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.
Point of Clarification. The flow of blood from the
right atrium to the left atrium via the foramen ovale enables
oxygenated blood from the placenta to reach the coronary arteries
in cerebral circulation, the two most metabolic, active organs
in the developing fetus.
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.