Sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node, sinus node or Keith–Flack node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximately 15 mm long, 3 mm wide, and 1 mm thick, located directly below and to the side of the superior vena cava.