What is the phase of the heart called when it is relaxed and filling with blood?

The phase of the heart referred to when it is relaxed and filling with blood is known as diastole. During diastole, the heart chambers—specifically the atria and ventricles—relax, allowing blood to flow into them from the veins. This phase is crucial for ensuring that the heart is adequately filled with blood before it contracts during the next phase, which is systole.

Diastole encompasses several important processes: the atrioventricular valves (the mitral and tricuspid valves) open, allowing blood to fill the ventricles, and the semilunar valves remain closed, preventing blood from flowing back into the heart from the arteries. The relaxation of the heart muscle during diastole is essential for the heart's function, as it prepares the heart to pump a sufficient volume of blood to the body during systole, when the heart contracts.

The other options do not accurately describe this phase. Systole refers to the contraction phase when the heart pumps blood out. Contraction is a term that generally describes the action of the heart during systole. The term "resting phase" may colloquially refer to the relaxation period, but it is not the correct technical term used in cardiology.