# Septal Development of the Heart > Cardiac septation for MBBS Anatomy: septum primum and secundum, foramen ovale, endocardial cushions, muscular and membranous interventricular septum, ASD VSD. - Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/septal-cardiac-development - Exam / course: MBBS · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Septal Development of the Heart", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/septal-cardiac-development ## Direct answer The four-chambered heart is built by walls that grow, perforate and overlap. In the common atrium the septum primum grows down toward the endocardial cushions, its leading edge closing the ostium primum while perforations near its upper edge coalesce into the ostium secundum; a stiffer septum secundum then grows on the atrium's right side, never completing, its free edge leaving the foramen ovale. The septum primum becomes the flap valve: fetal right-to-left flow pushes it open, and at birth the rising left atrial pressure slams it shut against the septum secundum. Below, the muscular interventricular septum grows up from the apex, its last gap closed by the membranous septum of cushion and conotruncal tissue by about the end of week seven. ## What you must remember - **Atrial sequence:** septum primum with ostium primum (closed by fusion with endocardial cushions) and ostium secundum (perforations in the primum's upper part); septum secundum with the foramen ovale — the secundum is muscular and incomplete, the primum the thin valve flap. - **Foramen ovale mechanics:** one-way fetal shunt (right atrium to left, bypassing lungs); after birth pulmonary venous return raises left atrial pressure and closes the flap — anatomical fusion follows in months to years; probe patency persists in about a quarter of adults. - **Endocardial cushions:** superior and inferior AV canal cushions fuse during the fifth week, dividing the atrioventricular canal into right and left atrioventricular openings, contributing the AV portion of the membranous septum and the valve leaflets' core. - **Ventricular septation:** the muscular septum grows from the floor of the ventricle; the remaining interventricular foramen closes by the membranous septum (endocardial cushion plus proximal conotruncal swellings) around the end of week seven. - **Commonest defects:** ostium secundum ASD (fossa ovalis region) is the commonest atrial defect; membranous VSD the commonest ventricular one; ostium primum defects sit with AV cushion defects and cleft mitral leaflet. - **Syndrome link:** atrioventricular septal defects are over-represented in Down syndrome, a standard clinical correlation in Indian paediatric postings. - **Postnatal consequence:** shunt direction depends on resistance — left-to-right initially (Eisenmenger progression can reverse it), which is why large ASDs and VSDs are closed before pulmonary vascular disease is fixed. ## Explaining a murmur by its embryology A child with a systolic murmur at the left sternal edge and a hyperdynamic right ventricle most often carries a membranous VSD — small ones often close spontaneously as tricuspid tissue seals them, so cardiologists watch rather than rush. Shift the story to a young woman with fixed splitting of the second sound and an exercise intolerance: secundum ASD, the ostium secundum made too large or the septum primum flap deficient, shunting left-to-right at atrial level for decades before atrial arrhythmia or paradoxical embolism betrays it; device or surgical closure normalises the prognosis. The cushion defects complete the picture. An infant with Down syndrome, a single AV valve on echocardiography and a common atrioventricular canal has a complete AV septal defect — the cushions never fused, leaving primum ASD, inlet VSD and a cleft mitral valve in one anatomy. The embryology predicts the repair (patch plus valve reconstruction) and the counselling — AVSD is the classical cardiac lesion of trisomy 21. Primum ASDs sit between the two: isolated, still cushion-derived, still with the mitral cleft. ## Where students slip The recurring error is placing the ostium secundum in the septum secundum — it is an upper perforation of the septum primum; the foramen ovale is the secundum's incomplete edge. Reversing them makes the valve mechanism unexplainable. The second slip is assigning the membranous septum to muscle: it is cushion and conotruncal tissue, which is precisely why outflow-ridge anomalies (and 22q11 deletions) drag membranous VSDs with them. The third is calling every patent foramen ovale a septal defect — probe patency is a normal variant in a quarter of adults, an ASD only when the flap fails to cover the defect with functional shunting; examiners distinguish these deliberately. ## Frequently asked questions ### What is the difference between the ostium secundum and the foramen ovale? The ostium secundum is the upper perforation in the septum primum; the foramen ovale is the gap left by the incomplete free edge of the septum secundum, covered by the primum flap. ### Why does the foramen ovale close at birth? Lung expansion suddenly increases pulmonary venous return and left atrial pressure, pushing the septum primum flap against the septum secundum and abolishing the shunt. ### Which septal defect is associated with Down syndrome? Atrioventricular septal (canal) defect from failed endocardial cushion fusion — primum ASD, inlet VSD and a common or cleft AV valve. ### From which tissues does the membranous septum form? From fused endocardial cushion tissue and the proximal conotruncal (bulbar) ridges, closing the interventricular foramen by about the end of the seventh week. ### Why can a small membranous VSD close on its own? Neighbouring tricuspid valve tissue can adhere to and seal the defect during infancy, a frequent outcome that paediatric cardiology exploits with watchful follow-up.