B18 - Foetal circulation

Question 1

Fetal circulation

(from physiology)

Answer 1

• The fetus receives the oxygenated blood through the v. Umbilicalis
• Most of the blood reaches the fetal liver, while a small part of it reaches the heart directly through the v. cava caudalis
• Finally blood goes from the liver → right ventricle
• Left and right atria communicate with each other through the foramen ovale
• In the fetus the left and right ventricles work as a parallelly coupled system, as most of the blood pumped out from the right ventricle circumvents the resistant lung tissue
• Blood goes through the ductus arteriosus → aorta and is then added to the systemic circulation
• Blood from aorta:
  
  • One third goes to the cranial part of the body
  • Rest is delivered to the caudal one
    
    • Half of this blood goes through the placenta
• After delivery:
  
  • Sudden increase of the pulmonary circulation sets in
  • The pulmonary resistance decreases
    
    • Because of the surfactant factors after the onset of breathing
  • More and more blood goes through the lung
  • Pressure decreases in the right atrium as compared to the left atrium
    
    • Leads to the closure of the foramen ovale = fossa ovale
    • Pressure decreases in both the a. pulmonaris and ductus arteriosus
  • Closes the ductus arteriosus = lig. arteriosum
• Muscles of the left ventricle develop rapidly and finally the serially coupled pulmonary and systemic circulation evolves

Question 2

How does the fetal circulation differ from the postnatal circulation

Answer 2

• The fetal circulation differs from the normal postnatal circulation, mainly because the lungs are not in use
  
  • The fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord

Question 3

Give the major shunts found in the fetal circulatory system

Answer 3

• For. ovale
  
  • ​Between right atrium and left atrium
  • A valve associated with this opening prevents backflow of blood
  • After birth: breathing increases aortic pressure → for. ovale close and becomes fossa ovalis
• Ductus arteriosus
  
  • ​Vessel that connects the pulmonary trunk → aorta
  • Most of the blood pumped from right ventricle to pulmonary trunk is diverted into the aorta
  • After birth: breathing decreases pressure in lungs → lungs and pulmonary vessels expand → smooth muscles in the wall of the ductus arteriosus constrict → closed
• Ductus venosus
  
  • ​Branch from v. umbilicalis
  • Allows the oxygenated blood from the placenta to bypass the fetal liver and go to the fetal heart
  • After birth: close slowly after umbilical cord is severed, allows v. porta to perforate all parts of the liver
• After birth:
  
  • For. ovale → fossa ovalis
  • Ductus arteriosus → lig. arteriosus
  • Ductus venosus → lig. venosus

Question 4

Draw the fetal circulation

Answer 4

1. A. umbilicalis (deoxygenated)
   
   • Nutrients and deoxygenated blood
   • Vena cava caudalis → placenta
2. V. umbilicalis (oxygenated)
   
   • Placenta → fetus
3. Ductus venosus (oxygenated)
   
   1. Continuation of the umbilical trunk
   2. Runs through the liver bypassing hepatic circulation → vena cava caudalis

• Vena cava caudalis
  
  • Receives umbilical blood from ductus venosus
  • ​Enters right atrium
• Aorta (oxygenated)
  
  • ​Delivered to fetal tissue

Question 5

Changes after birth

Answer 5

• V. umbilicalis → lig. teres hepatis
• A. umbilicalis → lig. teres hepatis
• Ductus venosus → lig. venosus
  
  • ​Allows vena porta to perfuse all parts of the liver
• Ductus arteriosus → lig. arteriosus
• For. ovale → fossa ovalis
  
  • ​Breathing increases aortic pressure → for. ovale close
  • Increased venous return from the lungs raises the pressure within the left atrium → for. ovale close