Why is the SA node considered the primary pacemaker of the heart?

The SA node is considered the primary pacemaker of the heart primarily because it has the highest intrinsic rate of spontaneous depolarization compared to any other cardiac tissue. This intrinsic property allows the SA node to generate electrical impulses at a frequency that dictates the overall rhythm of the heart, typically around 60 to 100 beats per minute in a healthy adult at rest.

The SA node's location in the right atrium is crucial, as it is strategically positioned to initiate the electrical impulse that spreads through the atria and subsequently activates the ventricles via the AV node. While it's important for other cardiac tissues, such as the AV node and Purkinje fibers, to have intrinsic rates of their own, these rates are slower than that of the SA node. Thus, if the SA node functions effectively, it overrides any competing pacemaker activity from the other cardiac tissues, solidifying its role as the heart's primary pacemaker.