In the list of phases of the cardiac cycle, what is the second phase?

• A. Diastole
• B. Atrial Systole
• C. Isovolumetric Contraction
• D. Ejection

Answer: (B)

The sequence of events in the cardiac cycle is organized by what the ventricles are doing: relaxing and filling, then contracting, then relaxing again. After the ventricles have largely filled during ventricular diastole, the atria contract to top off ventricular filling. That atrial contraction is called atrial systole, and it typically appears as the second phase in many standard lists of the cycle. It provides the final “atrial kick” that adds a bit more blood to the ventricles before they start to contract. Once atrial systole finishes, the ventricles begin to contract (isovolumetric contraction) and then eject blood, followed by relaxation phases.