Final answer:
The relaxation phase of the heart is called diastole, which occurs when the heart chambers fill with blood after contraction. This phase follows the systolic phase where the heart contracts to pump blood around the body.
Step-by-step explanation:
The relaxation phase of the heart is known as diastole. During diastole, the heart chambers fill with blood as they relax after contraction.
The cardiac cycle includes both phases of heart activity: systole and diastole. Systole is the phase where the heart contracts to pump blood out. Specifically, ventricular systole occurs when the ventricles contract to push blood out of the heart, following the closing of the atrioventricular valves and preceding the closing of the semilunar valves. Ventricular relaxation immediately follows this phase. The regulation and coordination of systole and diastole are crucial for efficient blood circulation throughout the body.