Fetal Circulation

Notes

Prenatal circulation

Oxygenated blood arrives via the placenta; thus, the fetus is dependent upon maternal oxygen stores.
High vasculature pressure in the lungs prevents significant pulmonary blood flow; thus, blood is shunted from the lungs, and the liver, as well, via special temporary structures.
Arteries carry blood Away from the heart.
Veins Return blood to the heart.

Superior vena cava and right heart flow:

Pathway:

Blood from the superior vena cava drains into the right atrium, then to the right ventricle.
Right ventricle pumps the blood into the pulmonary trunk (aka, artery).
The majority of low oxygenated blood is shunted directly to the aorta via the ductus arteriosus.

Within the pulmonary trunk, a small amount of blood is sent through the high pressure pulmonary arteries to the right and left lungs.

Inferior vena cava and left heart flow:

Inferior vena cava delivers oxygenated blood from placenta to left heart.
Inferior vena cava drains oxygen-poor blood from the trunk and lower extremities.

Pathway:

Oxygen-rich blood leaves the placenta via the left umbilical vein.
Most of this blood is shunted directly to the inferior vena cava, where it mixes with relatively oxygen-poor blood returning from the trunk and lower extremities (oxygen-rich blood bypasses liver vasculature).
De-oxygenated blood from trunk/lower extremities ascends through the IVC and mixes with the oxygenated blood from the left umbilical vein.

Blood from the inferior vena cava passes from right atrium, through the foramen ovale, to the left atrium (bypasses right ventricle/pulmonary system).

This blood mixes with the small amount of blood returning from the pulmonary veins.

After the left ventricle pumps the blood into the aorta, it is first sent to the Head, Neck, and Upper Extremities. Thus, the developing brain receives mixed blood with a relatively high oxygen concentration.
After the aortic branches, remaining blood intermixes with blood from the ductus arteriosus (from the SVC, which, again, is deoxygenated).
Blood passing through the descending aorta is sent to the trunk and lower extremities.
Via the umbilical arteries, blood returns to the placenta (where it is oxygenated).

Post-natal circulation:

Structures that shunt blood from the liver and lungs regress after birth.
The first post-natal breath alters pulmonary pressures and opens the pulmonary vasculature, allowing blood flow.

Summary of Post-natal changes:

The ductus arteriosus, which carried blood from the pulmonary trunk to the aorta, becomes the ligamentum arteriosus.
The ductus venosus, which shunted blood from the hepatic vasculature directly to the inferior vena cava, becomes the ligamentum venosum.
The umbilical arteries, which delivered low-oxygen blood from the fetus to the placenta, become the medial umbilical ligaments.
The left umbilical vein, which delivered high-oxygen blood from the placenta to the fetus, becomes the ligamentum teres.
The foramen ovale, which allowed direct passage from the right to left atrium, closes; it is represented by the fossa ovalis, which can be felt in the dorsal aspect of the interatrial septum.