The Cardiac Cycle - A Complete Explanation

The Trigger: Electrical Initiation

The Seven Phases of the Cardiac Cycle

Phase A - Atrial Systole (Atrial Contraction)

• ECG event: P wave (atrial depolarization)
• What happens: The atria contract, actively ejecting blood through the open mitral and tricuspid (AV) valves into the relaxed ventricles
• About 70% of ventricular filling has already occurred passively before atrial contraction. Atrial systole adds the final ~30% ("atrial kick")
• Left atrial pressure rises, which is reflected as the "a wave" on the jugular venous pulse (JVP)
• The fourth heart sound (S4) can occur here - not audible normally, but heard when ventricular compliance is reduced (e.g. ventricular hypertrophy)
• The SA node narrows the venae cavae and pulmonary vein orifices during contraction, limiting backflow into the veins (though some regurgitation does occur)
• Semilunar valves (aortic and pulmonary): CLOSED
• AV valves (mitral and tricuspid): OPEN

Phase B - Isovolumetric Ventricular Contraction (IVC)

• ECG event: QRS complex (ventricular depolarization)
• What happens: The ventricles begin contracting. Ventricular pressure rises sharply and rapidly
• As soon as ventricular pressure exceeds atrial pressure, the mitral and tricuspid valves snap shut - this produces S1 (first heart sound - "lubb")
• The aortic and pulmonary valves are still closed because ventricular pressure hasn't yet exceeded aortic pressure (~80 mmHg) or pulmonary artery pressure (~10 mmHg)
• Therefore, all four valves are closed - no blood enters or leaves, so ventricular volume stays constant (isovolumetric)
• This phase lasts ~0.05 seconds
• The tension in the ventricular wall increases enormously with no change in fiber length - analogous to an isometric contraction

Phase C - Rapid Ventricular Ejection

• ECG event: ST segment
• What happens: Ventricular pressure finally exceeds aortic pressure (~80 mmHg in left, ~10 mmHg in right). The aortic and pulmonary semilunar valves open
• Blood is ejected rapidly into the aorta and pulmonary artery - this is the fast ejection phase
• Left ventricular pressure peaks at ~120 mmHg; right ventricular pressure peaks at ~25 mmHg
• Aortic pressure also rises rapidly, reaching ~120 mmHg (systolic blood pressure)
• Ventricular volume falls steeply from the End-Diastolic Volume (EDV, ~130 mL) toward its minimum
• AV valves remain closed; semilunar valves are open

Phase D - Reduced Ventricular Ejection

• ECG event: T wave (ventricular repolarization begins)
• What happens: Ejection continues but at a slower rate as the ventricle begins to relax
• Aortic pressure starts to fall as blood runs off into the peripheral arteries
• Ventricular volume reaches its minimum - the End-Systolic Volume (ESV, ~50 mL)
• The ejection fraction (EF) = (EDV - ESV) / EDV = (130 - 50) / 130 ≈ 65% - a key index of ventricular function
• Late in systole, aortic pressure actually slightly exceeds left ventricular pressure, but forward momentum briefly keeps blood flowing forward

Phase E - Isovolumetric Ventricular Relaxation (IVR)

• ECG event: After T wave (electrical silence)
• What happens: Ventricular muscle fully relaxes. Ventricular pressure drops rapidly
• The momentum of ejected blood is overcome; aortic and pulmonary valves snap shut - this produces S2 (second heart sound - "dubb"), the dicrotic notch appears on the aortic pressure tracing
• All four valves are again closed - volume remains constant (isovolumetric)
• This period of protodiastole (~0.04 s) followed by IVR ends when ventricular pressure falls below atrial pressure, allowing the AV valves to open
• Duration: ~0.08 seconds

Phase F - Rapid Ventricular Filling

• ECG event: Isoelectric period (before next P wave)
• What happens: Ventricular pressure drops below atrial pressure; mitral and tricuspid valves open
• Blood that had been accumulating in the atria during systole rushes passively into the ventricles - most of ventricular filling (about 70-80%) happens here
• Ventricular volume increases rapidly; pressure remains low
• The third heart sound (S3) may occur at the peak of rapid filling - physiologically normal in young people, but a sign of heart failure or volume overload in older adults
• The "v wave" on the JVP tracing corresponds to venous filling of the atrium during ventricular systole, followed by rapid descent when the AV valves open

Phase G - Reduced Ventricular Filling (Diastasis)

• ECG event: Late diastole
• What happens: Ventricular filling slows as the ventricle becomes progressively distended
• AV valve cusps drift toward the closed position (but do not close)
• Pressures in the ventricle and atrium equilibrate; flow slows to a trickle
• This phase is shortened disproportionately when heart rate increases (tachycardia mainly steals from diastole, not systole)
• The next P wave fires, triggering atrial systole again, and the cycle repeats

Pressure-Volume Loop of the Left Ventricle

• d → a: Diastolic filling (volume increases, pressure rises slowly)
• a → b: Isovolumetric contraction (pressure rises sharply, volume constant)
• b → c: Ejection (volume falls, pressure rises to peak then falls)
• c → d: Isovolumetric relaxation (pressure falls, volume constant)

Summary Table

Key Numerical Values (at rest, HR ~72 bpm)

Effect of Increasing Heart Rate

Right vs Left Heart

• Right atrial systole occurs before left atrial systole
• Left ventricular contraction starts before right ventricular contraction
• However, because pulmonary artery pressure (~10-25 mmHg) is much lower than aortic pressure (~80-120 mmHg), the right ventricle ejects before the left
• During inspiration, the aortic valve closes slightly before the pulmonary valve - this produces physiological splitting of S2