Final answer:
The ventricles are completely depolarized during the QRS complex on the electrocardiogram (ECG), which is the phase preceding ventricular contraction.
Step-by-step explanation:
The ventricles are completely depolarized during the QRS complex portion of the electrocardiogram (ECG). This is when the ventricular contraction occurs due to the spread of the electrical signal throughout the ventricles, which is essential for the pumping of blood to the lungs and body. The QRS complex is indicative of the ventricles undergoing depolarization and precedes the ventricular contraction that pumps blood out of the heart.
A normal ECG tracing shows various peaks and waves associated with the cardiac cycle. The P wave corresponds to atrial depolarization, followed by atrial contraction. The T wave represents ventricular repolarization, marking the beginning of ventricular relaxation. During the QRS complex, not only are the ventricles depolarized but the repolarization of the atria also occurs, which is, however, masked on the ECG due to the dominance of the stronger ventricular depolarization signal.
The QRS complex represents the depolarization of the ventricles, which triggers their contraction. The large size of the ventricular cardiac muscle requires a stronger electrical signal, resulting in a distinctive QRS complex on the ECG.
Learn more about QRS complex