What is the term for the heart when it is contracting?

The term that refers to the heart when it is contracting is "systole." During systole, the heart's ventricles contract to pump blood out of the heart and into the arteries. This phase is crucial for maintaining blood circulation throughout the body and is characterized by the rise in blood pressure during the contraction.

In contrast, the other terms represent different concepts: "diastole" describes the phase of relaxation when the heart muscle is filling with blood, "cardiac output" refers to the volume of blood the heart pumps per minute, and "arrhythmia" is used to describe any deviation from the normal heart rhythm. Understanding the phases of the cardiac cycle is essential for interpreting EKGs and assessing heart function.