Anatomy of Cardiac Cycle: A Complete Guide for Medical Students

1. What is the Cardiac Cycle?

• The cardiac cycle is the complete sequence of events in the heart from one heartbeat to the next.
• It includes contraction (systole) and relaxation (diastole) of both atria and ventricles.

Duration:

• At rest (heart rate 72/min): ~0.8 seconds

---

2. Phases of Cardiac Cycle

Total = 0.8 seconds

Phase	Duration	Description
Atrial Systole	0.1 sec	Atria contract → Blood to ventricles
Ventricular Systole	0.3 sec	Ventricles contract → Blood to aorta & pulmonary artery
Complete Cardiac Diastole	0.4 sec	Both atria & ventricles relax, chambers fill with blood

3. Subphases of Ventricular Systole

1. Isovolumetric Contraction:
• All valves closed
• Pressure builds up
2. Ejection Phase:
• Semilunar valves (aortic & pulmonary) open
• Blood is pumped out

---

4. Subphases of Ventricular Diastole

1. Isovolumetric Relaxation:
• All valves closed
• Ventricles relax
2. Rapid Filling Phase:
• AV valves (mitral & tricuspid) open
• Blood flows rapidly into ventricles
3. Slow Filling Phase (Diastasis):
• Blood fills slowly as pressure equalizes

---

5. Heart Sounds During Cycle

Sound	Cause	Phase
S₁ (Lub)	AV valves close	Start of ventricular systole
S₂ (Dub)	Semilunar valves close	Start of diastole
S₃	Rapid filling of ventricles	Early diastole
S₄	Atrial contraction	Late diastole (usually abnormal)

6. Pressure Changes

• Atrial pressure rises during atrial contraction.
• Ventricular pressure increases in systole and decreases in diastole.
• Aortic pressure rises during ventricular ejection.

---

7. Volume Changes

• End Diastolic Volume (EDV): ~120 mL (just before contraction)
• End Systolic Volume (ESV): ~50 mL (after contraction)
• Stroke Volume (SV): EDV - ESV = ~70 mL

---

8. ECG Relation

ECG Wave	Event
P wave	Atrial depolarization (before atrial systole)
QRS complex	Ventricular depolarization (before systole)
T wave	Ventricular repolarization (during diastole)

9. Mnemonic to Remember Cycle Flow

"A Vicious Rapid System Ends Dangerously"
➡️ Atrial systole → Ventricular systole → Rapid ejection → Slow ejection → End systole → Diastole