A Level Biology Practice Exam

During which phase of the cardiac cycle does the heart relax and the chambers fill with blood?

1. Systole

2. Diastole

3. Coronary

4. Atrial contraction

The correct answer is: Diastole

The phase of the cardiac cycle in which the heart relaxes and the chambers fill with blood is known as diastole. During diastole, the heart muscles are in a relaxed state, allowing the atria and ventricles to expand and fill with blood returning from the body and lungs. Blood flows passively into the atria and then into the ventricles due to the pressure differences, contributing to a continuous flow of blood throughout the heart chambers. In contrast, during systole, the heart is actively contracting to pump blood out of the ventricles into the arteries. Coronary refers to the blood supply to the heart itself rather than a phase of the cardiac cycle. Atrial contraction occurs at the end of diastole, where the atria contract to push the last bit of blood into the ventricles but does not characterize the overall filling phase of the cycle. Thus, diastole serves as the critical phase where relaxation and filling occur, making it the correct choice.