Isovolumetric Relaxation: As the ventricles begin to relax, pressure drops rapidly. When ventricular pressure falls below arterial pressure, the semilunar valves close, but the AV valves remain shut until ventricular pressure falls below atrial pressure.
Ventricular Filling: This phase begins when the AV valves open, allowing blood to flow from the atria into the ventricles. Approximately 80% of filling occurs passively due to the pressure gradient before the atria even contract.
Atrial Systole: The final stage of diastole involves atrial contraction, which provides the 'atrial kick' to push the remaining 20% of blood into the ventricles. This phase concludes with the End-Diastolic Volume (EDV), the maximum volume the heart holds before the next beat.
Valve Mechanics: Valves are passive structures that respond to pressure. They open when the upstream pressure is higher than the downstream pressure and close when the downstream pressure becomes higher, preventing regurgitation.
First Heart Sound (): Known as 'lubb', this sound is caused by the turbulence created when the atrioventricular (AV) valves close at the onset of ventricular systole.
Second Heart Sound (): Known as 'dupp', this sound is caused by the closure of the semilunar valves at the onset of ventricular diastole as blood attempts to flow back into the relaxing ventricles.
| Phase | AV Valves | Semilunar Valves | Ventricular Volume |
|---|---|---|---|
| Atrial Systole | Open | Closed | Increasing (Final 20%) |
| Isovolumetric Contraction | Closed | Closed | Constant (EDV) |
| Ventricular Ejection | Closed | Open | Decreasing |
| Isovolumetric Relaxation | Closed | Closed | Constant (ESV) |
| Ventricular Filling | Open | Closed | Increasing (Passive 80%) |
The 'Iso' Rule: Whenever a phase is labeled 'isovolumetric', it means all four valves are closed and the volume of blood in the chamber cannot change. This occurs twice: once at the start of systole and once at the start of diastole.
Pressure Triggers: Always remember that valves do not open because of muscle pulling on them; they open because the pressure in the chamber behind them exceeds the pressure in the vessel or chamber in front of them.
Volume Calculations: Be prepared to calculate Stroke Volume () using the formula: . This represents the actual amount of blood pumped per beat.
Timing Misconceptions: Atrial systole happens at the end of ventricular diastole, not at the start of the cycle. Most ventricular filling is passive and does not require atrial contraction.
Simultaneous Contraction: A common error is assuming the whole heart contracts at once. In reality, the atria and ventricles contract sequentially to allow for proper filling and ejection timing.
Valve Opening Mechanism: Students often think valves open due to electrical signals. While electricity triggers the muscle to contract, it is the resulting pressure change that physically moves the valve flaps.
Emptying the Heart: Many believe the heart pumps out all its blood during systole. In a healthy heart, only about 50-70% of the blood is ejected (the ejection fraction), leaving the ESV behind.
