The cardiac septa are the walls that divide the heart into its four chambers. They develop from different embryologic structures and have distinct anatomical features that are clinically relevant.
Interatrial Septum
The interatrial septum separates the right and left atria. It is oriented obliquely, running from right-posterior to left-anterior.
Anatomical Features
Septum Primum Component: The lower portion of the interatrial septum, derived from the embryologic septum primum.
Septum Secundum Component: The upper portion, derived from the embryologic septum secundum. It is thicker and more muscular.
Fossa Ovalis: An oval depression in the lower part of the septum, representing the remnant of the foramen ovale. It is the thinnest part of the interatrial septum.
Limbus of Fossa Ovalis (Annulus Ovalis): A prominent crescent-shaped rim that surrounds the fossa ovalis superiorly and anteriorly. It is formed by the free edge of the septum secundum.
Relations
- Right side: Covered by endocardium; the triangle of Koch (containing the AV node) is adjacent inferiorly
- Left side: Covered by endocardium; has a shallow depression corresponding to the fossa ovalis
Embryologic Development
The interatrial septum forms from two overlapping structures:
- Septum primum: Grows from the roof of the common atrium toward the endocardial cushions
- Septum secundum: Grows to the right of the septum primum, leaving an opening (foramen ovale)
The foramen ovale allows oxygenated blood from the placenta to bypass the fetal pulmonary circulation. After birth, increased left atrial pressure closes the foramen ovale, leaving the fossa ovalis as its remnant.
Interventricular Septum
The interventricular septum separates the right and left ventricles. It has two distinct components:
Muscular Interventricular Septum
The thick, muscular portion comprising the majority of the septum (approximately 90%).
Structure:
- Thicker on the left ventricular side (15-20 mm)
- Thinner on the right ventricular side (5-10 mm)
- Contains branching networks of myocardial fibers
Function:
- Contributes to left ventricular ejection
- Provides structural support for both ventricles
- Contains conducting tissue (left bundle branch on the left side)
Arterial Supply:
- Septal perforator branches from the LAD artery
- Posterior septal branches from the PDA
Venous Drainage:
- Anterior cardiac veins
- Thebesian veins
Membranous Interventricular Septum
A thin, fibrous portion located superiorly, just below the aortic valve.
Structure:
- Composed of fibrous tissue (no muscle)
- Continuous with the fibrous skeleton of the heart
- Small (approximately 1 cm in diameter)
Relations:
- Right side: Bounded by the septal leaflet of the tricuspid valve
- Left side: Lies beneath the right coronary and non-coronary cusps of the aortic valve
- Superior: Central fibrous body
- Anterior: Supraventricular crest
The membranous septum is the most common site for ventricular septal defects.
Embryologic Development
The interventricular septum forms from:
- Muscular ventricular septum: Grows from the floor of the common ventricle upward
- Membranous ventricular septum: Forms from the endocardial cushions and neural crest cells
Closure of the interventricular foramen completes the separation of the ventricles.
Clinical Significance
Atrial Septal Defects (ASD)
Types include:
- Ostium secundum ASD (most common, 75%): Defect in the region of the fossa ovalis
- Ostium primum ASD: Defect in the lower part of the septum, associated with AV canal defects
- Sinus venosus ASD: Defect near the superior vena cava orifice
- Coronary sinus ASD: Defect involving the coronary sinus
Ventricular Septal Defects (VSD)
Types include:
- Membranous VSD (most common, 80%): Defect in the membranous septum
- Muscular VSD: Can occur anywhere in the muscular septum (multiple defects possible)
- Outlet VSD: Beneath the aortic valve, associated with tetralogy of Fallot
- Inlet VSD: Near the tricuspid valve, associated with AV septal defects
Small VSDs may close spontaneously. Large defects cause left-to-right shunting, leading to pulmonary overcirculation and heart failure.