Fetal circulation differs fundamentally from adult circulation because the placenta, not the lungs, serves as the organ of gas exchange. The fetal circulatory system includes unique shunts that bypass the nonfunctional fetal lungs.
Overview of Fetal Circulation
Key Differences from Adult Circulation
| Feature | Fetal | Adult |
|---|---|---|
| Gas exchange organ | Placenta | Lungs |
| Pulmonary circulation | High resistance, low flow | Low resistance, high flow |
| Systemic circulation | Low resistance (placenta) | Higher resistance |
| Pressures | RV > LV (right dominant) | LV > RV (left dominant) |
| Shunts present | Foramen ovale, ductus arteriosus, ductus venosus | None (closed) |
| Oxygenation source | Placenta | Lungs |
Fetal Circulatory Structures
The Placenta
The placenta is the fetal organ of gas exchange, nutrient delivery, and waste removal.
Structure:
- Fetal side: Chorionic villi with fetal capillaries
- Maternal side: Intervillous space with maternal blood
- Separation: Two cell layers (syncytiotrophoblast and fetal endothelium)
Umbilical cord vessels:
- One umbilical vein: Carries oxygenated blood from placenta to fetus
- Two umbilical arteries: Carry deoxygenated blood from fetus to placenta
The Three Fetal Shunts
| Shunt | Structure | Length | Function |
|---|---|---|---|
| Ductus venosus | Bypass through the liver | 2-3 cm | Connects umbilical vein to IVC |
| Foramen ovale | Opening between atria | Flap valve | Bypasses pulmonary circulation |
| Ductus arteriosus | Connection: pulmonary trunk to aorta | 10-15 mm | Bypasses pulmonary circulation |
Ductus Venosus
Location: Connects the umbilical vein to the inferior vena cava
Structure:
- Narrow channel through the liver
- Sphincter-like smooth muscle
- Regulates blood flow through the liver
Function:
- 40-60% of umbilical venous blood bypasses the liver
- Remaining blood perfuses the hepatic sinusoids
- Delivers oxygenated blood to the IVC
Closure:
- Functional: Minutes after birth (loss of umbilical flow)
- Anatomic: 1-2 weeks after birth
- Remnant: Ligamentum venosum (on the liver)
Foramen Ovale
Location: Interatrial septum
Structure:
- Flap valve created by the septum primum and septum secundum
- Inferior vena cava directs blood toward the foramen ovale
- Valve opens from right to left
Function:
- Directs oxygenated blood from IVC to left atrium
- Bypasses the right ventricle and pulmonary circulation
- Accounts for approximately 30-40% of combined cardiac output
Direction of shunting:
- Fetal: Right-to-left (IVC blood to LA)
- Atrial pressures: Right atrial > Left atrial (fetal)
- Adult: Left-to-right or closed
Closure:
- Functional: Minutes after birth (increased LA pressure)
- Anatomic: 3-12 months (septal fusion)
- Remnant: Fossa ovalis
Ductus Arteriosus
Location: Between the pulmonary trunk and the descending aorta
Structure:
- Muscular artery (thick wall)
- High sensitivity to oxygen and prostaglandins
- 10-15 mm long, 5-10 mm diameter
Function:
- Diverts blood from the pulmonary trunk to the aorta
- Bypasses the high-resistance pulmonary circulation
- Accounts for approximately 50-60% of right ventricular output
Direction of shunting:
- Fetal: Right-to-left (pulmonary trunk to aorta)
- Due to high pulmonary vascular resistance
Closure:
- Functional: 10-15 hours after birth (smooth muscle contraction)
- Anatomic: 2-3 weeks (fibrotic obliteration)
- Remnant: Ligamentum arteriosum
Mechanism of closure:
- Increased oxygen tension (constriction)
- Decreased prostaglandin E2 (PGE2 levels fall after birth)
- Endothelin-1 release
- Bradykinin release
Blood Flow in Fetal Circulation
Oxygen Saturation Levels
| Location | O₂ Saturation | PO₂ (mmHg) |
|---|---|---|
| Umbilical vein | 80-90% | 30-40 |
| Ductus venosus | 80-90% | 30-35 |
| Inferior vena cava (below DV) | 70-75% | 25-30 |
| Right atrium | 65-70% | 22-27 |
| Left atrium (via foramen ovale) | 70-75% | 25-30 |
| Left ventricle | 65-70% | 22-27 |
| Aorta (pre-ductal) | 65-70% | 22-25 |
| Pulmonary artery | 55-60% | 18-22 |
| Ductus arteriosus | 55-60% | 18-20 |
| Aorta (post-ductal) | 60-65% | 20-24 |
| Umbilical arteries | 50-55% | 15-20 |
Flow Distribution
| Structure | % of Combined Ventricular Output |
|---|---|
| Umbilical-placental | 40-45% |
| Pulmonary | 8-10% |
| Brain | 15-20% |
| Heart | 3-5% |
| Kidneys | 2-4% |
| GI tract | 5-6% |
| Lower body | 15-20% |
Directional Flow Patterns
Flow from the Placenta to the Heart
- Umbilical vein carries oxygenated blood from the placenta
- Ductus venosus: ~50% of umbilical venous blood bypasses the liver
- Portal sinus: Remaining blood enters the liver
- Inferior vena cava: Receives blood from the ductus venosus and the lower body
- Crista dividens: The eustachian valve directs IVC blood toward the foramen ovale
Preferential Streaming
The fetal circulation uses preferential streaming to deliver the most oxygenated blood to the brain and heart:
Well-oxygenated blood path:
- Umbilical vein → Ductus venosus → IVC → Foramen ovale → LA → LV → Aorta (pre-ductal) → Carotid arteries → Brain
Less-oxygenated blood path:
- SVC → RA → RV → Pulmonary trunk → Ductus arteriosus → Descending aorta → Umbilical arteries → Placenta
Changes at Birth
Sequence of Events
Phase 1: Delivery and first breath
- Chest compression during vaginal delivery
- Expansion of the lungs (first breath)
- Dramatic decrease in pulmonary vascular resistance
- Increased pulmonary blood flow (10× increase)
Phase 2: Shunt closure
- Ductus venosus: Loss of placental flow → passive closure
- Foramen ovale: Increased LA pressure (from increased pulmonary venous return) + decreased RA pressure (loss of placental flow) → functional closure
- Ductus arteriosus: Increased PO₂ → smooth muscle constriction → functional closure within hours
Phase 3: Permanent closure
- Anatomic remodeling and fibrosis over weeks to months
- Ligamentous remnants remain
Cardiorespiratory Transition at Birth
| Event | Before Birth | After Birth |
|---|---|---|
| Gas exchange | Placenta | Lungs |
| Pulmonary vascular resistance | High (fetal) | Low (aerobic) |
| Systemic vascular resistance | Low (placenta) | High (placenta removed) |
| Right ventricular pressure | Equal to LV | 1/6 of LV |
| Foramen ovale | Open (right→left) | Closed |
| Ductus arteriosus | Open (right→left) | Closed |
| Ductus venosus | Open | Closed |
| Umbilical vessels | Patent | Obliterated |
Persistent Fetal Circulation (PPHN)
Persistent pulmonary hypertension of the newborn occurs when the pulmonary vascular resistance remains high after birth, causing right-to-left shunting across the foramen ovale or ductus arteriosus.
Causes:
- Meconium aspiration syndrome
- Congenital diaphragmatic hernia
- Sepsis
- Birth asphyxia
- Pulmonary hypoplasia
Clinical Significance of Fetal Remnants
| Fetal Structure | Adult Remnant | Clinical Significance |
|---|---|---|
| Umbilical vein | Ligamentum teres (round ligament of liver) | Recanalized in portal hypertension (caput medusae) |
| Ductus venosus | Ligamentum venosum | Divides left and caudate lobes |
| Foramen ovale | Fossa ovalis | Patent foramen ovale (PFO) in 25% of adults |
| Ductus arteriosus | Ligamentum arteriosum | PDA if patent; protects recurrent laryngeal nerve |
| Umbilical arteries | Medial umbilical ligaments | Part of the anterior abdominal wall |
| Allantois | Urachus (median umbilical ligament) | Urachal cyst or sinus if patent |