Fetal
Circulation
During
pregnancy, the fetal circulatory system works
differently than after birth:
- The fetus is connected
by the umbilical cord to the placenta, the organ
that develops and implants in the mother's uterus
during pregnancy.
- Through the blood vessels
in the umbilical cord, the fetus receives all
the necessary nutrition, oxygen, and life support
from the mother through the placenta.
- Waste products and
carbon dioxide from the fetus are sent back
through the umbilical cord and placenta to the
mother's circulation to be eliminated.
Blood
from the mother enters the fetus through the vein
in the umbilical cord. It goes to the liver and
splits into three branches. The blood then reaches
the inferior vena cava, a major vein connected
to the heart.
Inside
the fetal heart:
- Blood enters the right
atrium, the chamber on the upper right side
of the heart. Most of the blood flows to the
left side through a special fetal opening between
the left and right atria, called the foramen
ovale.
- Blood then passes into
the left ventricle (lower chamber of the heart)
and then to the aorta, (the large artery coming
from the heart).
- From the aorta, blood
is sent to the head and upper extremities. After
circulating there, the blood returns to the
right atrium of the heart through the superior
vena cava.
- About one-third of
the blood entering the right atrium does not
flow through the foramen ovale, but, instead,
stays in the right side of the heart, eventually
flowing into the pulmonary artery.
Because
the placenta does the work of exchanging oxygen
(O 2 ) and carbon dioxide (CO 2
) through the mother's circulation, the fetal
lungs are not used for breathing. Instead of blood
flowing to the lungs to pick up oxygen and then
flowing to the rest of the body, the fetal circulation
shunts (bypasses) most of the blood away from
the lungs. In the fetus, blood is shunted from
the pulmonary artery to the aorta through a connecting
blood vessel called the ductus arteriosus.
With
the first breaths of air the baby takes at birth,
the fetal circulation changes. A larger amount
of blood is sent to the lungs to pick up oxygen.
- Because the ductus
arteriosus (the normal connection between the
aorta and the pulmonary valve) is no longer
needed, it begins to wither and close off.
- The circulation in
the lungs increases and more blood flows into
the left atrium of the heart. This increased
pressure causes the foramen ovale to close and
blood circulates normally.
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