ventricles contract.
  • Be precise about direction in ventricular systole: the impulse spreads through Purkyne tissue from the apex upwards, which is why blood is pushed towards the exit valves at the top of the ventricles.
  • For cardiac cycle questions, structure each phase as chamber activity → pressure change → valve action → blood flow. Four elements in order answer almost any part of the question.
  • Never say the valves open themselves. Say they are forced open by a pressure difference, and close when the pressure difference reverses.

Why students lose marks here

A student wroteThe atrioventricular node starts the heartbeat.
Why it loses marksThe SAN is the pacemaker. The AVN is a relay station: it receives the wave of excitation and delays it. Naming the wrong node loses the mark and usually the explanation mark as well.
The answer that scoresThe sinoatrial node in the wall of the right atrium initiates the electrical impulse and sets the rhythm of the heartbeat. The atrioventricular node receives the wave of excitation and delays it by about 0.1 seconds so the atria can empty before the ventricles contract.
A student wroteThe ventricles contract from the top downwards so blood can be squeezed out.
Why it loses marksThe contraction direction is reversed. The impulse travels down the septum through the bundle of His and then up through the Purkyne tissue, so the ventricles contract from the apex upwards, pushing blood towards the valves at the top.
The answer that scoresThe impulse passes down the bundle of His in the septum and then spreads up the ventricle walls through Purkyne tissue, so the ventricles contract from the apex upwards. This forces blood towards the pulmonary and aortic semilunar valves at the top of the ventricles so it can be ejected.
A student wroteThe valves open to let blood through when the chambers contract.
Why it loses marksThis ignores pressure, which is the mechanism the examiner wants. The valves have no muscle — they are opened and closed by differences in pressure between the chambers.
The answer that scoresValves are forced open by a pressure difference. In atrial systole, atrial pressure exceeds ventricular pressure, so the tricuspid and bicuspid valves open. In ventricular systole, ventricular pressure exceeds atrial pressure and the atrioventricular valves are forced shut, while the rising ventricular pressure opens the pulmonary and aortic semilunar valves.
A student wroteDiastole is when the heart stops beating to rest.
Why it loses marksDiastole is not a pause or a stoppage — it is an active filling phase. Blood is flowing into the atria and then into the ventricles throughout it.
The answer that scoresDiastole is the relaxation phase of the cardiac cycle. The atria and ventricles relax, ventricular pressure falls, the semilunar valves close to prevent backflow, the tricuspid and bicuspid valves open, and blood flows from the vena cava and pulmonary vein into the atria and on into the ventricles so the heart refills.