What is the phase of the heart cycle when the ventricles relax called?

The phase of the heart cycle when the ventricles relax is known as diastole. During diastole, the myocardium of the ventricles is relaxed, allowing the chambers of the heart to fill with blood. This process is crucial for maintaining adequate blood circulation throughout the body, as it ensures that the ventricles are filled with enough blood to pump effectively during the next contraction phase.

In contrast, systole is the phase when the ventricles contract to pump blood out of the heart. The contraction phase is a less formal term often used to describe this period of systole, but it does not specifically denote the relaxation and filling aspect of the heart cycle. The filling phase typically refers to the initial phase of diastole when the ventricles are in the process of filling with blood, which is also part of diastole but does not encompass the entire relaxation phase. Therefore, diastole is the correct and specific term for the relaxation phase of the heart cycle.