Final answer:
The right and left atria contract after receiving an action potential from the sinoatrial (SA) node, which initiates the depolarization and subsequent contraction of these chambers.
Step-by-step explanation:
The right and left atria depolarize and contract following the arrival of the action potential from the sinoatrial (SA) node.
The process begins with the SA node generating an action potential, which then spreads across the atria causing them to contract. This is known as atrial systole. The electrical impulse then travels to the atrioventricular (AV) node, where there is a brief pause to allow the atria to finish contracting and effectively pump blood into the ventricles. After the delay, the impulse continues down the bundle of His, through the right and left bundle branches, and finally spreads throughout the ventricles via the Purkinje fibers, resulting in ventricular contraction. The delay at the AV node ensures that the atria and ventricles contract in a coordinated manner, optimizing the pumping efficiency of the heart.