G1v / 15A02: Describe the foetal circulation and changes that occur at birth

15A02: Exam Report

Describe the foetal circulation and the changes that occur at birth.

55% of candidates passed this question.

This topic is well covered in standard texts and has been asked previously. Better answers displayed knowledge of the key concepts, such as the parallel circulations in the foetus and preferential flow of better oxygenated blood to the brain and upper limbs.

Some candidates spent time on the maternal and placental circulations which were not required to answer the question asked.

G1v / 15A02: Describe the foetal circulation and changes that occur at birth

Foetus: conception → birth

Foetal Circulation

Parallel System

  • 1 umb v (oxygenated)

60% Ductus Venosus (#Shunt 1) bypasses liver & IVC   ///               40% Portal V → liver/GI

  • RA

  • Foramen Ovale (#Shunt 2)

  • Ascending Aorta

  • Coronary + Cerebral circulation

  • SVC (deoxygenated)

  • RV

  • Pulmonary trunk

5% lungs

high PVR because

↓PaO2 & collapsed  

////        95% Ductus Arteriosus (Shunt #3)

              Desc Aorta                           

↓            

            Iliac Arteries

 ↓

↓2 Umbilical A

 ↓

Placenta

Placenta

Circulatory Changes at Birth

1. Removal of Low Resistance Placenta

  • Clamped umbilical cord
  • ↑↑SVR
  • ↑LVEDP
  • ↑LAP

2. Inflation of lungs

  • Passage through birth canal squeezes fluid from lungs
  • Asphyxia → activation of central + peripheral ChemoR
  • First breath
  • ↓PVR
  • ↑Pulmonary BF
  • ↓RVEDP

3. Closure Ductus Arteriosus

  • ↑PaO2
  • ↓Local prsostaglandin formation
  • DA closure in 15hrs (local constriction of T. Media smooth m. in DA)

4. Foramen Ovale closure

  • ↑Pulmonary BF
  • ↑LVEDP
  • ↑LAP
  • LAP > RAP = closure F. Ovale

5. Closure Ductus Venosus

  • Loss of venous return from Umbilical vessels
  • Closes in 1st week of life
  • Open @ birth → allows umb v. catheterisation
  • Remnant KA Ligamentum Venosum

Infant ventricles = smaller + ↓ compliance = HR 110 – 150bpm