Cardiac Cycle Timeline (0.8 seconds at 75 bpm)
T = 0.0 s — SAN Fires
- Sinoatrial node generates electrical impulse
- Impulse spreads through both atria via gap junctions
- P wave begins on ECG (atrial depolarization)
T = 0.0-0.1 s — ATRIAL SYSTOLE (0.1 s)
- Both atria contract simultaneously
- AV valves (tricuspid + mitral) are OPEN → remaining blood pushed into ventricles
- Semilunar valves CLOSED
- AVN receives impulse → delays it 0.1 s
T = 0.1 s — Impulse Reaches Bundle of His
- After AVN delay, impulse conducted down Bundle of His → left + right branches → Purkinje fibres
- QRS complex appears on ECG (ventricular depolarization)
T = 0.1-0.13 s — ISOVOLUMETRIC CONTRACTION (initial ventricular systole)
- Ventricles begin contracting → pressure rises rapidly
- AV valves CLOSE (ventricular pressure > atrial pressure) — produces first heart sound "LUB"
- Semilunar valves still CLOSED (ventricular pressure < arterial pressure yet)
- All four valves momentarily CLOSED
- Volume is constant (isovolumetric)
T = 0.13-0.4 s — VENTRICULAR EJECTION PHASE (rest of ventricular systole)
- Ventricular pressure exceeds aortic and pulmonary artery pressure
- Semilunar valves OPEN → blood ejected into aorta and pulmonary artery
- AV valves remain CLOSED
- Ventricles empty (~70 mL ejected per beat = stroke volume)
T = 0.4 s — Ventricular Systole Ends; Semilunar Valves Close
- Ventricular pressure drops below arterial pressure
- Semilunar valves CLOSE — produces second heart sound "DUB"
- T wave on ECG (ventricular repolarization begins)
T = 0.4-0.8 s — JOINT DIASTOLE (0.4 s)
- All four chambers RELAX simultaneously
- AV valves OPEN → blood flows from veins → atria → passively into ventricles
- Semilunar valves CLOSED
- Ventricular filling: ~70% passive, ~30% from next atrial systole
T = 0.8 s — New Cycle Begins
- SAN fires again → next heartbeat begins