Fetal circulation

Fetal circulationdiffers from the adult circulation due to the presence of certain vessels and shunts. 

These shunts will close after birth and most of these fetal vessels will be seen as remnants in the adult circulation. The function of these shunts is to direct oxygen-rich venous blood to the systemic circulation and to ensure oxygen-deplete venous blood bypasses the underdeveloped pulmonary circulation.There There are three shunts in the fetal circulation:

• ductus venosus (DV)
• foramen ovale (FO)
• ductus arteriosus (DA)

Pathway for oxygenated blood

The lungs finish their development after birth. Prior to this, the lung's function is taken over by the placenta, which therefore becomes the oxygen-transfer organ during fetal life.

Oxygen-rich blood is carried by the umbilical vein from the placenta to the fetus. The umbilical vein enters at the umbilicus and divides into two branches:

• right umbilical umbilical branch passes to the right side of the liver and is joined by the portal vein; the blood from the liver eventually eventually drains into the inferior vena cava (IVC) via the hepatic veins
• left umbilical branch gets shunted via the ductus venosus into the IVC

Appropriate channeling of blow flow is required to ensure sufficient oxygen_{and nutrient supply to vital organs. This is achieved by unique features of blood vessels which help create  differences in velocities and direction of blow flow.}

Role of ductus venosus

The ductus venous diameter is one-third the diameter of the umbilical vein hence the blood is shunted to the IVC under pressure. This increased pressure propels the blood in the IVC to the right atrium and directly into the left atrium via another shunt, the foramen ovale. The blood then passes into the systemic circulation via the left ventricle and aorta.

Pathway for deoxygenated blood

The carbon dioxide-rich blood from the brain and upper extremities returns to the right atrium via the superior vena cava. This blood passes into the right ventricle and is then shunted directly into the descending aorta from the pulmonary artery via the ductus arteriosus. This occurs because the lungs are not developed and the pulmonary arteries offer high resistance to blood flow.

At the same time blood from the hepatic circulation and lower extremities returns via the IVC to the heart and it passes into descending aorta through the path of right ventricle, pulmonary artery and ductus arteriosus.

All this blood passes into circulation and is carried via umbilical arteries back to placenta for oxygenation.

Note: a very very small amount of blood does get filtered by the lungs and reaches the left atrium via pulmonary veins.