During which phase of the heart cycle are the atria and ventricles relaxed?

The correct response refers to diastole, which is the phase of the heart cycle when both the atria and ventricles are in a relaxed state. During diastole, the heart chambers fill with blood as the pressure in the heart decreases, allowing blood to flow from the veins into the atria and then into the ventricles. This relaxation is crucial for the proper functioning of the heart, as it maximizes blood volume in the chambers and prepares the heart for the next contraction, which occurs during systole.

In contrast, systole refers to the phase when the heart muscles contract to pump blood out of the ventricles and into the arteries, resulting in the opposite state of relaxation. Atrial contraction specifically describes the period when the atria contract to push blood into the ventricles, which again does not correspond to the overall state of relaxation in both atria and ventricles. Cardiac output is a measure of the amount of blood the heart pumps in a given time, but it does not refer to a specific phase of the heart cycle. Thus, diastole is the appropriate choice for this question.