The statements below show some events in a cardiac cycle, but they are not arranged in the correct sequence. Which statement identifies the fourth event to happen during the cardiac cycle? 1. The impulse passes through Purkyne tissue. 2. A wave of excitation spreads across the atria. 3. The atrioventricular node holds back the impulse for a fraction of a second. 4. The sinoatrial node contracts. 5. The wave of excitation moves upwards from the base of the ventricles. 6. The ventricles contract. 7. The atria contract.
- A1
- B3
- C4
- D7