During which phase of the cardiac cycle does the heart muscle relax?

The heart muscle relaxes during diastole. This phase is crucial as it allows the chambers of the heart to fill with blood after they have contracted. In diastole, the atrioventricular (AV) valves open, allowing blood to flow from the atria into the ventricles. This relaxation is essential for maintaining a continuous flow of blood throughout the body, ensuring that the next contraction during systole is effective and that the heart can pump blood efficiently.

During systole, the heart muscle contracts, pumping blood out of the heart. The isometric phase refers to the period within systole when the heart muscle contracts without a change in volume; however, the heart muscle is not relaxed during this time. The contraction phase also describes the active pumping of blood, which, like systole, involves tension in the cardiac muscle rather than relaxation. Understanding these phases and their roles is key to comprehensively grasping cardiac physiology.