SN1 SHORT NOTE 5M ★ High-yield

Describe the fetal circulation and the changes that occur at birth.

Anatomy · 1st Professional — Pre-Clinical

The fetal circulation is adapted to a placenta that oxygenates and lungs that do not, and depends on three shunts.

Shunt	Connects	Purpose
Ductus venosus	Umbilical vein to the inferior vena cava	Bypasses the liver, carrying about half the oxygenated blood straight to the heart
Foramen ovale	Right atrium to left atrium	Bypasses the lungs; directs the best-oxygenated blood to the left side and so to the brain and heart
Ductus arteriosus	Pulmonary trunk to the descending aorta	Bypasses the lungs; carries about 90% of right ventricular output

Pulmonary vascular resistance is high before birth because the lungs are collapsed and fluid-filled, so blood takes the path of least resistance through the shunts
Only about 10% of the cardiac output goes to the lungs
The most oxygenated blood is in the umbilical vein; oxygen saturation falls progressively as the streams mix

Fetal structure	Adult remnant	Mechanism of closure
Umbilical vein	Ligamentum teres (in the falciform ligament)	Cessation of placental flow
Ductus venosus	Ligamentum venosum	Closes within days
Foramen ovale	Fossa ovalis	Left atrial pressure exceeds right, pressing the flap shut; anatomical fusion over months
Ductus arteriosus	Ligamentum arteriosum	Functional closure in 10–15 hours by a rise in oxygen tension and a fall in prostaglandins
Umbilical arteries	Medial umbilical ligaments (proximal parts remain as superior vesical arteries)	Contraction of smooth muscle

Patent ductus arteriosus — common in prematurity and after congenital rubella; gives a continuous "machinery" murmur and a left-to-right shunt. Closed with indomethacin or ibuprofen, which inhibit prostaglandin synthesis
Prostaglandin E is given to keep the duct open in duct-dependent lesions such as transposition or severe coarctation, until surgery can be done — the same pharmacology used in the opposite direction
Patent foramen ovale — probe-patent in about 25% of adults; may permit paradoxical embolism and is implicated in cryptogenic stroke
Persistent pulmonary hypertension of the newborn — pulmonary resistance fails to fall, so the fetal shunts persist and the infant remains cyanosed
Umbilical vein catheterisation is possible for several days after birth and is used for emergency access in neonatal resuscitation

All questions in Systemic Embryology