The Cardiac Cycle
Understanding the pressure-volume loop and the mechanical events of the cardiac cycle is foundational for cardiac physiology.
1. Key Phases
| Phase | Valvular Status | Description |
|---|---|---|
| Isovolumetric Contraction | All closed | Ventricular pressure rises; volume constant. |
| Ventricular Ejection | Semilunar open | Blood leaves the ventricle; volume decreases. |
| Isovolumetric Relaxation | All closed | Ventricular pressure falls; volume constant. |
| Ventricular Filling | AV valves open | Passive filling followed by atrial kick. |
2. Exam Must-Knows
- Heart Sounds: S1 (mitral/tricuspid closure) marks the start of systole. S2 (aortic/pulmonic closure) marks the end of systole.
- Pressure-Volume Loops: Area inside the loop = Stroke Work. Changing afterload shifts the top-left corner; changing preload shifts the bottom-right corner.
- Ejection Fraction (EF):
. Normal is >55%.
Physiology Board Hint: Focus on the isovolumetric phases. In isovolumetric contraction, pressure spikes rapidly because the chamber is a closed system. Pathologies like aortic stenosis shift the pressure-volume loop to the right (higher pressure needed to eject), while mitral regurgitation shifts it significantly to the right (volume overload).