Fetal Circulation

O₂ goes through syncytiotrophoblast of chorionic villus, into the cytotrophoblast, then through the myxomatous stroma of the chorionic villus, then into the blood vessel. The blood vessels of the chorionic villus all coalesce to form the umbilical vein. This has the highest O2 content. It goes to the liver and it can go two ways: 1) into the hepatic sinusoids and recollects into the hepatic vein and gets dumped into the IVC; and 2) ductus venosus and straight into the IVC. Then it goes up the right side of the heart, from where it will  go straight across, through the foramen ovale (which is open in all fetuses) and into the left atrium. So, all this oxygenated blood will go directly from the right atrium through the foramen ovale into the left atrium, then the left ventricle and out the aorta.

What about SVC blood valve? Blood coming from the superior part of the right atrium will go straight down, through the tricuspid valve into the right ventricle. Now, it will go out the pulmonary artery. This is a PROBLEM b/c the pulmonary vessels are too thick and it’s encountering this tremendous amount of pressure. To counter this problem, kept the patent ductus open (which is kept open by the PGE2, a vasodilator, made by the placenta) – so, there is a right to left shunt and blood can get out of the pulmonary artery and dumped back into the aorta.

Then, when the baby is born and takes its first breath, the pulmonary vessels (that were all shut), all open within a millisecond, and blood is going through those pulmonary arteries and gas exchange is occurring through the lungs in literally seconds. Also, the patent ductus closes and forms the ligamentum arteriosum.

This is normal fetal circulation. Vessels with the least O2 are the 2 umbilical arteries, and the one with the most amount of O2 is the umbilical vein.