What is the primary event of diastole?

• A. Atria contract
• B. Ventricles relax
• C. Systole occurs
• D. Valves close

Answer: (B)

During diastole, the heart muscle relaxes, especially the ventricles. This relaxation lowers ventricular pressure, allowing the atrioventricular valves to open and blood to flow from the atria into the ventricles, which is how filling occurs. Atrial contraction can help top off filling near the end of diastole, but the defining event is ventricular relaxation and filling. The other options describe actions not characteristic of the diastolic phase: atrial contraction is a later part of diastole but not the main event, ventricles contracting is systole, and valve closure relates to pressure changes at transitions between phases rather than the primary diastolic event.