What is the pathway of fetal circulation?

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Pathway of Fetal Circulation

Fetal circulation is a unique parallel system where oxygenated blood from the placenta bypasses the lungs and liver through specialized shunts (foramen ovale, ductus arteriosus, and ductus venosus) to preferentially deliver oxygen-rich blood to the brain and heart.

Oxygenation and Return from Placenta

  • Oxygenated blood returns from the placenta via the umbilical vein to the fetus 1
  • The umbilical vein carries the most oxygen-rich blood in the fetal circulation, as gas exchange occurs at the placental interface 2

Hepatic Bypass via Ductus Venosus

  • The ductus venosus acts as a critical bypass shunt, allowing approximately 57-65% of umbilical venous blood to bypass the hepatic microcirculation and flow directly to the inferior vena cava 3, 2
  • This shunt increases blood volume delivery to the heart at the expense of the liver 4
  • During hypoxic stress, the percentage of blood shunting through the ductus venosus increases from 57% to 65%, facilitating greater oxygen delivery to vital organs 2
  • The ductus venosus connects the umbilical vein to the inferior vena cava within the fetal liver parenchyma 5

Preferential Streaming in Inferior Vena Cava

  • Oxygenated blood from the ductus venosus preferentially streams within the inferior vena cava to facilitate delivery to the brain and heart 1
  • This streaming mechanism ensures well-oxygenated blood reaches the myocardium and brain preferentially over other organs 5

Right Atrium and Foramen Ovale

  • Blood entering the right atrium from the inferior vena cava (containing ductus venosus blood) preferentially crosses the foramen ovale to the left atrium 4
  • This right-to-left shunt at the atrial level directs oxygen-rich blood toward the left ventricle 6
  • Less oxygenated blood from the superior vena cava preferentially flows through the tricuspid valve to the right ventricle 1

Ventricular Output and Ductus Arteriosus

  • The right ventricle pumps blood into the pulmonary artery, but most of this blood bypasses the high-resistance pulmonary circulation through the ductus arteriosus 4
  • The ductus arteriosus shunts blood from the pulmonary artery directly to the descending aorta 4
  • The left ventricle receives oxygen-rich blood from the left atrium and pumps it to the ascending aorta, supplying the brain, heart, and upper body 4

Distribution to Vital Organs

  • The brain, heart, and adrenal glands receive preferentially oxygenated blood through adaptive circulatory mechanisms 4
  • During hypoxemia, this "brain-sparing" reflex becomes more pronounced, with increased impedance in umbilical arteries and decreased impedance in middle cerebral arteries 4
  • Approximately 27-39% of fetal cardiac output is derived from umbilical venous blood, with this percentage increasing during hypoxic stress 2

Return to Placenta

  • Deoxygenated blood returns to the placenta via two umbilical arteries that branch from the internal iliac arteries 4
  • These arteries carry blood with the lowest oxygen content in the fetal circulation back to the placenta for reoxygenation 4

Ineffective Shunting and Recirculation

  • Approximately 34% of cardiac output represents ineffective circulation due to recirculation shunts 2
  • About 22% of umbilical venous blood recirculates back to the placenta without perfusing fetal tissues (left-to-right shunt analogue) 2
  • Approximately 23% of systemic venous blood recirculates to the fetal body rather than returning to the placenta for oxygenation (right-to-left shunt analogue) 2

Adaptive Responses to Hypoxemia

  • Progressive Doppler changes occur with worsening placental function: increased umbilical artery impedance → brain sparing (decreased middle cerebral artery impedance) → increased ductus venosus shunting → abnormal ductus venosus waveforms → umbilical venous pulsations 4
  • Increased right ventricle afterload causes further shunting to the left ventricle, improving left ventricular output to vital organs 4
  • When ductus venosus becomes abnormal with decreased, absent, or reversed A-wave flow, the risk for stillbirth increases dramatically 4

References

1
Research

Perinatal oxygen delivery and cardiac function.

Advances in pediatrics, 1985

4
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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