3.5.4 The Cardiac Cycle

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The cardiac cycle describes the sequence of events that occur in one heartbeat. The heart is myogenic, meaning it generates its own electrical impulses to coordinate contraction.

Key Structures Involved:

Sinoatrial Node (SAN):

Located in the wall of the right atrium.
Acts as the pacemaker, initiating a wave of electrical excitation that causes the atria to contract simultaneously.

Atrioventricular Node (AVN):

Located at the base of the atria.
Delays the electrical impulse, allowing the atria to fully contract and empty before ventricular contraction begins.

Bundle of His and Purkyne Fibres:

Electrical signals pass from the AVN to the Bundle of His, a specialised group of fibres in the septum.
The Bundle of His splits into Purkyne fibres, which spread the impulse upwards from the apex of the heart.
This ensures that the ventricles contract from the bottom upwards, efficiently ejecting blood into the arteries.

Stages of the Cardiac Cycle:

Cardiac Diastole:

Both the atria and ventricles relax.
Elastic recoil reduces pressure in the chambers.
Blood flows into the atria from the vena cava (right atrium) and pulmonary vein (left atrium).
As atrial pressure increases, the atrioventricular (AV) valves open, and blood begins to flow passively into the ventricles.
The semilunar valves are closed to prevent backflow.

Atrial Systole:

The atria contract, forcing the remaining blood into the ventricles.
The ventricles remain relaxed during this phase.

Ventricular Systole:

The ventricles contract, increasing pressure within them.
The AV valves close to prevent backflow into the atria.
The semilunar valves open, allowing blood to flow:
From the right ventricle into the pulmonary artery.
From the left ventricle into the aorta.

Timing of the Cardiac Cycle:

The atria contract first, ensuring the ventricles are filled with blood.
A delay at the AVN ensures the ventricles contract after the atria have emptied.
The ventricles contract from the apex upwards, maximising blood ejection.

Key Terms:

Myogenic: The heart's ability to contract without external stimulation.
Sinoatrial Node (SAN): The pacemaker of the heart, initiating the heartbeat.
Atrioventricular Node (AVN): Delays the signal, ensuring atria and ventricles contract in sequence.
Bundle of His: Conducts the electrical signal from the AVN to the ventricles.
Purkyne Fibres: Spread the signal through the ventricles, ensuring coordinated contraction.

Summary of Key Events:

Phase	What Happens?	Valves Involved
Cardiac Diastole	Atria and ventricles relax; blood flows passively into atria and ventricles.	AV valves open, semilunar valves closed.
Atrial Systole	Atria contract, forcing remaining blood into ventricles.	AV valves open, semilunar valves closed.
Ventricular Systole	Ventricles contract; AV valves close to prevent backflow, and semilunar valves open to allow blood to exit the heart.	AV valves closed, semilunar valves open.

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Tips for Exams:

Be able to describe the role of each structure (SAN, AVN, Bundle of His, Purkyne fibres) in coordinating the cardiac cycle.
Understand the timing and role of valves during each phase of the cycle.
Be prepared to explain why the ventricles contract from the apex upwards.

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Summary:

The cardiac cycle ensures efficient blood flow through the heart and to the body.
It is controlled by the SAN, AVN, and conduction pathways.
The sequence of contraction and relaxation (systole and diastole) involves precise coordination to prevent backflow and maximise blood ejection.

The Cardiac Cycle Simplified Revision Notes for A-Level AQA Biology

3.5.4 The Cardiac Cycle

Key Structures Involved:

Stages of the Cardiac Cycle:

Timing of the Cardiac Cycle:

Key Terms:

Summary of Key Events:

Tips for Exams:

Summary:

500K+ Students Use These Powerful Tools to Master The Cardiac Cycle For their A-Level Exams.

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