close. Closure of these valves give the second heart sound (S2). The ventricles then start to relax, the mitral and tricuspid valves begin to open, and the cycle begins again.

In summary, when the ventricles are in systole and contracting, the atria are relaxed and collecting returning blood. When, in late diastole, the ventricles become fully dilated (understood in imaging as LVEDV and RVEDV), the atria begin to contract, pumping blood to the ventricles. The atria feed a steady supply of blood