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Foramen ovale to left atrium and left ventricle

Heart & shunts · SaO2 ~65% R → L only

Flap-valve opening between the atria. The ductus venosus-enriched IVC stream is split by the crista dividens (free lower edge of septum secundum); most of it pushes septum primum aside and crosses into the left atrium, then the left ventricle and ascending aorta.

Traced from the start

  1. Placenta
  2. Umbilical vein
  3. Ductus venosus
  4. Foramen ovale to left atrium and left ventricle

Detail

Saturation
~65% in the left atrium and ventricle (PO2 ~25-28 mmHg) after mixing with the small pulmonary venous return
Why it is open
Right atrial pressure exceeds left atrial pressure because high pulmonary vascular resistance keeps pulmonary venous return, and so left atrial filling, low
Left ventricular share
~40-45% of combined ventricular output, almost all of it to coronaries, brain and upper limbs
Closure at birth
Functional within the first breaths: pulmonary venous return rises (left atrial pressure up) while cord clamping cuts IVC return (right atrial pressure down); septum primum fuses to septum secundum over the first year
Adult remnant
Fossa ovalis; probe-patent foramen ovale persists in ~25% of adults (hover to see)

When it goes wrong

Transposition of the great arteries at birth

Two parallel circuits; survival depends on mixing at the foramen ovale and ductus, so alprostadil (PGE1) is started and a Rashkind balloon atrial septostomy enlarges the atrial communication

Persistent pulmonary hypertension of the newborn (meconium aspiration, sepsis, diaphragmatic hernia)

Pulmonary resistance never falls, so right-to-left shunting continues across the foramen ovale and ductus: cyanosis that oxygen barely improves, treated with ventilation and inhaled nitric oxide

Premature narrowing of the foramen ovale in utero

Left heart underfilling contributes to hypoplastic left heart syndrome; right heart dilatation and hydrops

Practise this structure

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