Sino-atrial node is called the pacemaker of our heart. Why?

The Sino – artrial node is a patch of nodal tissue present in the right upper corner of the right atrium. These nodal tissues have the ability to generate action potential without any external stimuli. The maximum action potential can be generated by SAN which is around 70-75 mm – and this action potential is responsible for initiating and maintaining the rhythmic contraction of the heart. Due to this ability of the SAN, it is called as the pace maker of the heart.