Blood in the umbilical vein has a Po₂ of ≈ 30 mm Hg and is ≈ 80% saturated with O₂.
Umbilical arteries have low O₂ saturation.
3 Important Shunts
Blood entering the fetus through the umbilical vein is conducted via the ductus venosus into the IVC, bypassing hepatic circulation.
Most of the highly oxygenated blood reaching the heart via the IVC is directed through the foramen ovale into the left atrium.
Deoxygenated blood from the SVC passes through the RA → RV → main pulmonary artery → ductus arteriosus → descending aorta; shunt is due to high fetal pulmonary artery resistance.
At Birth (see below)
Infant takes a breath → decreases resistance in pulmonary vasculature → increases left atrial pressure compared to right atrial pressure → foramen ovale closes (now called fossa ovalis).
Increase in O₂ (from respiration) and decrease in prostaglandins (from placental separation) → closure of ductus arteriosus.
NSAIDs (e.g., indomethacin, ibuprofen) or acetaminophen help close the patent ductus arteriosus → forms the ligamentum arteriosum (remnant of ductus arteriosus).
