Which heart phase is not included in the cardiac cycle?

The correct choice identifies isovolumetric relaxation as a phase not typically categorized in the cardiac cycle in the most commonly taught basic frameworks. The cardiac cycle primarily consists of two main phases: diastole and systole. Diastole refers to the phase when the heart muscle relaxes and the chambers fill with blood, while systole involves contraction when the heart muscles contract and blood is pumped out of the chambers.

Isovolumetric relaxation, while it does occur during the cardiac cycle, is often considered a part of the transition between the ejection of blood (systole) and filling the heart with blood (diastole) rather than being a standalone phase. It describes a brief period where the ventricles are relaxing after systole but before the chambers fill with blood, ensuring all heart valves are closed and the volume of blood in the heart does not change.

In contrast, the recognized phases of diastole and contraction more clearly encompass the essential actions of the heart within the cycle, and thus are more prominent in discussions regarding heart physiology.