Fetal Circulation
High-Yield Summary
- Placenta exchanges gas/nutrients/waste by diffusion without mixing maternal and fetal blood; also an immune barrier and endocrine organ.
- Fetal hemoglobin (HbF) has higher O₂ affinity than adult hemoglobin, extracting oxygen efficiently from maternal blood.
- Umbilical arteries carry deoxygenated blood away from fetus (toward placenta); umbilical vein carries oxygenated blood to the fetus — reverse of typical artery/vein oxygenation.
- 3 shunts bypass non-functional fetal lungs/liver: foramen ovale (RA→LA, bypasses lungs), ductus arteriosus (pulmonary artery→aorta, bypasses lungs), ductus venosus (umbilical vein→IVC, bypasses liver).
Key Terms
- Fetal hemoglobin (HbF)
- Higher O₂ affinity than adult hemoglobin; pulls oxygen across the placenta more effectively.
- Foramen ovale
- Opening between right and left atria; shunts blood past the lungs.
- Ductus arteriosus
- Connects pulmonary artery to aorta; reroutes blood past the lungs.
- Ductus venosus
- Lets oxygenated umbilical vein blood bypass the liver, flowing into the inferior vena cava.
Umbilical Vessels
| Vessel | Carries |
|---|---|
| Umbilical arteries (2) | Deoxygenated blood, fetus → placenta |
| Umbilical vein (1) | Oxygenated blood, placenta → fetus |
The Three Fetal Shunts
| Shunt | Path / Bypasses |
|---|---|
| Foramen ovale | Right atrium → left atrium; bypasses lungs |
| Ductus arteriosus | Pulmonary artery → aorta; bypasses lungs |
| Ductus venosus | Umbilical vein → inferior vena cava; bypasses liver |
Common MCAT Trap
- Umbilical vessels are named by direction (to/from fetus), NOT by oxygenation — umbilical vein carries oxygenated blood, umbilical arteries carry deoxygenated blood (opposite of systemic convention).
- Two shunts bypass the lungs (foramen ovale, ductus arteriosus) but only one bypasses the liver (ductus venosus) — don't mix up which organ each one skips.
Quick Recall
Which fetal shunt allows blood to bypass the liver?
Why does the umbilical vein carry oxygenated blood, unlike typical veins?
Why does fetal hemoglobin help the fetus, functionally?