Final answer:
The pacemaker of the heart is the Sinoatrial (SA) node due to its intrinsic depolarization rate that sets the heart's rhythm. The Atrioventricular (AV) node is the part of the cardiac conduction system that fires the slowest, allowing for a delay between atrial and ventricular contractions.
Step-by-step explanation:
The pacemaker of the heart is the sinoatrial (SA) node, a specialized clump of myocardial conducting cells. This node has the highest intrinsic rate of depolarization, initiating the sinus rhythm and thereby setting the pace for the entire heart's contraction cycle. The other parts of the cardiac conduction system—atrioventricular (AV) node, Bundle of His, and Purkinje fibers—follow the electrical impulses that originate from the SA node.
The atrioventricular (AV) node, on the other hand, is designed to slow down the electrical current before it passes onto the ventricles. This delay ensures that the atria have time to empty their blood into the ventricles before ventricular contraction occurs. Among the options given, the AV node would have the slowest rate of firing compared to the atrioventricular bundle, bundle branches, and Purkinje fibers, which serve to rapidly conduct the impulses through the ventricles thereafter.
In summary, the final answer to the question about the pacemaker of the heart is the Sinoatrial (SA) node (d). As for the component of the heart conduction system with the slowest rate of firing, it is the Atrioventricular (AV) node (a).