Figure 2-9. Fetal circulation before birth.
a. The umbilical vein transports blood rich in oxygen and nutrients from the
placenta to the fetal body. This vein travels along the anterior abdominal wall of the
fetus to the liver, and at the porta hepatis, the umbilical vein divides into two branches.
b. About 1/2 of the blood passes into the liver and the rest enters a shunting
vessel called the ductus venosus that bypasses the liver. The ductus venosus travels a
short distance and joins the inferior vena cava.
c. There, the oxygenated blood from the placenta is mixed with deoxygenated
blood from the lower parts of the fetal body. This blood continues through the vena
cava to the right atrium.
d. As the blood relatively high in oxygen enters the right atrium of the fetal heart,
a large proportion of it is shunted directly into the left atrium through an opening in the
atrial septum called the foramen ovale.
e. The more highly oxygenated blood that enters the left atrium through the
foramen ovale is mixed with a small amount of deoxygenated blood returning from the
pulmonary veins. This mixture moves into the left ventricle and is pumped into the