Umbilical cord and fetal circulation
Umbilical cord
The umbilical cord contains two umbilical arteries and one umbilical vein, all embedded in Wharton’s jelly.
- The umbilical vein carries oxygenated blood from the placenta to the fetus.
- The umbilical arteries carry deoxygenated blood from the fetus back to the placenta.
Fetal circulation
The umbilical vein carries oxygenated blood from the placenta to the fetus. It then divides into:
- The larger ductus venosus
- A smaller portal sinus
The ductus venosus bypasses the liver and drains oxygenated blood from the umbilical vein into the IVC. The portal sinus carries oxygenated blood to the liver. Deoxygenated blood from the liver drains into the IVC through the hepatic veins.
The fetal right atrium (RA) receives:
- Partly oxygenated blood from the IVC
- Deoxygenated blood from the SVC
The foramen ovale connects the RA and left atrium (LA). Blood then flows from the LA to the left ventricle (LV) and into the ascending aorta, which supplies the rest of the body.
Most of the oxygenated blood reaching the RA is shunted across the foramen ovale to the LA. Most of the deoxygenated blood in the RA is passed to the right ventricle (RV) and into the pulmonary arteries. Because the fetus is not breathing, pulmonary resistance is high, so blood is shunted away from the lungs.
The pulmonary artery shunts blood to the descending aorta through the ductus arteriosus. Finally, deoxygenated blood is carried by the umbilical arteries back to the placenta for oxygenation.
Organogenesis occurs from 3rd week to 8th week of the embryonic period. This is the period most susceptible to teratogenic drugs, toxins, and radiation.