What separates the right and left atria?

• A. Interventricular septum
• B. Interatrial septum
• C. Fossa ovalis
• D. Pectinate muscle

Answer: (B)

The separation between the right and left atria is the interatrial septum. This wall forms the boundary that divides the two atrial chambers. During fetal development, parts of this septum come together to close the opening called the foramen ovale, which allows blood to pass between atria. After birth, this opening normally closes, leaving a small depression on the septum called the fossa ovalis. The fossa ovalis is a marker of where the foramen ovale used to be, not the structure that actually separates the atria. The other features mentioned don’t bisect the atria: the interventricular septum sits between the ventricles, pectinate muscles are muscular ridges in the atrial walls, and the fossa ovalis is just a remnant marker on the interatrial septum.