Cardiovascular

The interatrial septum and the foramen ovale

6 steps · 3 anomalies · Weeks 4 to 5 to At birth

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Students find this one impossible because they try to picture two septa at once. Do it in order instead. One thin septum grows down and gets a hole in it; a second thick septum grows down beside it and leaves a gap of its own; the two holes overlap so blood can cross before birth, and the flap slams shut when the baby takes its first breath. Say it in that order and it makes sense every time.

Weeks 4 to 5

Step 1 of 6

What happens: A thin crescent, the septum primum, grows down from the roof of the common atrium towards the endocardial cushions.
What it forms: A gap below its free edge — the ostium primum.
What remains in the adult: The septum primum becomes the valve of the foramen ovale.

Nothing in this set goes wrong at this step — it is here because the story does not make sense without it.

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Senior’s 30-second takeaway

The interatrial septum and the foramen ovale — the whole bench in four lines. Read this on the way in.

If you see this: A question naming septum primum or secundum · A fixed, widely split second heart sound · A low atrial defect with a valve problem
If asked to identify it: Septum primum grows down and gets a hole; septum secundum grows down on its right and leaves the foramen ovale.
If asked one more: The flap opens leftwards, so blood can only cross from right to left before birth.
If asked why it matters: A probe patent foramen ovale persists in about a quarter of adults and can allow a paradoxical embolus.

Then stop. Let the examiner ask the next question.

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Senior’s 30-second takeaway

Read this first