The cardiac cycle represents a single heartbeat, comprising sequential contraction and relaxation of the atria and ventricles. This intricate process ensures efficient blood circulation, driven by pressure changes and governed by heart valves. Understanding the cardiac cycle’s phases, pressure-volume relationships, and physiological regulation is crucial for mastering cardiovascular physiology.
This article will cover:
✔ Phases of the cardiac cycle
✔ Hemodynamic changes during each phase
✔ Heart sounds and their correlation with valve activity
✔ Clinical significance & pathological variations
1. Phases of the Cardiac Cycle
The cardiac cycle is divided into systole (contraction) and diastole (relaxation) and consists of five sequential phases:
|
Phase |
Ventricular Activity |
Valvular Status |
Hemodynamic Events |
|
1. Atrial Systole |
Atria contract |
AV valves open, SL valves closed |
Ventricular filling boosted |
|
2. Isovolumetric Contraction |
Ventricles begin contracting |
All valves closed |
First heart sound (S1); Pressure rises |
|
3. Ventricular Ejection |
Ventricles contract fully |
SL valves open, AV valves closed |
Blood ejected into arteries |
|
4. Isovolumetric Relaxation |
Ventricles relax |
All valves closed |
Second heart sound (S2); Pressure falls |
|
5. Ventricular Filling |
Passive ventricular filling |
AV valves open, SL valves closed |
Rapid filling of ventricles |
A. Breakdown of Each Phase
1. Atrial Systole (End of Diastole) – "Topping Off" Ventricular Volume
✔ The atria contract, pushing the final 20-30% of blood into the ventricles.
✔ The P wave on ECG represents atrial depolarization.
✔ Ventricular pressure slightly increases, but AV valves remain open.
✔ In conditions like atrial fibrillation, this phase is lost, reducing ventricular preload.
2. Isovolumetric Contraction – Pressure Builds, but No Ejection Yet
✔ Ventricles start contracting, sharply increasing pressure.
✔ AV valves snap shut, producing S1 ("lub").
✔ All valves remain closed → No blood movement (isovolumetric).
✔ The QRS complex on ECG represents ventricular depolarization.
3. Ventricular Ejection – The Power Stroke
✔ When ventricular pressure exceeds arterial pressure, the semilunar (SL) valves open.
✔ Blood is forcefully ejected into the aorta and pulmonary artery.
✔ Stroke volume (SV) is determined here.
✔ The T wave on ECG represents ventricular repolarization.
4. Isovolumetric Relaxation – Ventricular Pressure Drops
✔ Ventricles relax, and pressure falls rapidly.
✔ The SL valves close, producing S2 ("dub").
✔ The dicrotic notch (small pressure rebound in the aorta) marks aortic valve closure.
✔ No blood enters or exits the ventricles (isovolumetric state).
5. Ventricular Filling – Cycle Restarts
✔ When ventricular pressure drops below atrial pressure, AV valves reopen.
✔ Blood passively fills the ventricles, preparing for the next cycle.
✔ This phase contributes most of ventricular filling (~70%).
2. Pressure-Volume Loop: The Hemodynamic Representation
The pressure-volume (PV) loop graphically represents the relationship between pressure and volume changes throughout the cardiac cycle:
|
Point |
Phase |
Key Event |
|
A → B |
Ventricular Filling |
Volume increases, slight pressure rise |
|
B → C |
Isovolumetric Contraction |
Pressure increases, no volume change |
|
C → D |
Ventricular Ejection |
Volume decreases, pressure peak reached |
|
D → A |
Isovolumetric Relaxation |
Pressure falls, no volume change |
🔹 End-Diastolic Volume (EDV): The maximum ventricular volume at the end of filling.
🔹 End-Systolic Volume (ESV): The minimum ventricular volume after ejection.
🔹 Stroke Volume (SV) = EDV - ESV
🔹 Ejection Fraction (EF) = (SV / EDV) × 100
📌 Clinical Relevance:
✔ In heart failure with reduced ejection fraction (HFrEF), SV & EF decrease, shifting the PV loop leftward.
✔ In aortic stenosis, the afterload increases, requiring higher pressure for ejection.
3. Heart Sounds: The Audible Markers of the Cycle
|
Heart Sound |
Cause |
Associated Phase |
|
S1 ("lub") |
AV valves closure |
Isovolumetric contraction |
|
S2 ("dub") |
SL valves closure |
Isovolumetric relaxation |
|
S3 (ventricular gallop) |
Rapid filling |
Ventricular filling (abnormal in adults) |
|
S4 (atrial gallop) |
Atrial contraction against a stiff ventricle |
Atrial systole (abnormal) |
📌 Clinical Relevance:
✔ Loud S1 → Mitral stenosis (forceful closure).
✔ Split S2 → Can indicate pulmonary hypertension or bundle branch blocks.
✔ S3 → Common in heart failure, indicating volume overload.
✔ S4 → Seen in hypertrophic cardiomyopathy and ventricular stiffness.
4. Clinical Correlations & Pathological Variations
|
Condition |
Effect on Cardiac Cycle |
|
Aortic Stenosis |
Increased afterload, prolongs isovolumetric contraction |
|
Mitral Regurgitation |
Continuous volume overload, no true isovolumetric phase |
|
Heart Failure (HFrEF) |
Reduced stroke volume and ejection fraction |
|
Atrial Fibrillation |
Loss of atrial systole, reducing ventricular preload |
5. Regulation of the Cardiac Cycle
✔ Preload (Frank-Starling Law): More ventricular filling → Stronger contraction.
✔ Afterload: Higher arterial resistance → Increased cardiac workload.
✔ Contractility: Higher sympathetic activity → Stronger ventricular contraction.
✔ Heart Rate: Increased sympathetic drive → Shorter diastole, faster cycle.
Conclusion
The cardiac cycle is a seamless interplay of pressure, volume, and valvular function, ensuring efficient blood circulation. Each phase has distinct mechanical and electrical events, with pathophysiological alterations leading to cardiovascular diseases.
In the next article, we will explore "Electrical Conduction of the Heart," covering SA node, AV node, bundle branches, and arrhythmias.
References
- Guyton AC, Hall JE. Textbook of Medical Physiology. 14th ed. Elsevier; 2020.
- Braunwald E. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 11th ed. Elsevier; 2018.
- Klabunde RE. Cardiovascular Physiology Concepts. 3rd ed. Lippincott Williams & Wilkins; 2021.
- Mayo Clinic. Heart Sounds and Murmurs. Available at: www.mayoclinic.org.
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