Between the two upper chambers lies the thin wall called the

• A. interatrial septum
• B. interventricular septum
• C. atrial septum
• D. coronary groove

Answer: (A)

The wall between the two upper chambers is the interatrial septum, the thin tissue that separates the right atrium from the left atrium. It’s relatively delicate compared to the thick wall between the ventricles, the interventricular septum, which must handle higher pressures. Sometimes people say “atrial septum,” but the precise name for the wall between the atria is interatrial septum. The coronary groove, by contrast, is just a surface sulcus containing vessels and is not a septal wall.