Fetal Physiology

Question 1

Describe the foetal circulation, especially in how it differs to an adult’s

Answer 1

• Blood from the placenta enters the large umbilical vein and passes mostly (some diverts to the liver) through the ductus venosus to the IVC. It mostly shoots across the right atrium through foramen ovale into the left side of the heart and to the periphery
• From the right heart, relatively deoxygenated blood goes through the pulmonary artery and a small amount of blood is diverted to the lungs, but most goes through the ductus arteriosus to the descending aorta then umbilical arteries to the placenta

Question 2

Describe the distribution of the foetal-placental circulation. Which parts will be conserved if supply is low? Which are optional?

Answer 2

• Placenta 40%, brain 30%, hindquarters 18%, lungs 4%
• Brain, heart, lung > liver, gut, kidney > bone, muscle

Question 3

What changes occur in the foetal circulation post-birth?

Answer 3

• Foramen ovale closes
• Ductus arteriousus closes
• Ductus venosus closes

Mechanisms: loss of placental blood flow (increase in systemic resistance), lungs inflate (decrease in pulmonary resistance). Resultant closure of FO occurs within minutes (sealing takes months to 2 years). After birth, DA constricts (increased O2, decreased PGE2) and in 1-8 days flow ceases. DV closes soon after birth in a similar way (unknown mechanism)

Question 4

At what gestation has foetal alveolar circulation developed enough for gas exchange?

Answer 4

22-24 weeks

Question 5

How is foetal lung growth facilitated by the foetus?

Answer 5

• Foetal lung liquid (produced in alveoli)
  
  • Switched off by adrenaline
• Foetal breathing (small diaphragmatic contraction)
  
  • Marker of distress if gasping

Question 6

What facilitates the first breath of a baby?

Answer 6

• Surfactant (can be deficient if premature)
• Cooling
• pH/pO2/pCO2
• Pain
• Position
• Auditory stimuli
• Visual stimuli