The structure indicated is the posterior interventricular artery (posterior descending artery) of the heart.

The posterior interventricular artery determines the “dominance” of the coronary blood supply to the heart. For example, if the posterior interventricular artery arises from the right coronary artery, the heart is said to be “right coronary dominant”. In the majority of people, the posterior interventricular artery will usually arise from the right coronary artery. If the posterior interventricular artery arises from the left coronary artery, it will branch from the circumflex coronary artery.

Co-dominance occurs when the posterior interventricular artery arises from an anastomosis between the left and right coronary arteries.

The posterior interventricular artery runs in the posterior interventricular sulcus and supplies the posterior third of the interventricular septum (the rest is supplied by the anterior interventricular artery)

Learn more about the blood supply to the heart in this anatomy tutorial.