Outline the phases of the cardiac cycle and what happens to AV and semilunar valves and chamber pressures during each phase.

• A. Atrial systole: AV valves open, semilunar valves closed; ventricles fill. Isovolumetric contraction: AV valves close; semilunar valves closed; pressure in ventricles rises. Ventricular ejection: semilunar valves open; AV valves closed; blood ejected. Isovolumetric relaxation: semilunar valves close; AV valves remain closed briefly; pressure falls. Ventricular filling: AV valves reopen; passive filling resumes.
• B. Atrial systole: AV valves closed; semilunar valves open; ventricles eject; Isovolumetric contraction: all valves closed; pressure constant.
• C. During atrial diastole, all valves open; ventricles fill.
• D. An action potential occurs only during ejection.

Answer: (A)