The Cardiac Septum: Anatomy and Function of the Heart's Dividing Walls

The septum (plural: septa) is the muscular wall dividing the heart into left and right sides, preventing mixing of oxygenated and deoxygenated blood.

Septal Structures

1. Interatrial Septum

• Separates right and left atria.
• Fossa ovalis: Remnant of fetal foramen ovale (allows right-to-left shunting in utero). Normally closes after birth.
• Patent Foramen Ovale (PFO): Persistent flap-like opening (~25% of adults). Usually asymptomatic; paradoxical emboli risk in cryptogenic stroke.
• Atrial Septal Defect (ASD): True defect (ostium secundum, primum, sinus venosus). Left-to-right shunt → RV volume overload.

2. Interventricular Septum

• Separates right and left ventricles.
• Thick, muscular (except membranous portion near aortic valve).
• Ventricular Septal Defect (VSD): Most common congenital heart defect. Left-to-right shunt → pulmonary overcirculation, Eisenmenger if untreated.

3. Atrioventricular (AV) Septum

• Separates atria from ventricles; contains AV valves (tricuspid, mitral) and conduction system (AV node, His bundle).
• AV Septal Defect (AVSD): Common in Down syndrome.

Functional Significance

• Complete separation of systemic (high pressure, oxygenated) and pulmonary (low pressure, deoxygenated) circulations.
• Enables dual-circuit circulation essential for high-metabolic-rate endothermy.
• Conduction system embedded in septum ensures coordinated contraction.

Clinical Correlates

• Hypertrophic Cardiomyopathy: Asymmetric septal hypertrophy → LVOT obstruction.
• Septal Infarction: LAD occlusion → septal Q waves, bundle branch blocks.
• Cardiac Surgery: Septal myectomy (HOCM), defect closure (device/surgical).

The septum is not just a wall—it's a dynamic structure integrating structure, conduction, and hemodynamics.