Which of the following structures forms the anatomical boundary between the right and left atria on the internal surface of the heart?

Explanation

## Interatrial Septum: Anatomical Boundary ### Definition The **interatrial septum** is the muscular wall that separates the right and left atria internally. It forms the partition between the two atrial chambers. ### Key Anatomical Features | Structure | Location | Function | |-----------|----------|----------| | Interatrial septum | Between right and left atria | Separates atrial chambers | | Fossa ovalis | Depression in interatrial septum | Remnant of foramen ovale | | Crista terminalis | Right atrial wall | Landmark between smooth and trabeculated regions | | Coronary sinus | Right atrium | Venous drainage into right atrium | **Key Point:** The interatrial septum is the direct anatomical boundary; the fossa ovalis is a depression *within* the septum, not the boundary itself. ### Clinical Significance **High-Yield:** The fossa ovalis is the thinnest part of the interatrial septum and is a common site for: - Patent foramen ovale (PFO) - Atrial septal defect (ASD) — ostium secundum type - Transseptal catheterization during cardiac procedures **Clinical Pearl:** During fetal life, the foramen ovale allows right-to-left shunting of blood, bypassing the non-functional lungs. After birth, the septum primum acts as a valve; if it fails to fuse completely, PFO results. ### Related Structures - **Crista terminalis**: A muscular ridge on the lateral wall of the right atrium; marks the junction between the smooth posterior wall (sinus venarum) and the trabeculated anterior wall (atrial appendage). - **Coronary sinus**: The venous drainage channel that opens into the right atrium, not a septal structure. [cite:Gray's Anatomy 42e Ch 8]