During the phase of the cardiac cycle called , the entire heart is relaxed and blood flows into all four chambers. 2. during the phase of the cardiac cycle called , the heart contracts beginning with the atria and then the ventricles. 3. when the ventricles contract, the snap closed. 4. a defective heart valve can be identified by the sound of a(n) . 5. the , or pacemaker, initiates the heartbeat by generating electrical impulses that spread across both atria. 6. impulses are delayed briefly at the and are then transmitted through special fibers to the base of the ventricles. strong contractions begin at the apex of the heart and drive blood up and out of the heart. 7. a(n) records the electrical changes that occur during the heartbeat and provides data on the health of the heart.