Fetal circulation and Fetal postnatal derivatives | Heart embryology | Cardiovascular system | Lec4
Channel: Dr. Jamil Lectures
Date: 2023-05-10

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In this lectutre we will discuss about Fetal Circulation before birth and after birth,Patent ductus arteriosus and fetal postnatal derivatives.


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Fetal Circulation
★ Before Birth
● Oxygenated blood enters the umbilical vein from mother circulation via placenta
→blood in the umbilical vein has a Po2 of 30 mm Hg and is 80% saturated with O2.
● The oxygenated blood of umbilical vein conducts via ductus venosus bypassing the
hepatic circulation and eventually enters the right atrium via inferior vena cava
→blood bypasses the hepatic circulation because liver is still not well developed in
fetus → so hepatic circulation has high resistance for blood → that’s why blood
moves into ductus venosus (low resistance) instead of hepatic circulation (high
resistance) → Deoxygenated blood from head and neck of fetus also drains into right
atrium via superior vena cava → so RA has mixed blood with less O2 saturation as
compared to umbilical vein (oxygenated from IVC and deoxygenated from SVC).
6
● In a fetus right atrial pressure is higher than left atrial
pressure,so 70% blood moves from right atrium to left
atrium via foramen ovale → left atrium to left ventricle
and eventually into the aorta that supplies mostly the
head and neck (head and neck gets more oxygenated
blood as compared to other body).
● 30% of right atrial blood enters the right ventricles and
then into pulmonary trunk → blood drains into aorta from
pulmonary trunk via ductus arteriosus bypassing the
lungs because high resistance in lungs → because in
fetus lungs are still not developed,non-functional and
cannot breath,so low O2 concentration in lungs (hypoxia)
→ normally hypoxia causes vasodilation in systemic
vessels but hypoxia leads to vasoconstriction in
pulmonary vessels → so pulmonary vasoconstriction
leads to increase in pulmonary resistance → blood
moves in ductus arteriosus (low resistance) instead of
lungs (high resistance).
Remember →Reason for Vasoconstriction of pulmonary vessels in Hypoxia→ The
pulmonary capillaries near hypoxic alveoli undergo vasoconstriction→this pushes
blood away from hypoxic alveoli and shunts blood towards well ventilated alveoli
→By this, alveoli with more O2 concentration will receive better blood supply for
effective ventilation (gas exchange).
● Due to high resistance in the lungs, Right side of the heart (right Atrium & ventricle)
needs to generate high pressure to pump blood into the high pressure pulmonary
circuit → that’s why the right side of heart has higher pressure than the left side of
heart in the fetus.
● Mixed blood in the aorta from RV and LV supplies to rest body parts and then enters
the umbilical artery that drains blood back into mother circulation→ umbilical artery
has low O2 saturation.
★ After Birth
● Umbilical Cord is cut down and connection between placenta & fetus is broken→ no
blood supply to fetus via umbilical vein and no blood drainage from fetus via umbilical
artery.
● Baby cries right after birth and inhales the air into lung → O2 concentration in alveoli
increases that leads to pulmonary vasodilation due to well ventilated alveoli for better
gas exchange → pulmonary vasodilation results in decreased pulmonary resistance
→Right heart has to generate less pressure to pump the blood into low pressure
pulmonary circulation →pressure on right side of heart (right Atrium & ventricle)
decreases but Left heart has to generate high pressure to supply the blood whole
body→that’s why left heart has higher pressure as compared to right heart after birth.
● The left atrium has higher pressure as compared to the right atrium,so septum
primum fuses with septum secundum and results in functional closure of the foramen
ovale immediately right after birth but anatomically closure occurs after 1 year of birth
and remnant of foramen ovale is called fossa ovalis.




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