Which structure is known as the heart's natural pacemaker?

The sinoatrial (SA) node is known as the heart's natural pacemaker because it is responsible for initiating the electrical impulses that regulate the heartbeat. This small group of cells, located in the right atrium of the heart, generates electrical signals that spread through the heart muscle, causing it to contract and pump blood.

The SA node has the highest intrinsic rate of depolarization, meaning it can stimulate heartbeats more frequently than other components of the conduction system. This property allows it to set the pace for the whole heart, ensuring that it beats in a coordinated manner and maintains a regular rhythm.

In contrast, the atrioventricular (AV) node and other structures like the Bundle of His and Purkinje fibers play supporting roles in the conduction system. The AV node acts as a relay station between the atria and ventricles, allowing for a slight delay in the electrical signal to ensure the atria contract fully before the ventricles do. The Bundle of His and Purkinje fibers are involved in conducting the impulse through the ventricles. However, they do not initiate the heartbeat themselves; their activity is driven by the signals from the SA node. Thus, the correct identification of the SA node as the natural pacemaker