Define cardiac cycle.

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Cardiac Cycle**: The cardiac cycle is defined as the series of events that occur from the beginning of one heartbeat to the beginning of the next heartbeat. It encompasses all the mechanical and electrical activities of the heart during this period.

2. **Phases of the Cardiac Cycle**: The cardiac cycle is primarily divided into two main phases:
- **Systole**: This is the phase where the cardiac muscles contract. During systole, the heart pumps blood into the arteries. This phase is crucial for the circulation of blood throughout the body.
- **Diastole**: This is the phase where the heart muscles relax. During diastole, the heart chambers fill with blood in preparation for the next contraction. This phase ensures that the heart has enough blood to pump during the next systolic phase.

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