The Cardiac Cycle

This lesson covers the cardiac cycle — the sequence of events that occurs during one complete heartbeat — as required by the Edexcel GCSE PE specification (1PE0 — Topic 1). You need to understand the phases of the cardiac cycle, how blood moves through the heart, and how the cycle relates to heart rate and exercise.

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What Is the Cardiac Cycle?

The cardiac cycle is the term used to describe one complete heartbeat — from the beginning of one contraction to the beginning of the next. At rest, the cardiac cycle takes approximately 0.8 seconds (at a heart rate of 75 bpm).

The cardiac cycle has three main phases:

1. Atrial systole — the atria contract
2. Ventricular systole — the ventricles contract
3. Diastole — the whole heart relaxes and fills with blood

Phase	What Happens	Duration (approx.)
Atrial systole	Both atria contract simultaneously, pushing blood into the ventricles through the AV valves	0.1 seconds
Ventricular systole	Both ventricles contract simultaneously, pushing blood out through the semilunar valves into the aorta and pulmonary artery	0.3 seconds
Diastole	The entire heart relaxes. The atria fill with blood from the veins. The ventricles also begin to fill passively	0.4 seconds

Exam Tip: Notice that diastole (relaxation) takes the longest — about half the total cycle. This is when the heart chambers fill with blood. During exercise, the diastole phase shortens (the heart beats faster), but the same sequence of events still occurs.

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The Three Phases in Detail

Phase 1: Atrial Systole

• Both the left and right atria contract at the same time.
• The right atrium pushes deoxygenated blood through the tricuspid valve into the right ventricle.
• The left atrium pushes oxygenated blood through the bicuspid valve into the left ventricle.
• The semilunar valves (pulmonary and aortic) are closed during this phase to prevent blood flowing backwards from the arteries.
• Atrial contraction only accounts for about 20-30% of ventricular filling — most blood flows into the ventricles passively during diastole.

Phase 2: Ventricular Systole

• Both the left and right ventricles contract at the same time, generating high pressure.
• The AV valves (tricuspid and bicuspid) close — this prevents blood flowing back into the atria. The closing of these valves produces the first heart sound ("lub").
• The right ventricle pushes deoxygenated blood through the pulmonary valve into the pulmonary artery towards the lungs.
• The left ventricle pushes oxygenated blood through the aortic valve into the aorta towards the body.
• The left ventricle generates much more pressure than the right because it must push blood much further.

Phase 3: Diastole (Cardiac Diastole)

• The entire heart relaxes.
• The semilunar valves (pulmonary and aortic) close — this prevents blood flowing back into the ventricles from the arteries. The closing of these valves produces the second heart sound ("dub").
• Blood flows into the atria from the veins: deoxygenated blood from the vena cava fills the right atrium; oxygenated blood from the pulmonary veins fills the left atrium.
• Blood begins to flow passively from the atria into the ventricles through the open AV valves.
• The cycle then repeats with the next atrial systole.