Final answer:
In an ECG, atrial contraction is represented by the P wave, which indicates the depolarization of the atria and precedes their contraction. The QRS complex and T wave represent ventricular depolarization and repolarization, respectively.
Step-by-step explanation:
In an ECG, atrial contraction is represented by the P wave. The P wave occurs as a result of the depolarization of the atria, which precedes atrial contraction. Approximately 25 ms after the start of the P wave, the atria begin to contract, pumping blood into the ventricles.
Subsequently, the large QRS complex represents the depolarization of the ventricles, which is followed by their contraction. The ventricles start contracting as the QRS peaks at the R wave. The T wave signifies ventricular repolarization and attributes to the ventricles starting their relaxation phase. The repolarization of the atria is not visible on an ECG because it is masked by the ventricular depolarization represented by the QRS complex.