Fetal Circulation

Introduction

Fetal circulation is a unique physiological system designed to meet the metabolic needs of the developing fetus in utero. Because the fetal lungs are not yet functional, the circulatory system must bypass them while still delivering well-oxygenated blood to vital organs like the brain and heart.

This is achieved through a series of right-to-left shunts and specialized vascular structures — the foramen ovale, ductus arteriosus, and ductus venosus — that direct blood flow in a way that optimizes oxygen delivery.

Understanding fetal circulation is crucial not only for embryology but also for pediatric cardiology, neonatology, and obstetric practice, as disorders of these shunts can result in serious perinatal complications.

1. Overview of Fetal Circulation

Fetal circulation differs from postnatal circulation in three major ways:

Oxygen Source: Oxygen is supplied by the placenta, not the lungs.
Right-to-Left Shunting: Blood bypasses the pulmonary circuit via foramen ovale and ductus arteriosus.
Special Vessels: Presence of umbilical vein, umbilical arteries, and ductus venosus to connect placenta and fetal systemic circulation.

2. Placental Circulation

2.1 Umbilical Vein

Single large vein that carries oxygenated, nutrient-rich blood from placenta to fetus.
Oxygen saturation of umbilical vein is approximately 80%, the highest in the fetal body.

2.2 Umbilical Arteries

Paired arteries carrying deoxygenated blood and waste products back to placenta.
Branch from internal iliac arteries of the fetus.

3. Ductus Venosus

3.1 Anatomy and Function

A vascular shunt connecting umbilical vein to the inferior vena cava (IVC).
Allows a significant portion of highly oxygenated placental blood to bypass the hepatic sinusoids and reach the heart rapidly.
A portion of umbilical venous blood still perfuses the liver, supporting fetal hepatic development.

3.2 Hemodynamic Advantage

Ensures that blood entering the right atrium is well oxygenated, preferentially directed toward left heart and brain.

4. Right Atrium — Preferential Streaming

Blood entering the right atrium is stratified:

Oxygen-rich blood from IVC (placental blood) is directed through foramen ovale to the left atrium.
Less oxygenated blood from SVC (upper body) flows into the right ventricle → pulmonary artery → ductus arteriosus → descending aorta.

This preferential streaming ensures that brain and myocardium receive the best-oxygenated blood.

5. Foramen Ovale

5.1 Structure

An opening between right and left atria, formed by the septum primum and septum secundum.
Functions as a one-way valve allowing blood to pass from RA → LA.

5.2 Role in Fetal Circulation

Allows highly oxygenated blood from placenta to bypass pulmonary circuit and enter systemic circulation directly.
LA receives this blood and pumps it into LV → ascending aorta → coronary and cerebral circulation.

6. Right Ventricle and Ductus Arteriosus

6.1 Right Ventricular Output

The RV pumps about two-thirds of combined cardiac output in fetal life.
However, because pulmonary vascular resistance is high, only about 10% of RV output enters lungs.

6.2 Ductus Arteriosus

A vascular shunt connecting pulmonary artery to descending aorta.
Diverts RV blood away from the high-resistance pulmonary circuit into systemic circulation.
Ensures that lower body and placenta receive adequate blood flow.

7. Oxygen Saturation Pattern

Oxygen content is not uniform throughout fetal circulation:

Highest O₂ saturation: Umbilical vein (~80%).
Lower saturation: SVC blood (~26%), mixed blood in RA/RV (~55%).
Brain and heart receive relatively oxygen-rich blood due to preferential streaming.

8. Fetal Cardiac Output

Total fetal cardiac output is referred to as combined cardiac output (CCO) since both ventricles pump in parallel.
Distribution:
- RV contributes ~60–65% of CCO.
- LV contributes ~35–40% of CCO.
This parallel arrangement allows highly oxygenated blood to be directed strategically.

9. Changes at Birth

At birth, dramatic changes occur:

First Breath: Lungs expand → pulmonary vascular resistance falls → increased pulmonary blood flow.
Increased LA Pressure: Foramen ovale flap closes functionally due to higher LA pressure.
Ductus Arteriosus Constriction: Functional closure within hours (prostaglandin levels fall, oxygen tension rises).
Ductus Venosus Closure: Blood flow from umbilical vein stops after cord clamping; ductus venosus collapses and becomes ligamentum venosum.

Eventually, these shunts fibrose into ligaments:

Foramen ovale → Fossa ovalis
Ductus arteriosus → Ligamentum arteriosum

10. Molecular Regulation

Prostaglandins (PGE2): Maintain ductus arteriosus patency in utero.
Bradykinin: Released from lungs at first breath, mediates ductal constriction.
Oxygen tension: Major factor in functional closure of ductus arteriosus.

11. Clinical Correlations

11.1 Patent Foramen Ovale (PFO)

Persistence of foramen ovale after birth.
Often asymptomatic, but can allow paradoxical emboli (stroke risk).

11.2 Patent Ductus Arteriosus (PDA)

Failure of ductus arteriosus to close.
Causes left-to-right shunt → pulmonary overcirculation → heart failure if untreated.
Treated with NSAIDs (indomethacin/ibuprofen) or surgical ligation.

11.3 Persistent Pulmonary Hypertension of the Newborn (PPHN)

High PVR prevents normal closure of shunts, leading to cyanosis.
Requires careful management with oxygen, nitric oxide, sometimes ECMO.

11.4 Ductus Venosus Agenesis

Rare anomaly leading to abnormal venous drainage pattern.
May cause heart failure or hydrops fetalis.

12. Importance in Congenital Heart Disease

Fetal circulation patterns can actually be protective in certain heart defects:

Transposition of great arteries: Mixing through foramen ovale/ductus arteriosus is life-saving until surgical correction.
Hypoplastic left heart syndrome: RV provides systemic output through ductus arteriosus until staged surgery.

13. Imaging of Fetal Circulation

Doppler ultrasound: Used to assess umbilical artery/vein flow, ductus venosus waveform, and fetal well-being.
Fetal echocardiography: Can detect abnormal shunts or structural defects prenatally.

14. Evolutionary Perspective

The presence of shunts in fetal circulation reflects an adaptive mechanism conserved across mammals. In species with placental development, similar shunts are used to bypass nonfunctional lungs and direct oxygen-rich blood to vital organs.

15. Summary Table of Fetal Shunts

Shunt	Location	Function	Postnatal Fate
Ductus venosus	Umbilical vein → IVC	Bypasses liver, delivers O₂ blood to RA	Ligamentum venosum
Foramen ovale	RA → LA	Bypasses RV/pulmonary circuit	Fossa ovalis
Ductus arteriosus	Pulmonary artery → Aorta	Diverts RV output from lungs to aorta	Ligamentum arteriosum