Fetal blood and maternal blood exchange oxygen, carbon dioxide, nutrients
and waste materials in the placenta, but do not mix. Fetal blood reaches
the placenta through the 
paired umbilical arteries, which are branches of the internal iliac
arteries. While the blood is circulating through the capillaries of the
chorionic villi within the 
placenta , the exchange process takes place. Blood rich in oxygen and
nutrients then leaves the placenta to return to the fetus through a 
single umbilical vein. The umbilical vein is the only fetal vessel
that carries this enriched blood. The remaining vessels of the fetus carry
a mixture of enriched and depleted blood.
Blood returning to the fetus via the umbilical vein is 80% saturated
with oxygen. Traveling from the placenta to the organs of the fetus, the
highly oxygenated blood in the umbilical vein slowly loses its oxygen concentration
by mixing with desaturated blood in several places: 1) 
in
the liver by mixing with a small amount of blood from the portal system,
2) in the 
inferior
vena cava which carries deoxygenated blood returning from the lower
extremities, pelvis and kidneys, 3) in the 
right
atrium by mixing with returning blood from the head and extremities,
4) at the entrance of the 
ductus
arteriosus into the descending aorta.
The blood returning from the placenta is first routed through
the 
ductus venosus causing most of it to bypass the liver and empty directly
into the inferior vena cava. Here it mixes with deoxygenated blood from
the lower limbs. It then enters the right atrium. A sphincter mechanism
in the ductus venosus near the entrance of the umbilical vein regulates
the flow of umbilical blood through the liver. A small portion of the blood
enters the 
liver
sinusoids and mixes with blood from the portal circulation. This sphincter
closes when venous return is high, in order to prevent an overloading of
the heart. A large amount of blood is 
directed
from the right atrium through the foramen ovale and into the left atrium.
A smaller amount is prevented from entering the foramen ovale by the septum
secundum or the crista dividens and remains in the right atrium to mix
with the desaturated blood returning from the head and arms via the superior
vena cava.
The blood in the left atrium mixes with a small amount of blood
returning from the lungs and then enters the left ventricle and ascending
aorta. The myocardium and brain are thereby supplied with well oxygenated
blood because the coronary and carotid arteries are the first to branch
from the ascending aorta. The desaturated blood from the superior vena
cava is directed into the pulmonary trunk from the right ventricle. The
high resistance in the pulmonary vessels causes a large portion of this
blood to flow through the 
ductus
arteriosus and into the descending aorta to mix with the blood from
the proximal aorta. The two umbilical arteries, arising from the internal
iliac arteries, facilitate the flow of blood toward the placenta. The oxygen
saturation in the umbilical arteries is approximately 58%.