The heart cycle
A resting adult's heart beats about 72 times a minute. One beat — about 0.8 s — is one heart cycle.
Atrial systole — 0.1 s
The atria contract and push blood into the ventricles.
Ventricular systole — 0.3 s
The ventricles contract and push blood into the arteries.
Diastole — 0.4 s
The whole heart relaxes and fills with blood.
The two sounds of a heartbeat, lub and dub, are the valves closing: first between the atria and ventricles, then at the arteries.
The ECG
An electrocardiogram records the electrical signals of the heart. The P wave goes with the atria contracting, the QRS complex with the ventricles contracting, and the T wave with the ventricles recovering.
Figure to be added