Summarize the two phases of the cardiac cycle during which all four heart valves are briefly closed
What will be an ideal response?
The first phase during which all four heart valves are closed, known as the isovolumetric contraction phase, occurs at the beginning of ventricular systole. Pressure rises in the ventricles as they begin to contract. The high pressure closes the AV valves, producing the heart sound S1. Since ventricular pressure is not yet great enough to push open the semilunar valves, they remain closed. Ventricular volume does not change during this shortest phase of the cardiac cycle. The second phase during which all four heart valves are closed, known as the isovolumetric relaxation phase, occurs at the beginning of ventricular diastole. Pressure declines in the ventricles causing the semilunar valves to snap shut, producing the heart sound S2. The pressure in the ventricles is still somewhat higher than that in the atria, so the AV valves remain closed. Once again, ventricular volume does not change during this brief phase.
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