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Diastole is the phase of the cardiac cycle during which the heart chambers, specifically the atria and ventricles, relax and fill with blood. This dilation allows the chambers to expand as they accommodate the blood returning from the body and lungs. During diastole, the heart is in a state of rest, enabling the chambers to accept and hold blood before the next contraction. This process is essential for maintaining an adequate blood supply to the body's tissues and overall cardiovascular function.

In contrast, systole refers to the phase of contraction where the heart pumps blood out of the chambers into the arteries. Arrhythmia is a term that describes irregular heartbeats, which does not directly relate to the dilation of heart chambers. A heart attack, or myocardial infarction, occurs when blood flow to a part of the heart is blocked, leading to damage of heart tissue, and is not a reflection of normal chamber activity or dilation. Thus, diastole is the phase that correctly represents the dilation of heart chambers.