Final answer:
The electrical impulse from the SA node first spreads to the atria then to the AV node, through the bundle of His, and down to the Purkinje fibers, eventually causing the ventricles to contract. The AV node has the slowest firing rate in the heart's conduction system.
Step-by-step explanation:
The electrical impulse governing heart rate and rhythm begins at the sinoatrial (SA) node, which is the heart's primary pacemaker located in the right atrium. From there, it spreads across the atria causing them to contract and then reaches the atrioventricular (AV) node.
The impulse pauses at the AV node for about 0.1 second, allowing the atria to fully empty their blood into the ventricles. After this pause, the impulse enters the bundle of His, moves down the right and left bundle branches, and finally spreads to the Purkinje fibers.
This network orchestrates the contraction of the ventricles, ensuring efficient blood pumping throughout the body. The correct answer for the spread of the impulse from the SA node is: A. Atria.
Regarding the component of the heart conduction system with the slowest rate of firing, the answer is a. atrioventricular node. This is due to the critical pause it introduces, allowing time for the atria to complete their contraction and effectively pump blood into the ventricles before the ventricular contraction occurs.