The Cardiac Cycle

The cardiac cycle describes the complete sequence of events that occurs during one heartbeat, from the moment the heart begins to fill with blood through to the point where it has emptied and begins to fill again. Understanding this cycle is essential for explaining how blood is pumped through the heart and body, and is a key part of the AQA GCSE PE specification (3.1.1.2). This lesson also covers the double circulatory system and the pathway blood takes around the body.

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What Is the Cardiac Cycle?

The cardiac cycle is the repeated sequence of contraction (systole) and relaxation (diastole) of the heart's chambers. One complete cardiac cycle — one heartbeat — takes approximately 0.8 seconds at a resting heart rate of 75 bpm.

The cycle has three main phases:

Phase	What Happens	Duration (approx.)
Atrial systole	Both atria contract, pushing blood into the ventricles	0.1 seconds
Ventricular systole	Both ventricles contract, pushing blood into the arteries	0.3 seconds
Diastole (cardiac diastole)	The whole heart relaxes and fills with blood	0.4 seconds

Exam Tip: The key terms are systole (contraction) and diastole (relaxation). AQA will expect you to use these terms correctly. Remember: systole = squeeze (both start with 's').

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Phase 1: Atrial Systole

During atrial systole, the muscular walls of both atria contract simultaneously. This increases the pressure inside the atria, which forces blood downwards through the atrioventricular (AV) valves:

• Blood moves from the right atrium through the tricuspid valve into the right ventricle
• Blood moves from the left atrium through the bicuspid (mitral) valve into the left ventricle

At this point, the semilunar valves (pulmonary and aortic) are closed because the pressure in the ventricles is still lower than the pressure in the arteries.

The ventricles are relaxed and expand to accommodate the incoming blood. By the end of atrial systole, the ventricles are full of blood — they are said to be at their end-diastolic volume.

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Phase 2: Ventricular Systole

Immediately after the atria have contracted, the ventricles contract. This is the most powerful phase of the cardiac cycle, particularly on the left side.

As the ventricles contract:

1. The pressure inside the ventricles rises rapidly, exceeding the pressure in the atria.
2. This pressure difference causes the AV valves (tricuspid and bicuspid) to snap shut, preventing blood from flowing back into the atria. The closure of these valves produces the first heart sound — the "lub" of the heartbeat.
3. As ventricular pressure continues to rise, it exceeds the pressure in the pulmonary artery and aorta.
4. The semilunar valves (pulmonary and aortic) open, and blood is ejected:
   • From the right ventricle through the pulmonary valve into the pulmonary artery (towards the lungs)
   • From the left ventricle through the aortic valve into the aorta (towards the body)

The amount of blood ejected from each ventricle per beat is the stroke volume.

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Phase 3: Diastole (Relaxation)

After the ventricles have emptied, they relax. This is the longest phase of the cardiac cycle:

1. As the ventricles relax, the pressure inside them drops below the pressure in the arteries.
2. Blood in the pulmonary artery and aorta briefly tries to flow back into the ventricles, but this causes the semilunar valves to snap shut, producing the second heart sound — the "dub" of the heartbeat.
3. With all valves initially closed, the heart muscle relaxes completely.
4. Blood begins to flow passively into the atria again from the veins (vena cava into the right atrium; pulmonary veins into the left atrium).
5. As the atria fill and pressure builds, the AV valves open passively and blood begins to trickle into the ventricles, even before atrial systole begins again.
6. The cycle then repeats with the next atrial contraction.

graph LR
    A[Atrial Systole] --> B[Ventricular Systole]
    B --> C[Diastole]
    C --> A

    style A fill:#3498db,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#27ae60,color:#fff

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Heart Sounds

The familiar "lub-dub" sound of the heartbeat is produced by the closure of the heart valves:

Sound	Valve(s) Closing	When
"Lub" (first sound)	Tricuspid and bicuspid (AV) valves	Start of ventricular systole
"Dub" (second sound)	Pulmonary and aortic (semilunar) valves	Start of diastole

Exam Tip: If asked what causes the heart sounds, always specify which valves are closing and when. Simply saying "the valves close" is not precise enough for full marks.

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The Conduction System

The heart has its own built-in pacemaker that controls the timing of the cardiac cycle. You do not need detailed knowledge of this for AQA GCSE PE, but you should know that:

• The sinoatrial (SA) node, located in the wall of the right atrium, acts as the natural pacemaker. It generates electrical impulses that cause the atria to contract.
• These impulses then travel to the atrioventricular (AV) node, which delays the signal briefly before passing it to the ventricles, ensuring the atria finish contracting before the ventricles begin.
• The impulse then spreads through the walls of the ventricles, causing them to contract from the bottom upwards, which efficiently ejects blood into the arteries.