Which phase follows Atrial Systole in the listed sequence?

• A. Isovolumetric Contraction
• B. Diastole
• C. Ejection
• D. Isovolumetric Relaxation

Answer: (A)

Right after atrial systole, the ventricles start to contract. The pressure inside the ventricles rises quickly, causing the atrioventricular valves to snap shut while the semilunar valves remain closed. Since both sets of valves are closed, the ventricular volume doesn’t change—this is the isovolumetric contraction phase. Only after ventricular pressure surpasses that in the aorta and pulmonary artery do the semilunar valves open and blood is ejected, transitioning to the next phase. The other phases—diastole (ventricular relaxation and filling), isovolumetric relaxation (ventricular pressure falls with all valves closed), and rapid/euvoled ejection—occur later in the cycle.