Fetal Circulation

Introduction

Fetal circulation is specially adapted to serve prenatal needs - the lungs do not provide gas exchange in utero, so the fetus relies entirely on the placenta for oxygenation and nutrition. Three unique shunts allow blood to bypass the liver and lungs: the ductus venosus, the foramen ovale, and the ductus arteriosus.

Diagram

Fetal Circulation - showing all three shunts, oxygen saturation color-coded

Fig. 13.46 - The Developing Human (Moore & Persaud). Red = high oxygen; purple = medium oxygen; blue = poor oxygen.

Features of Fetal Circulation

1. Gas Exchange Organ: Placenta (not Lungs)

The placenta acts as the lung, kidney, and gut for the fetus
Fetal blood is oxygenated at the placenta via the extensive arteriocapillary-venous network within the chorionic villi
Maternal and fetal blood do NOT normally mix; gas exchange occurs across the placental membrane

2. Umbilical Vessels

1 umbilical vein: carries oxygenated blood (~80% saturated) from placenta to fetus
2 umbilical arteries: carry deoxygenated blood (~58% saturated) from fetus back to placenta
These are contained in the umbilical cord

The Three Fetal Shunts

Shunt 1: Ductus Venosus (Bypasses the Liver)

Highly oxygenated, nutrient-rich blood enters the fetus via the umbilical vein
On approaching the liver, approximately 50% passes directly into the ductus venosus - a short vessel connecting the umbilical vein to the inferior vena cava (IVC)
This blood bypasses the hepatic sinusoids entirely
The remaining 50% enters the portal vein and liver sinusoids, then drains into the IVC via hepatic veins
A physiologic sphincter near the umbilical vein regulates flow - during uterine contractions, it prevents cardiac overload

Shunt 2: Foramen Ovale (Right Atrium → Left Atrium, Bypasses Lungs)

Blood arriving in the IVC (now mixed, but still relatively well-oxygenated) enters the right atrium
The crista dividens (inferior border of septum secundum) deflects most of this blood through the foramen ovale directly into the left atrium
In the left atrium, it mixes with a small amount of poorly oxygenated pulmonary venous blood
Blood then flows: Left atrium → Left ventricle → Ascending aorta
Since the coronary and carotid arteries are the first branches off the ascending aorta, the heart and brain receive the best-oxygenated blood in the fetus

Shunt 3: Ductus Arteriosus (Pulmonary Trunk → Aorta, Bypasses Lungs)

Deoxygenated blood from the SVC returns to the right atrium
It mixes with a small amount remaining from the IVC stream and passes to the right ventricle → pulmonary trunk
Pulmonary vascular resistance is very HIGH in fetal life (lungs are fluid-filled and collapsed)
Therefore most of this blood bypasses the lungs via the ductus arteriosus - connecting the pulmonary trunk to the descending aorta
It mixes with blood from the proximal aorta
Blood in the descending aorta then supplies the lower body and returns to the placenta via the two umbilical arteries

Sites of Mixing of Oxygenated and Deoxygenated Blood (Exam Favourite)

Five sites where mixing occurs:

Site	Description
I. Liver	Umbilical venous blood mixes with portal venous blood
II. IVC	Mixes with deoxygenated blood from lower limbs, kidneys, pelvis
III. Right atrium	Mixes with deoxygenated blood from SVC (head/upper limbs)
IV. Left atrium	Mixes with small amount of pulmonary venous return
V. Descending aorta	Ductus arteriosus blood mixes with blood from proximal aorta

Circulatory Changes at Birth

At birth, two major events trigger circulatory transition:

Clamping of the umbilical cord - stops placental blood flow, drops right atrial pressure
First breath - lungs expand, pulmonary vascular resistance falls dramatically, pulmonary blood flow increases, raising left atrial pressure

Closure of Shunts

Fetal Structure	Trigger for Closure	Adult Remnant
Ductus venosus	Loss of umbilical blood flow	Ligamentum venosum
Foramen ovale	LA pressure > RA pressure; septum primum pressed against septum secundum	Fossa ovalis (functional closure at birth; anatomical by ~3 months)
Ductus arteriosus	Rise in PaO₂ (oxygen causes smooth muscle contraction); fall in prostaglandins	Ligamentum arteriosum (functional closure within days; anatomical by 12 weeks)
Umbilical vein	Cord clamping	Ligamentum teres hepatis (round ligament of liver)
Umbilical arteries	Cord clamping	Medial umbilical ligaments (proximal parts → superior vesical arteries)

Diagram: Neonatal Circulation After Birth

Neonatal circulation - all three shunts closed, adult pattern established

Fig. 13.47 - The Developing Human. After birth, pulmonary and systemic circulations become fully separated.

Clinical Correlates (Bonus Points)

Patent Ductus Arteriosus (PDA): Failure of DA to close, common in preterm neonates and in congenital rubella. Treated with indomethacin (PG synthesis inhibitor) or surgical ligation
Patent Foramen Ovale (PFO): Present in ~25% adults; usually asymptomatic but can cause paradoxical embolism
Persistent Fetal Circulation (Persistent Pulmonary Hypertension of Newborn - PPHN): High pulmonary vascular resistance persists after birth; shunts remain open; presents with cyanosis
Erythroblastosis fetalis: Anti-D antibodies cross the placenta (the only immunoglobulin that does so); causes hemolytic disease of the newborn

Summary Points for Exam

Umbilical vein carries oxygenated blood; umbilical arteries carry deoxygenated blood (one vein, two arteries - rule of odd is oxygenated)
Three shunts: ductus venosus (bypasses liver), foramen ovale (bypasses lungs - right to left), ductus arteriosus (bypasses lungs - pulmonary trunk to aorta)
Heart and brain get the best-oxygenated blood via ascending aorta
Liver gets intermediate oxygenation (receives both umbilical venous and portal blood)
Lower body and placenta receive the least oxygenated blood via descending aorta
At birth: lungs expand → PVR falls → LA pressure rises → foramen ovale closes; PaO₂ rises → ductus arteriosus constricts

Sources: The Developing Human: Clinically Oriented Embryology (Moore & Persaud); Langman's Medical Embryology

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