Hello!! The answer would be A - The SA node. It is known as the "pacemaker" because it initiates the heart's rhythmic contractions. I hope I helped!! Have a great day. :)
The heart beat is triggered by electrical impulses and among them, the sinoatrial node (SA node) is the one that receives the name of the “pacemaker” of the heart. The impulse of the SA node begins with a bundle in the right atrium, which spreads through the walls of the atria and causes a contraction. The SA node is the principal factor that affects the rate and rhythm of the heartbeat.
This atomic model has changed over time. Scientists used the model to make predictions. Sometimes the results of their experiments were a surprise and they did not fit with the existing model. Scientists changed the model so that it could explain the new evidence.
