Fetal Circulation
by Lisa McCabe.
Hello, my name is Lisa McCabe.
I'm a Clinical Nurse Specialist
at Children's Hospital Boston
in the Cardiovascular Program.
We are now going to talk
about fetal circulation
and the transition to
postnatal circulation that
occurs after birth.
The specific
structures associated
with fetal circulation
include the placenta,
the umbilical vein, the ductus
venosus, the foramen ovale,
the ductus arteriosus, and
the umbilical arteries.
Blood rich in
nutrients and oxygen,
supplied via the placenta,
flows through the umbilical vein
to the ductus venosus.
Blood flows from the ductus
venosus into the inferior vena
cava, up to the right atrium.
This blood mixes
with blood returning
to the heart from the upper
body via the superior vena cava,
and from the lower body
via the inferior vena cava.
Once in the right
atrium, some of the blood
flows to the right ventricle
and some of the blood
flows through the foramen
ovale, through the left atrium,
and into the left ventricle,
where it is then pumped out
into the aorta to the body.
Blood that flows into
the right ventricle
is then pumped into
the pulmonary artery.
Because the lungs are
fluid-filled instead
of air-filled, the
vessels in the lungs
are narrow, creating higher
resistance to blood flow
into the lungs.
Due to this high
resistance to blood flow
in the pulmonary circulation
and the low resistance
to blood flow in the
systemic circulation,
blood pumped into the pulmonary
artery by the right ventricle
is more likely to flow
into the ductus arteriosus
and then into the aorta.
Due to the high blood flow
from the superior vena
cava and the inferior vena
cava up to the right atrium,
pressures in the right
atrium are higher than
in the left atrium.
This promotes blood flow
through the foramen ovale
at the atrial level.
Only 8% of right ventricular
output flows into the lungs,
providing nutrients for the
developing lung tissues.
Most of the blood passes
through the ductus arteriosus
into the aorta and
out to the body.
The umbilical arteries allow
blood to flow from the body
back to the placenta
to be enriched
with oxygen and nutrients.
Once the baby is born
and takes a breath,
the umbilical cord is
clamped and the placenta
is removed from the
systemic circulation.
Immediately, the transition from
fetal to postnatal circulation
begins.
With the elimination of the
placenta from the circulation,
systemic vascular
resistance begins to rise.
With each breath, more
alveoli in the lungs
expand, and the
vessels surrounding
them dilate in response
to the presence of oxygen.
Pulmonary pressures
begin to decrease.
Although pulmonary pressures are
lower than systemic pressures,
within minutes after birth
it is six to eight weeks
before pulmonary vascular
resistance decreases to normal.
In the postnatal
circulation, blood no longer
flows through the foramen
ovale or the ductus arteriosus.
The increase in left
atrial pressure after birth
forces the septum primum
against the septum secundum,
functionally closing
the foramen ovale.
Within three months, the
foramen ovale permanently
closes as fibrin deposits
fuse the layers of septal wall
together.
Point of clarification.
While the patent foramen ovale,
or PFO, may close by this time,
it also may not.
Autopsy studies in adults
indicate that 15 to 25%
of adults have a patent foramen
ovale which never closed.
The shunt via the PFO after
birth is usually small.
The ductus arteriosus begins
to close shortly after birth,
once the infant
begins to breathe.
Normally the ductus closes
completely in 4-10 days.
The ductus venosus is
open at the time of birth,
making central venous
access possible
through the umbilical vein.
As fibrin infiltrates
the ductus venosus,
it usually closes
within 3-7 days.
After it closes, the remnant
is known as ligamentum venosum.
Within a week after
birth, the umbilical vein
and umbilical arteries are
infiltrated with fibrin
and also become ligaments.
Thank you for your time.
This concludes our lecture
on fetal circulation.
