[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Heart and vessels](https://durgical.com/en/embryology/#a-heart) › Atrial septation: septum primum, septum secundum and foramen ovale [Embryology](https://durgical.com/en/embryology/) · [Heart and vessels](https://durgical.com/en/embryology/#a-heart) # Atrial septation: septum primum, septum secundum and foramen ovale - Process Two overlapping walls divide the atrium and leave a door, the foramen ovale, so blood can cross to the left before birth. Updated 2026-11-01 **See it in 3D:** [Interatrial septum](https://durgical.com/en/anatomy/interatrial-septum/) · [Right atrium](https://durgical.com/en/anatomy/right-atrium/) · [Left atrium](https://durgical.com/en/anatomy/left-atrium/) · [Pulmonary veins](https://durgical.com/en/anatomy/pulmonary-veins/) · [Coronary sinus](https://durgical.com/en/anatomy/coronary-sinus/) · [Superior vena cava](https://durgical.com/en/anatomy/superior-vena-cava/) · [Inferior vena cava](https://durgical.com/en/anatomy/inferior-vena-cava/) ## Key points - Septum primum grows toward the endocardial cushions; the gap it leaves is the ostium primum. - Before the ostium primum closes, cell death opens the ostium secundum in the upper septum primum. - Septum secundum grows to the right of the primum and leaves the foramen ovale open. - The septum primum is the valve of the foramen ovale; it closes by pressure at birth and later fuses (fossa ovalis). - Secundum ASD is the most common (excess resorption of the primum or a short secundum). A patent foramen ovale persists in about one in four adults. ## What it is Between weeks 4 and 6 the primitive atrium is divided by two septa: the thin septum primum and the thick muscular septum secundum to its right. Between them lies the foramen ovale, a one-way valve that lets blood pass right to left during fetal life. At birth left atrial pressure rises and closes the valve; errors in this process give atrial septal defects. ## Key facts - End of week 4: the septum primum grows down from the atrial roof. - Week 5: the ostium secundum opens; the ostium primum closes as the septum primum fuses with the cushions. - Weeks 5–6: the septum secundum grows but never completes: its free edge rims the foramen ovale. - At birth: pulmonary resistance falls, left atrial pressure rises and the septum primum is pressed against the secundum. - Anatomic fusion happens over the first months to years; if it does not, a patent foramen ovale remains. ## Structure - Septum primum: thin membrane that forms the floor of the fossa ovalis. - Septum secundum: thick muscular fold that forms the limbus (rim) of the fossa ovalis. - Foramen ovale: the oblique passage between the two septa. - The sinus venosus is absorbed into the right atrium and the pulmonary veins into the left atrial wall. ## Step by step - The septum primum grows down from the atrial roof toward the endocardial cushions. - The space between its edge and the cushions is the ostium primum. - Perforations in the upper septum primum merge into the ostium secundum. - The septum primum fuses with the cushions and closes the ostium primum. - The septum secundum grows to its right, partly covering the ostium secundum and leaving the foramen ovale. - In the fetus, blood from the inferior vena cava pushes the valve open and enters the left atrium. - At birth, higher left atrial pressure shuts the valve; fusion follows later. ## Markers - TBX5 - NKX2-5 - GATA4 ## Clinical notes - Secundum ASD: the most common; pulmonary flow murmur and a fixed split second heart sound. - Primum ASD: part of the atrioventricular canal (cushion) defect spectrum. - Sinus venosus ASD: near the superior vena cava, often with anomalous drainage of right pulmonary veins. - A patent foramen ovale allows paradoxical embolism: a venous clot crosses to the left side and causes a stroke. ## Memory aid The primum is the curtain and the secundum is the frame: as long as the curtain swings only to the left, the fetus breathes through the placenta. ## Test yourself Think of the answer, then tap the question to see it. ## How does the ostium secundum form? By programmed cell death in the upper septum primum, before the ostium primum closes. ## Which structure acts as the valve of the foramen ovale? The septum primum, which closes against the septum secundum when left atrial pressure rises at birth. ## What is the most common type of atrial septal defect and its cause? Secundum ASD: excessive resorption of the septum primum or underdevelopment of the septum secundum. ## Why can a patent foramen ovale cause a stroke? It lets a venous clot pass right to left (paradoxical embolism), especially when right atrial pressure rises. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/atrial-septation-septum-primum-septum-secundum-and-foramen-ovale/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Interatrial septum](https://durgical.com/en/anatomy/interatrial-septum/) - [Right atrium](https://durgical.com/en/anatomy/right-atrium/) - [Left atrium](https://durgical.com/en/anatomy/left-atrium/) - [Pulmonary veins](https://durgical.com/en/anatomy/pulmonary-veins/) - [Coronary sinus](https://durgical.com/en/anatomy/coronary-sinus/) - [Superior vena cava](https://durgical.com/en/anatomy/superior-vena-cava/) - [Inferior vena cava](https://durgical.com/en/anatomy/inferior-vena-cava/) ## Related - [Atrial septal defect and patent foramen ovale](https://durgical.com/en/embryology/atrial-septal-defect-and-patent-foramen-ovale/) - [Atrioventricular septal defect (endocardial cushion defect)](https://durgical.com/en/embryology/atrioventricular-septal-defect/) - [Ventricular septation and the endocardial cushions](https://durgical.com/en/embryology/ventricular-septation-and-the-endocardial-cushions/) - [Embryonic heart structures and their adult derivatives (the table to know)](https://durgical.com/en/embryology/embryonic-heart-structures-and-their-adult-derivatives/) - [Fetal circulation and the changes at birth](https://durgical.com/en/embryology/fetal-circulation-and-the-changes-at-birth/) - [Heart tube: formation and looping](https://durgical.com/en/embryology/heart-tube-formation-and-looping/) ## Related conditions - [Atrial septal defect](https://durgical.com/en/conditions/atrial-septal-defect/) - [Ischaemic stroke](https://durgical.com/en/conditions/ischaemic-stroke/) [← All of embryology](https://durgical.com/en/embryology/) Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given. Sources: Langman's Medical Embryology, 13th ed., ch. 13: Cardiovascular System (Wolters Kluwer, 2015); [Atrial Septal Defect (ASD) | Congenital Heart Defects (CHDs) | CDC](https://www.cdc.gov/heart-defects/about/atrial-septal-defect.html) (Centers for Disease Control and Prevention, 2024). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/atrial-septation-septum-primum-septum-secundum-and-foramen-ovale/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/atrial-septation-septum-primum-septum-secundum-and-foramen-ovale/