Ductus venosus, foramen ovale, ductus arteriosus
Mnemonic
Three shunts, each bypassing an organ that is not yet working:
- Ductus venosus - bypasses the liver, taking oxygenated blood from the umbilical vein to the inferior vena cava. Becomes the ligamentum venosum
- Foramen ovale - bypasses the right ventricle and lungs, from right atrium to left. Becomes the fossa ovalis
- Ductus arteriosus - bypasses the lungs, from pulmonary trunk to aorta. Becomes the ligamentum arteriosum
“Venosus misses the liver, ovale and arteriosus miss the lungs.”
The umbilical vein carries oxygenated blood and the umbilical arteries carry deoxygenated blood, which is the reverse of the usual rule and a reliable examination question.
Expansion
- Ductus venosus - bypasses the liver, carrying oxygenated blood from the umbilical vein to the inferior vena cava
- Foramen ovale - bypasses the lungs, shunting blood from the right atrium to the left atrium
- Ductus arteriosus - bypasses the lungs again, connecting the pulmonary trunk to the aorta
The organising principle is that the placenta is the fetal lung and the fetal liver is largely bypassed, so blood is repeatedly diverted away from the pulmonary and hepatic circulations towards the systemic side.
At birth, the first breath drops pulmonary vascular resistance and raises left atrial pressure, which closes the foramen ovale, while rising oxygen tension and falling prostaglandins close the ductus arteriosus.