Final answer:
The Atrioventricular (AV) node is the site that slows down the electrical impulse in the heart to ensure the ventricles fill with blood before contracting.
Step-by-step explanation:
The site that slows down the electrical impulse in the heart to allow the ventricles to fill with blood is the Atrioventricular (AV) node. This node, a clump of specialized myocardial conductive cells, is located in the inferior portion of the right atrium within the atrioventricular septum. The AV node introduces a critical pause before depolarization and transmission of the electrical impulse to the atrioventricular bundle. This pause, attributed to the small diameter of the cells of the node and less efficient conduction between nodal cells, takes about 100 milliseconds and is essential for proper heart function. It ensures that the atria finish contracting and pump blood into the ventricles before the impulse activates the ventricular contraction.