Hi Nargis,

SA node (sinoatrial node) – known as the heart's natural pacemaker. The impulse starts in a small bundle of specialized cells located in the right atrium, called the SA node. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles.

The SA node is labelled 1.

Hope this will help,please feel free to ask if you have any further queries.

Thank You.