Which phase occurs immediately after atrial systole?

Which phase occurs immediately after atrial systole?

• A. Isovolumetric contraction.
• B. Atrial filling.
• C. Isovolumetric relaxation.
• D. Ventricular systole.

Answer: (A)

After atrial systole, the ventricles begin to contract while both sets of valves are momentarily closed. This is isovolumetric contraction. In this phase, ventricular pressure rises but the volume stays the same because the AV valves and the semilunar valves are closed. Once ventricular pressure exceeds the pressure in the arteries, the semilunar valves open and ejection begins. So the immediate next phase is the contraction with no volume change. Atrial filling happens before atrial systole, and isovolumetric relaxation occurs after ejection when the ventricles relax with all valves closed. Ventricular systole encompasses this contraction up to and including ejection, not the exact next step after atrial systole.

After atrial systole, the ventricles begin to contract while both sets of valves are momentarily closed. This is isovolumetric contraction. In this phase, ventricular pressure rises but the volume stays the same because the AV valves and the semilunar valves are closed. Once ventricular pressure exceeds the pressure in the arteries, the semilunar valves open and ejection begins. So the immediate next phase is the contraction with no volume change. Atrial filling happens before atrial systole, and isovolumetric relaxation occurs after ejection when the ventricles relax with all valves closed. Ventricular systole encompasses this contraction up to and including ejection, not the exact next step after atrial systole.