Final answer:
The QRS complex on an ECG indicates the depolarization of the ventricles, preceding the contraction of the ventricles known as ventricular systole.
Step-by-step explanation:
The large wave that results from the depolarization of the ventricles and precedes the contraction of the ventricles is the QRS complex on an electrocardiogram (ECG). The QRS complex is essential in the cardiac cycle as it represents ventricular depolarization, an electrical event that triggers the subsequent muscular contraction, which is the ventricular systole.
During ventricular systole, the ventricles contract to push blood to the lungs and the rest of the body, marking the peak of arterial blood pressure. Importantly, the QRS complex consists of three distinct points - Q, R, and S - which together indicate this critical phase of heart function. The depolarization begins after the end of the atrial contraction represented by the P wave, and it occurs just before the ventricles contract, indicating the heart is ready to pump blood.