interventricular septum

The interventricular septum is the dividing wall between the heart's two lower chambers. Picture a duplex house with a solid central wall: it keeps the two pumping rooms — and their very different blood supplies — completely separate.

This muscular partition separates the right ventricle from the left ventricle, ensuring that oxygen-poor blood on the right does not mix with oxygen-rich blood on the left. Most of it is thick muscle (the muscular septum), while a small upper portion near the valves is thin and fibrous (the membranous septum).

The septum is not just a passive wall: it carries part of the heart's electrical wiring, including the bundle of His and the bundle branches that conduct each heartbeat to the ventricles. A hole in this wall, called a ventricular septal defect, is one of the most common congenital heart abnormalities.