Final answer:
During atrial contraction, also known as atrial systole, the atrial muscles contract from the superior portion of the atria towards the atrioventricular septum, increasing pressure within the atria and pumping blood into the ventricles.
Step-by-step explanation:
Atrial contraction, also known as atrial systole, occurs after depolarization of the atrial muscles, represented by the P wave of the ECG. During atrial systole, the atrial muscles contract from the superior portion of the atria towards the atrioventricular septum, increasing the pressure within the atria. This pressure forces blood to be pumped into the ventricles through the open atrioventricular valves, contributing to the filling of the ventricles. Atrial systole lasts about 100 milliseconds and ends before ventricular systole, as the atrial muscles return to diastole.