Final answer:
Ventricular repolarization shows up as a positive deflection on an ECG due to the direction of the voltage change relative to the ECG electrodes. The lead, like lead II, records a positive deflection as the wave of repolarization moves towards its positive electrode, while cells regain their negative charge.
Step-by-step explanation:
The T wave on an electrocardiogram (ECG) represents the repolarization of the ventricles. Ventricular repolarization appears as a positive deflection in an ECG because of the direction of the voltage change relative to the recording electrodes. During repolarization, the cells inside the ventricles regain their negative charge, but the way an ECG lead 'sees' this activity depends on the vectors of the changing electrical fields in relation to its position. Lead II, for example, is positioned in such a way that the wave of repolarization moving through the ventricles towards the positive electrode causes a positive deflection on the ECG. The detailed vector mechanics and the orientation of the heart can explain the specific positive deflection of the T wave observed during ventricular repolarization.