PATHOLOGY

Detailed explanation-1: -The electrical depolarization results in simultaneous contraction of the atria, thus forcing any residual blood from the upper chambers into the lower chambers of the heart. The atrial contraction causes a further increase in atrial pressures.

Detailed explanation-2: -Every cardiac cycle (heartbeat) begins with the sinus node sending an electrical signal across both atria through specialized conduction tracts, which causes the atrial muscle to depolarize and contract.

Detailed explanation-3: -The P wave of the ECG represents atrial depolarization, which is followed by contraction and an increase in pressure in the atria (atrial systole). The AV valves are open, and there is no valve between the atria and veins, so this small increase in pressure is also evident in the ventricle (a wave) and in the veins.