Final answer:
In a healthy heart, atrial systole or the contraction of the atria occurs at the beginning of systole.
Step-by-step explanation:
In a healthy heart, the atrial contraction, also known as atrial systole, occurs at the beginning of systole. As the atrial muscles contract, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular valves. This contributes to approximately 20-30% of ventricular filling, with the remaining 70-80% occurring during diastole.