What defines a complete cardiac cycle?

A complete cardiac cycle encompasses all the mechanical and electrical events that occur in the heart during the time from the beginning of one heartbeat to the beginning of the next. This cycle includes the phases of diastole (when the heart muscle relaxes and the chambers fill with blood) and systole (when the heart muscle contracts and pumps blood out).

The cardiac cycle is crucial for maintaining an effective circulation of blood throughout the body and is characterized by distinct phases such as atrial and ventricular contraction and relaxation. Thus, the definition provided in the correct answer encapsulates the entirety of these processes, making it the most accurate choice. The time frame between the initiation of one heartbeat and the initiation of the next heartbeat covers all the activities—electrical stimulation, contraction, and relaxation of various heart chambers—necessary for effective heart function.

The other options focus on specific aspects of heart function rather than the complete process of a cardiac cycle. For instance, while relaxation of the heart muscle is a critical part of the cycle, it does not completely describe the cycle on its own. Likewise, contraction of the atria only represents only a segment of the full cycle, neglecting the vital role of ventricular contraction and the complete relaxation phase. The time between heartbeats,