Cardiovascular System
Every heartbeat is a beautifully coordinated mechanical and electrical event designed to pump blood efficiently. This repeating sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles is called the cardiac cycle. It’s not just rhythm — it’s precision. In less than a second, the heart fills, contracts, empties, and resets — ready to do it all over again.
⏱️ Duration of the Cardiac Cycle
At a heart rate of 75 beats per minute, one cardiac cycle = 0.8 seconds.
| Phase | Time (seconds) |
|---|---|
| Atrial systole | 0.1 |
| Ventricular systole | 0.3 |
| Complete cardiac diastole | 0.4 |
🧩 Phases of the Cardiac Cycle
Let’s break the 0.8-second symphony into its key movements.
1. Atrial Systole (0.1 sec)
Event: Atria contract → blood pushed into ventricles.
- AV valves (mitral & tricuspid): Open
- Semilunar valves (aortic & pulmonary): Closed
- Adds the final 20–30% of ventricular filling (the “atrial kick”).
- Atrial pressure: Slight rise
Clinical pearl: In conditions like atrial fibrillation, loss of atrial systole → ↓ cardiac output (especially in elderly or heart disease patients).
2. Isovolumetric Contraction (0.05 sec)
Event: Ventricles start contracting, but no blood leaves yet.
- AV valves: Close → causes first heart sound (S₁ — “lub”)
- Semilunar valves: Still closed
- Volume: Constant (isovolumetric)
- Pressure: Rises sharply
Key concept: S₁ marks the onset of ventricular systole. You can feel it as the apex beat.
3. Ventricular Ejection Phase (0.25 sec)
Event: Pressure in ventricles > pressure in arteries → semilunar valves open.
- Blood flows: RV → pulmonary artery; LV → aorta.
- AV valves: Remain closed
- Ejection fraction: About 60% of end-diastolic volume (≈70 mL).
Clinical link: ↓ Ejection fraction (<50%) = systolic heart failure.
Phases of ejection:
- Rapid ejection: First 1/3 of phase — most blood leaves quickly.
- Reduced ejection: Last 2/3 — slower as pressure equalizes.
4. Isovolumetric Relaxation (0.08 sec)
Event: Ventricles relax but no filling yet — all valves closed.
- Semilunar valves: Close → causes second heart sound (S₂ — “dub”)
- AV valves: Still closed
- Volume: Constant (no inflow yet)
- Pressure: Falls rapidly
High-yield point: S₂ marks the end of ventricular systole and the start of diastole.
5. Ventricular Filling Phase (0.4 sec total)
Now the heart prepares for the next beat. Blood flows into relaxed ventricles in three phases:
- Rapid filling phase: AV valves open → blood rushes in (passive filling). May produce a soft third heart sound (S₃) in some young individuals (pathological in adults — sign of volume overload).
- Diastasis: Slow filling as pressure equalizes between atria and ventricles.
- Atrial systole: Final “atrial kick” completes filling (as in phase 1).
Clinical tip: A stiff ventricle (e.g., in hypertrophic cardiomyopathy) → produces S₄ due to strong atrial contraction against resistance.
💓 Summary of Valve Status During Cardiac Cycle
| Phase | AV Valves | Semilunar Valves |
|---|---|---|
| Atrial systole | Open | Closed |
| Isovolumetric contraction | Closed | Closed |
| Ventricular ejection | Closed | Open |
| Isovolumetric relaxation | Closed | Closed |
| Ventricular filling | Open | Closed |
🧠 Shortcut Mnemonic: “OCCCO” → sequence of open/closed valves through the cycle.
⚙️ Pressure and Volume Changes
During one cardiac cycle:
- End-diastolic volume (EDV): ~120 mL
- End-systolic volume (ESV): ~50 mL
- Stroke volume (SV): EDV − ESV = ~70 mL
- Ejection fraction (EF): SV ÷ EDV ≈ 60%
Clinical application: ↓ EF → heart failure; ↑ EDV → volume overload (as in mitral regurgitation).
🔊 Heart Sounds and Their Significance
| Sound | Cause | Timing | Clinical Relevance |
|---|---|---|---|
| S₁ (“Lub”) | AV valve closure | Start of systole | Loud in mitral stenosis |
| S₂ (“Dub”) | Semilunar valve closure | End of systole | Split in inspiration (normal) |
| S₃ | Rapid ventricular filling | Early diastole | Normal in youth, CHF in adults |
| S₄ | Atrial contraction into stiff ventricle | Late diastole | LV hypertrophy, hypertension |
🔁 Cardiac Cycle Timing (At HR = 75 bpm)
| Phase | Duration (sec) | Event Summary |
|---|---|---|
| Atrial systole | 0.1 | Atria contract, ventricles fill |
| Isovolumetric contraction | 0.05 | AV valves close (S₁) |
| Ventricular ejection | 0.25 | Blood ejected, semilunar valves open |
| Isovolumetric relaxation | 0.08 | Semilunar valves close (S₂) |
| Ventricular filling | 0.4 | AV valves open, blood fills ventricles |
💡 Clinical Correlations
- Heart Murmurs: Caused by turbulent blood flow due to valve defects. Systolic murmurs: Aortic stenosis, mitral regurgitation. Diastolic murmurs: Aortic regurgitation, mitral stenosis.
- Heart Failure: When cardiac output fails to meet body demands. Left heart failure: Pulmonary congestion. Right heart failure: Systemic edema.
- Tachycardia (↑ HR): Shortens diastole → ↓ ventricular filling → ↓ stroke volume.
- Bradycardia (↓ HR): Prolongs diastole → ↑ filling → ↑ stroke volume.
🧠 High-Yield Summary Table
| Term | Definition / Normal Value |
|---|---|
| Cardiac cycle | One complete heartbeat (0.8 sec) |
| Stroke volume (SV) | 70 mL |
| Heart rate (HR) | 75 bpm |
| Cardiac output (CO) | HR × SV = ~5 L/min |
| Ejection fraction | ~60% |
| S₁ | Closure of AV valves |
| S₂ | Closure of semilunar valves |