Fetal circulation
fetal circulation anatomy physiology overview
Fetal Circulation
The Fetal Circulation Diagram
Step-by-Step Blood Flow
1. Placenta → Umbilical Vein
- Oxygenated blood (~80% O₂ saturation) leaves the placenta via the single umbilical vein, traveling in the umbilical cord to the fetal abdomen.
2. Ductus Venosus (Liver Bypass)
- On approaching the liver, approximately 50% of umbilical venous blood is shunted directly into the ductus venosus - a fetal vessel connecting the umbilical vein to the inferior vena cava (IVC).
- The other 50% flows into the portal vein and hepatic sinusoids, then drains into the IVC via hepatic veins.
- A physiologic sphincter near the ductus venosus regulates flow, preventing cardiac overload during high umbilical vein flow (e.g., uterine contractions).
3. Inferior Vena Cava → Right Atrium
- The IVC carries a mix of well-oxygenated (from the ductus venosus) and poorly oxygenated blood (from the lower limbs, pelvis, kidneys). Combined saturation ~67%.
- This blood enters the right atrium.
4. Foramen Ovale (Lung Bypass - Step 1)
- The crista dividens (lower edge of the septum secundum) splits the IVC stream in the right atrium:
- The larger, better-oxygenated portion is directed through the foramen ovale directly into the left atrium (right-to-left shunt).
- The smaller, less-oxygenated portion remains in the right atrium, where it mixes with poorly oxygenated blood from the superior vena cava (SVC) and coronary sinus.
- From the left atrium, blood passes to the left ventricle and out the ascending aorta, supplying the coronary arteries, head, neck, and upper limbs with the best-oxygenated blood in the fetal circulation.
5. Right Ventricle → Pulmonary Trunk
- The mixed (medium-oxygenation) blood remaining in the right atrium goes through the tricuspid valve into the right ventricle, then is pumped into the pulmonary trunk.
- Pulmonary vascular resistance is very high in fetal life because the lungs are collapsed and fluid-filled. Pulmonary artery pressure is slightly higher than aortic pressure.
- Only ~10% of right ventricular output enters the lung circulation.
6. Ductus Arteriosus (Lung Bypass - Step 2)
- The remaining ~90% of pulmonary trunk blood is shunted through the ductus arteriosus into the descending aorta, bypassing the lungs entirely.
- The blood in the descending aorta is medium-oxygenated (~60% saturation), supplying the gut, kidneys, lower limbs, and pelvis.
7. Descending Aorta → Umbilical Arteries → Placenta
- Two umbilical arteries (branches of the internal iliac arteries) carry deoxygenated blood from the descending aorta (~58% saturation) back to the placenta for reoxygenation.
- About 65% of descending aortic blood re-enters the placenta; the remaining 35% supplies the lower body.
The Three Key Fetal Shunts
| Shunt | Connects | Bypasses | Adult Remnant |
|---|---|---|---|
| Ductus venosus | Umbilical vein → IVC | Liver | Ligamentum venosum |
| Foramen ovale | Right atrium → Left atrium | Lungs (right heart) | Fossa ovalis |
| Ductus arteriosus | Pulmonary trunk → Descending aorta | Lungs | Ligamentum arteriosum |
Sites of Blood Mixing (O₂ Dilution)
- Liver - umbilical vein mixes with portal blood
- IVC - ductus venosus blood mixes with deoxygenated lower limb drainage
- Right atrium - IVC blood mixes with SVC blood
- Left atrium - foramen ovale blood mixes with small amount of pulmonary venous return
- Descending aorta - blood from the ascending aorta mixes with ductus arteriosus shunt
Why the Brain and Heart Get the Best Blood
Fetal Haemoglobin (HbF)
- Producing HbF (subunit composition: 2α + 2γ chains), which has a higher affinity for oxygen than adult HbA, due to weaker binding of 2,3-BPG to the γ chains.
- Maintaining relative polycythaemia (high red cell count).
- These adaptations allow adequate O₂ delivery despite saturations that would be hypoxic postnatally.
Neonatal Circulation - Changes at Birth
- Clamping of the umbilical cord - removes placental circulation; right atrial pressure falls.
- Lung expansion - dramatic drop in pulmonary vascular resistance; pulmonary blood flow increases markedly.
- Rise in left atrial pressure (from increased pulmonary return) exceeds right atrial pressure.
Closure of the Shunts
| Shunt | Closure Mechanism | Timing | Adult Structure |
|---|---|---|---|
| Foramen ovale | Left atrial pressure > right atrial pressure pushes septum primum against septum secundum | Functional: within hours; anatomic: weeks to months | Fossa ovalis |
| Ductus arteriosus | Rise in pO₂ at birth triggers smooth muscle contraction; prostaglandin E₂ withdrawal contributes | Functional: 10-15 hours; anatomic: 1-3 months | Ligamentum arteriosum |
| Ductus venosus | Sphincter contraction when umbilical flow ceases | Functional: shortly after cord clamping | Ligamentum venosum |
| Umbilical arteries | Smooth muscle contraction | Functional: immediately | Medial umbilical ligaments (proximal); superior vesical artery |
| Umbilical vein | Passive collapse | Within minutes | Ligamentum teres hepatis (round ligament) |
Clinical Relevance
- Patent ductus arteriosus (PDA): Failure of the DA to close causes a left-to-right shunt (aorta → pulmonary artery), leading to pulmonary overcirculation. More common in premature infants (where PGE₂ levels remain elevated). Treated with indomethacin (NSAIDs block PGE₂) or surgical ligation.
- Patent foramen ovale (PFO): Failure of anatomic closure in ~25% of adults; usually asymptomatic but can cause paradoxical embolism and cryptogenic stroke.
- Persistent pulmonary hypertension of the newborn (PPHN): Failure of pulmonary vascular resistance to fall at birth, causing continued right-to-left shunting through the foramen ovale and DA.
- Ductus-dependent congenital heart lesions: Some defects (e.g., pulmonary atresia, critical aortic stenosis) rely on a patent DA for survival. Prostaglandin E₁ infusion is used to keep the ductus open until surgical repair.
- The Developing Human: Clinically Oriented Embryology, Figs. 13.46-13.50 (pp. 869-875)
- Langman's Medical Embryology, Fig. 13.52 (pp. 236-237) - the 5 mixing sites
- Ganong's Review of Medical Physiology, 26e, pp. 614-615 - oxygen saturations and physiology
Give in a flowcharts
A detailed medical flowchart of fetal circulation on a clean white background. The flowchart shows the complete step-by-step pathway of blood flow in the fetus. The chart flows top to bottom with clearly labeled boxes and arrows. Use color coding: red boxes for oxygenated blood, blue boxes for deoxygenated blood, purple boxes for mixed blood, and yellow boxes for the three special shunts. Flow pathway: Start at top: PLACENTA (red box, "O2 saturation ~80%") ↓ arrow labeled "Umbilical Vein" LIVER LEVEL box (purple) splits into TWO paths with a Y-split: - LEFT PATH: "~50% → Portal Vein → Hepatic Sinusoids → Hepatic Veins" (blue box) - RIGHT PATH: "~50% → DUCTUS VENOSUS ★ SHUNT 1" (yellow box, "Bypasses liver") Both paths converge at: INFERIOR VENA CAVA (purple box, "Mixed blood ~67% O2 sat") ↓ RIGHT ATRIUM (purple box) Split with CRISTA DIVIDENS label, two paths: - LEFT PATH: "Larger, better-oxygenated stream → FORAMEN OVALE ★ SHUNT 2" (yellow box) → LEFT ATRIUM → LEFT VENTRICLE → ASCENDING AORTA (red box) → splits to: "Head, Neck, Brain, Arms (BEST oxygenated)" and "Coronary Arteries" - RIGHT PATH: "Smaller, less-oxygenated stream + SVC blood" → RIGHT VENTRICLE → PULMONARY TRUNK (blue box) → splits: - "~10% → Lungs → Pulmonary Veins → Left Atrium" - "~90% → DUCTUS ARTERIOSUS ★ SHUNT 3" (yellow box, "High pulmonary resistance") → DESCENDING AORTA (purple box, "~60% O2 sat") DESCENDING AORTA splits: - "35% → Gut, Kidneys, Lower Limbs" - "65% → Umbilical Arteries (2)" → back to PLACENTA (completing the circuit with a curved arrow) At the bottom right, a separate small legend box: "★ THREE FETAL SHUNTS: 1. Ductus Venosus → bypasses liver 2. Foramen Ovale → bypasses lungs (right heart) 3. Ductus Arteriosus → bypasses lungs Adult remnants: 1. → Ligamentum Venosum 2. → Fossa Ovalis 3. → Ligamentum Arteriosum" Use clear sans-serif font, bold labels, thick arrows with arrowheads. Title at top: "FETAL CIRCULATION FLOWCHART" in large bold dark blue text. Make it look like a professional medical study chart.
A clean professional medical flowchart titled "CHANGES AT BIRTH - Neonatal Transition" showing what happens to fetal shunts when a baby is born. White background, neat boxes and arrows, dark text. THREE TRIGGERS at the top in orange boxes side by side: "1. Cord Clamping → ↓ Right Atrial Pressure" "2. Lung Expansion → ↓ Pulmonary Vascular Resistance" "3. ↑ Pulmonary blood flow → ↑ Left Atrial Pressure" These three arrows all point down to: "LEFT ATRIAL PRESSURE > RIGHT ATRIAL PRESSURE" (bold red box) That leads to three separate columns below showing closure of each shunt: COLUMN 1 - FORAMEN OVALE: Box: "Septum primum pushed against Septum secundum" ↓ "Functional closure: within hours" ↓ "Anatomic closure: weeks-months" ↓ Green box: "Adult: FOSSA OVALIS" COLUMN 2 - DUCTUS ARTERIOSUS: Box: "↑ pO2 → smooth muscle contraction + ↓ PGE2" ↓ "Functional closure: 10-15 hours" ↓ "Anatomic closure: 1-3 months" ↓ Green box: "Adult: LIGAMENTUM ARTERIOSUM" COLUMN 3 - DUCTUS VENOSUS: Box: "Sphincter contraction → umbilical flow ceases" ↓ "Functional closure: shortly after cord clamping" ↓ Green box: "Adult: LIGAMENTUM VENOSUM" Below all three columns, a red warning box: "CLINICAL NOTE: Failure to close → Congenital Heart Disease • PDA (Patent Ductus Arteriosus): treat with Indomethacin or surgery • PFO (Patent Foramen Ovale): paradoxical embolism, cryptogenic stroke • PPHN: persistent right-to-left shunting → neonatal hypoxia" Also add a small side box: "Umbilical vessels: • Umbilical arteries → Medial umbilical ligaments • Umbilical vein → Ligamentum teres hepatis (Round ligament)" Title: "NEONATAL CIRCULATORY TRANSITION AT BIRTH" Large bold dark blue title at top. Professional medical education style.
Flowchart 1 - Fetal Blood Flow Pathway
Flowchart 2 - Changes at Birth (Neonatal Transition)
Quick Reference - The Complete Circuit in One Line:
Create a visual poster comparing fetal vs. neonatal circulation
A large, professionally designed medical education poster comparing FETAL vs NEONATAL circulation. Portrait orientation, clean white background with a dark navy blue header bar. TITLE at top in large bold white text on navy bar: "FETAL vs. NEONATAL CIRCULATION — A Comparative Overview" Subtitle in smaller white text: "Key structural and physiological differences before and after birth" The poster is divided into two main side-by-side columns separated by a vertical dividing line with a VS badge in the center. LEFT COLUMN — "FETAL CIRCULATION" with a warm coral/red header: - Small icon of a fetus/womb at top - Subheading: "Gas exchange via PLACENTA" - A simplified anatomical heart diagram showing: • Foramen ovale OPEN (arrow showing right-to-left flow, labeled "R→L shunt") • Ductus arteriosus OPEN between pulmonary trunk and descending aorta (labeled "bypasses lungs") • Ductus venosus illustrated as a vessel from umbilical vein to IVC (labeled "bypasses liver") • Umbilical vein entering from below (labeled "oxygenated, 80% sat") • Umbilical arteries exiting to placenta (labeled "deoxygenated, 58% sat") • Pulmonary vessels shown with "HIGH resistance — collapsed lungs" • Right ventricle wall slightly thicker than left - Color coding: red for oxygenated, blue for deoxygenated, purple for mixed - Key facts box below the diagram: "• Lungs: collapsed, fluid-filled, not functioning • Pulmonary vascular resistance: HIGH • Right atrial pressure > Left atrial pressure • 3 shunts active: DV, FO, DA • O2 source: Placenta (HbF — high O2 affinity) • Right ventricle wall = thicker (works harder) • ~10% blood reaches lungs only" RIGHT COLUMN — "NEONATAL CIRCULATION" with a cool teal/blue header: - Small icon of a newborn baby at top - Subheading: "Gas exchange via LUNGS" - A simplified anatomical heart diagram showing: • Foramen ovale CLOSED, labeled "→ Fossa ovalis" • Ligamentum arteriosum where DA was, labeled "DA closed → Lig. arteriosum" • Ligamentum venosum where DV was, labeled "DV closed → Lig. venosum" • Ligamentum teres hepatis labeled for umbilical vein remnant • Pulmonary vessels shown with "LOW resistance — lungs expanded" • Large pulmonary blood flow arrows • Left ventricle wall becomes thicker over weeks • No umbilical vessels (cord clamped) - Key facts box below the diagram: "• Lungs: expanded, air-filled, functioning • Pulmonary vascular resistance: LOW (drops at first breath) • Left atrial pressure > Right atrial pressure • All 3 shunts functionally closed • O2 source: Lungs (HbA gradually replaces HbF) • Left ventricle wall = thicker (by 1 month) • 100% blood reaches lungs" CENTER BOTTOM — a comparison table with 3 columns (Feature | Fetal | Neonatal): Rows: O2 source | Placenta | Lungs Lung function | Non-functional | Functional Pulmonary resistance | High | Low Foramen ovale | Open (R→L) | Closed (Fossa ovalis) Ductus arteriosus | Open | Closed (Lig. arteriosum) Ductus venosus | Open | Closed (Lig. venosum) Umbilical vessels | Active | Obliterated Dominant ventricle | Right | Left Haemoglobin type | HbF | HbA (gradual switch) Right vs Left BP | RA > LA | LA > RA BOTTOM FOOTER BAR in navy blue: "TRIGGER for transition: 1st breath → ↑ pO2 → DA closes | Cord clamp → ↓ RA pressure → FO closes | ↑ Pulmonary flow → ↑ LA pressure" And small text: "PDA = patent DA persists (treat with indomethacin) | PFO = patent FO in ~25% adults" Use professional medical illustration style, clean sans-serif fonts, clear color-coded anatomy, high contrast. Make it visually striking and suitable for medical students. Large A2 poster format feel.
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