Heart
The ECG shows atrial systole. This is called a P-wave. Then ventricular systole happens. This is called the QRS or QRS-complex. It is called a complex because there are three different waves in it. The Q-wave, R-wave, and S-wave. Then the ECG shows ventricular diastole. This is called the T-wave. Atrial diastole happens then too. But it is not seen separate from ventricular diastole.