HEART through the lense of ECG graph

How does the electric spark translates into a heartbeat? What information does an ECG graph actually show us? 00:00 - Introduction 01:09 - Heart 03:17 - Why Atrium Exists? 05:43 - Ventricles 09:30 - Electric Spark 14:07 - Excitation-Contraction Coupling 15:00 - ECG Sequence 16:00 - Main Part 34:25 - Master Summary Table 36:25 - Time Required 37:47 - Cardiac Output 38:56 - Question