Final answer:
The first part of the cardiac cycle is atrial depolarization, represented by the P wave on an ECG, leading to atrial contraction or atrial systole. It's critical for filling the ventricles with blood before their own contraction.
Step-by-step explanation:
The first part of the cardiac cycle is called atrial depolarization. This phase is represented by the P wave on an electrocardiogram (ECG), which is followed by atrial contraction, also referred to as atrial systole. The cardiac cycle also includes ventricular depolarization, which corresponds to the QRS complex on an ECG and is followed by ventricular contraction (ventricular systole). Additionally, ventricular repolarization is represented by the T wave, which precedes ventricular relaxation or diastole.
Specifically, during atrial depolarization, the atria contract to push blood into the ventricles. This phase of the cardiac cycle is critical because it ensures that the ventricles are filled with blood before they contract themselves. Therefore, the correct answer to the question is option (a) atrial depolarization.