Foetal Circulation - A Simple, Detailed Explanation

Why Is Foetal Circulation Different?

1. Bring oxygen-rich blood from the placenta to the foetal body
2. Bypass the lungs (since they are non-functional)
3. Partially bypass the liver (to reduce resistance and speed up circulation)

The 3 Special Shunts

Step-by-Step Blood Flow (Follow the Journey)

Step 1 - Oxygenated Blood Leaves the Placenta

• The placenta (not the lungs) oxygenates the foetal blood
• Blood leaving the placenta travels via the umbilical vein - this is the only vein in the body that carries oxygenated blood (the opposite of adult anatomy!)
• Oxygen saturation here: ~80% (Ganong's, p.614)

Step 2 - Ductus Venosus (Liver Bypass)

• As the umbilical vein approaches the liver, the blood splits:
  • ~50% goes through the ductus venosus - a direct shortcut into the inferior vena cava (IVC), bypassing the liver
  • ~50% enters the liver sinusoids and then joins the IVC via hepatic veins
• A physiological sphincter near the umbilical vein regulates this flow - it prevents the heart from being overloaded during uterine contractions
• After mixing in the IVC with deoxygenated blood from the lower limbs and abdomen, the saturation drops to about ~67%

Step 3 - Blood Enters the Right Atrium

• The IVC blood (mixed, but still relatively well-oxygenated) enters the right atrium
• The crista dividens (lower border of the septum secundum) acts like a traffic director:
  • It guides most of the IVC blood through the foramen ovale directly into the left atrium (the better-oxygenated blood is preferentially sent to the left side)
  • A small amount stays in the right atrium and mixes with poorly oxygenated blood from the superior vena cava (SVC) - blood returning from the head and arms

Step 4 - Foramen Ovale (Right-to-Left Atrial Shunt)

• The foramen ovale is a hole in the wall between the two atria
• It allows blood to jump from the right atrium → left atrium, completely bypassing the lungs
• This is kept open in foetal life because right atrial pressure > left atrial pressure (the lungs are collapsed, so very little blood returns from them to the left atrium)

Step 5 - Left Atrium → Left Ventricle → Ascending Aorta

• Blood in the left atrium mixes with the tiny amount returning from the (non-functioning) lungs
• It passes to the left ventricle, then pumps into the ascending aorta
• The first branches off the ascending aorta are the coronary arteries (heart muscle) and the carotid arteries (brain) - so the heart and brain receive the best-oxygenated blood in the foetus. Very smart design!

Step 6 - The Right Side: SVC Blood → Right Ventricle → Pulmonary Artery

• The deoxygenated blood from the SVC (head and arms) enters the right atrium, goes to the right ventricle, and is pumped into the pulmonary artery
• In a foetus, the pulmonary vascular resistance is very HIGH (lungs are fluid-filled and collapsed), so the pressure in the pulmonary artery is actually higher than in the aorta
• Result: most blood cannot enter the lungs and instead takes the shortcut...

Step 7 - Ductus Arteriosus (Lung Bypass)

• The ductus arteriosus connects the pulmonary artery to the descending aorta
• Most of the blood from the right ventricle flows through this duct into the descending aorta, completely bypassing the lungs
• This relatively desaturated blood now supplies the lower body and trunk of the foetus
• O2 saturation here: ~60%

Step 8 - Blood Returns to the Placenta

• From the descending aorta, blood flows into the two umbilical arteries (note: these are arteries but they carry deoxygenated blood - another reversal from adult anatomy!)
• The umbilical arteries wrap around the bladder and travel through the umbilical cord back to the placenta to be re-oxygenated

Where Does Mixing of Oxygenated and Deoxygenated Blood Occur?

Summary Schematic (Fetus vs Newborn vs Adult)

What Happens at Birth? (Circulatory Changes)

1. Lungs Expand → Pulmonary Resistance Falls

• The baby's first breath fills the lungs with air
• Pulmonary vascular resistance drops dramatically
• Blood now rushes into the lungs instead of bypassing them

2. Foramen Ovale Closes (Functionally)

• More blood now returns from the lungs to the left atrium, raising left atrial pressure
• Simultaneously, the umbilical cord is cut, removing the low-resistance placental circuit - right atrial pressure falls
• The pressure reversal (left > right) pushes the septum primum against the septum secundum, functionally closing the foramen ovale
• Permanent anatomical closure happens over weeks to months (the fused tissue becomes the fossa ovalis)

3. Ductus Arteriosus Closes

• The rise in arterial oxygen tension (PaO2) triggers muscular contraction of the ductus arteriosus wall
• Prostaglandins (especially PGE2), which kept the ductus open during foetal life, are now cleared by the lungs
• Functional closure occurs within hours; permanent fibrous closure forms the ligamentum arteriosum within weeks

4. Ductus Venosus Closes

• With cord clamping, blood flow through the umbilical vein stops
• The ductus venosus collapses and becomes the ligamentum venosum
• The umbilical vein becomes the ligamentum teres (round ligament of the liver)
• The umbilical arteries become the medial umbilical ligaments

Adult Remnants of Foetal Structures

Key Oxygen Saturation Values to Remember

Why Does the Foetus Tolerate Such Low O2 Saturations?

• Foetal haemoglobin (HbF) has a higher affinity for oxygen than adult HbG - it picks up oxygen more readily at the placental level
• The Bohr effect: maternal blood has higher CO2 (and thus lower pH), which causes maternal Hb to release oxygen more readily - this oxygen is then picked up by HbF
• Foetal tissues have a remarkable but poorly understood resistance to hypoxia (Ganong's, p.615)