Circulatory Adaptations at Birth

Question 1

Normal Fetal Circulation from Placenta to Heart

Answer 1

• Fetus receives oxygenated blood from the placenta through the umbilical vein
• This blood bypasses the liver by traveling through the ductus venosus, which connects the umbilical vein to the inferior vena cava (IVC)
• Oxygenated blood from the ductus venosus combines with deoxygenated blood from the IVC
• This mixed blood travels through the IVC and mixes with more deoxygenated blood from the superior vena cava (SVC)
• The blood then enters the R atrium of the heart

Question 2

Normal Fetal Circulation - Heart

Answer 2

• Once in the RA, most blood will be shunted to the LA via the foramen ovale (fetal lungs are not functional so RV is mostly bypassed)
• Blood will then travel from the LA to LV and then be distributed to the body by the aorta
• Some blood does enter the RV from the RA
  
  • Some proceeds to the pulmonary trunk
  • Most of it is shunted away from the pulmonary aa. and into the aorta via the ductus arteriosus

Question 3

Normal Fetal Circulation - Heart to Placenta

Answer 3

• Blood from the aorta circulates throughout the body and then returns to the placenta via the umbilical arteries, which carry deoxygenated blood back to the placenta.
• In the placenta, this blood is reoxygenated to restart the fetal cardiovascular cycle.

Question 4

Transition to Normal Neonatal Circulation

Answer 4

• First breath → ⇡alveolar O₂ pressures → vasodilation in the pulmonary vessels
• Obstetrical clamping → spontaneous constriction and change of umbilical vein to ligamentum teres and umbilical aa. to median umbilical ligaments
• 10-15 hours after birth ductus arteriosus constricts and becomes the ligamentum arteriosum
• Ductus venosus also constricts and becomes the ligamentum venosum
• ⇡LAP and ⇣RAP cause the foramen ovale to close and become the fossa ovalis

Question 5

Normal Neonatal Circulation - Body to Heart

Answer 5

• Obstetrical clamping leads to spontaneous constriction of umbilical vessels and eventually conversion of the ductus venosus to the ligamentum venosum
• LE blood therefore drains into the IVC and blood does not bypass the liver, which then drains into the IVC (all of this deoxygenated blood)
• Deoxygenated blood from both the IVC and SVC enter the RA of the heart

Question 6

Normal Neonatal Circulation - Heart

Answer 6

• LAP>RAP causes the foramen ovale to close and become fossa ovalis
  
  • deoxygenated blood then flows from RA to RV
• ⇡alveolar pressures → vasodilation of pulmonary vessels → blood can flow from RV to pulm. aa.
  
  • 10-15 hours after birth, ductus arteriosus contricts and becomes ligamentum arteriosus, so that all blood flows from the RV → pulmonary trunk → pulmonary arteries
• Oxygenated blood returns to the heart via four pulmonary veins and enters the LA → LV

Question 7

Normal Neonatal Circulation - Heart to Body

Answer 7

• Oxygenated blood from LV → aorta → systemic circulation/body

Question 8

Fetal Blood Flow

Answer 8

• Chorionic villi in the placenta
  
  • Derived from the fetus
  • Primary site for O2 exchange
  • Umbilical vein derived from villus vessels
• Umbilical vein
  
  • Vessel with the highest Po2 in the fetal circulation
• ​Inferior vena cava blood drains into the right atrium.
  
  • Most blood is directly shunted into the left atrium through the foramen ovale.
• Superior vena cava (SVC) blood
  
  • Most blood from the SVC is directed from the right atrium into the RV.
• Pulmonary artery blood
  
  • Blood from the pulmonary artery is shunted through a patent ductus arteriosus (PDA) into the aorta.
• Ductus arteriosus is kept open by prostaglandin E2, a vasodilator synthesized by the placenta.