1. Fetal Circulation (Before Birth)
Definition:
Fetal circulation is a specialized blood flow system that allows oxygen and nutrients to be supplied from the placenta instead of the fetal lungs.
Key Features of Fetal Circulation:
- Oxygenated blood comes from the placenta (Not the lungs).
- Three fetal shunts bypass the lungs & liver.
- Higher oxygenated blood is directed to the brain & heart.
Pathway of Blood Flow in Fetal Circulation:
- Placenta → Umbilical Vein → Carries oxygenated blood to the fetus.
- Ductus Venosus → Bypasses liver, directs blood to the inferior vena cava (IVC).
- Right Atrium (RA) → Blood splits into two pathways:
- Foramen Ovale: Shunts blood from RA to Left Atrium (LA), bypassing the lungs.
- Right Ventricle (RV) → Pulmonary Artery → Ductus Arteriosus → Aorta, bypassing lungs.
- Aorta → Supplies oxygenated blood to fetal organs.
- Deoxygenated blood returns via Umbilical Arteries to Placenta.
Key Fetal Circulatory Shunts & Their Functions:
|
Shunt |
Function |
Closure After Birth |
|
Ductus Venosus |
Bypasses the liver |
Closes within 1 week |
|
Foramen Ovale |
Shunts blood RA → LA
(Bypasses lungs) |
Closes functionally at birth, anatomically by 3 months |
|
Ductus Arteriosus |
Shunts blood Pulmonary Artery → Aorta
(Bypasses lungs) |
Closes by 24–48 hours
(due to ↑ O₂ & ↓ Prostaglandins) |
2. Changes in Circulation at Birth
Triggering Factors for Neonatal Circulatory Changes:
- First Breath (Lung Expansion) → Reduces pulmonary resistance.
- Umbilical Cord Clamping → Stops placental circulation.
- Increased Oxygen Levels → Closes fetal shunts.
Major Cardiovascular Changes at Birth:
|
Before Birth (Fetal Circulation) |
After Birth (Neonatal Circulation) |
|
High pulmonary resistance |
Low pulmonary resistance (Lungs expand) |
|
Low systemic resistance |
High systemic resistance (Cord clamping) |
|
Foramen Ovale Open |
Foramen Ovale Closes |
|
Ductus Arteriosus Open |
Ductus Arteriosus Closes |
|
Ductus Venosus Open |
Ductus Venosus Closes |
Closure of Fetal Shunts:
- Foramen Ovale Closure → Due to increased LA pressure.
- Ductus Arteriosus Closure → Due to increased O₂ & decreased prostaglandins.
- Ductus Venosus Closure → Becomes ligamentum venosum.
3. Clinical Conditions Related to Fetal Circulation
1. Patent Ductus Arteriosus (PDA)
- Failure of ductus arteriosus to close → Persistent shunting of blood.
- Signs: Continuous machine-like murmur.
- Treatment: NSAIDs (Indomethacin) or surgical closure.
2. Patent Foramen Ovale (PFO)
- Foramen ovale remains open → Causes right-to-left shunting.
- Risk Factor for Stroke (Paradoxical Embolism).
3. Persistent Pulmonary Hypertension of the Newborn (PPHN)
- Failure of pulmonary resistance to decrease after birth.
- Causes: Meconium aspiration, Perinatal asphyxia.
- Management: Oxygen, Nitric Oxide (Pulmonary vasodilator).
Summary Table: Fetal Circulation & Neonatal Changes
|
Feature |
Fetal Circulation |
Neonatal Circulation |
|
Gas Exchange |
Placenta |
Lungs |
|
Main Oxygen Source |
Umbilical Vein |
Pulmonary Veins |
|
Ductus Arteriosus |
Open |
Closes in 1–2 days |
|
Foramen Ovale |
Open |
Closes functionally at birth |
|
Ductus Venosus |
Open |
Closes in 1 week |
Key Takeaways
- Fetal circulation relies on three shunts (Ductus Arteriosus, Foramen Ovale, Ductus Venosus).
- At birth, the lungs expand, resistance drops, and the shunts close.
- PDA & PFO can persist after birth, requiring medical intervention.
