Foetal circulation

[Ref: BL8:p264-268; WG21:p631-632]

• Systemic and pulmonary circulations in parallel, not in series.
• HbF = higher affinity to O2
• P50 value is 18-20mmHg

From umbilical vein to liver

About half of oxygenated umbilical venous blood goes through liver

The other half bypass liver
* Goes into IVC via ductus venosus
* Joins the venous blood from lower limbs
* Then hepatic vein blood join IVC

From liver to ventricles

The streams of blood stay mostly separate in IVC

Blood with mostly umbilical venous blood
--> Passes through the right atrium
(then shunted through foramen ovale)
--> Enters left atrium, then left ventricle

Rest of the IVC blood joins the SVC blood
--> Passes through the right atrium
--> Enters the right ventricle

From the ventricles to aorta

Because of the large pulmonary resistance

• Only 1/10 of pulmonary artery blood goes through lungs
• The other 9/10 of pulmonary artery blood 
  --> Passes ductus arteriosus
  --> Enters aorta (distal to the origins of carotid and subclavian arteries)

LV output goes into ascending aorta

• Better oxygenated
• Most goes to head and upper limbs
• Rest then joins the blood from ductus arteriosus
  * Less well-oxygenated
  * Supplies placenta and the rest of body

 

Oxygen supply

[BL8:p266]

Umbilical vein

• SatO2 = 80%

Superior vena cava

• SatO2 = 25%

Inferior vena cava (IVC)

• SatO2 = 27%
• SatO2 = 67% (after ductus venosus)

Right ventricle

• SatO2 = ??50%

Ductus arteriosus

• SatO2 = 52%

Ascending aorta

• SatO2 = 62%

Descending aorta

• SatO2 = 58% (after ductus arteriosus)

Umbilical arteries

• SatO2 = 58%