What best defines the cardiac cycle?

• A. A complete heartbeat consisting of contraction and relaxation of both atria and both ventricles.
• B. A heartbeat that only contracts the atria.
• C. The electrical impulses of the heart.
• D. The circulation of blood through the lungs.

Answer: (A)

The cardiac cycle is the sequence of mechanical events that make one heartbeat, involving contraction and relaxation of both atria and both ventricles. It includes atrial systole to push blood into the ventricles, ventricular systole to eject blood to the lungs and body, and diastole when the chambers relax and fill again. While electrical impulses coordinate these actions, the cycle itself is defined by the coordinated contraction and relaxation of all four chambers. Descriptions focusing only on electrical activity, or on only part of the heart, or on pulmonary circulation, don’t capture the full sequence of events that constitutes one heartbeat.