Fetal Circulation

Question 1

Describe the temporary structures that are in place in order to allow fetal circulation to take place

Answer 1

• the ductus venosus
  
  • -> connects the umbilical vein to the inferior vena cava
• the foramen ovale
  
  • -> opening between the right and left atria
• the ductus arteriosus
  
  • -> leads from the bifurcation of the pulmonary artery the descending aorta
• the hypogastric arteries
  
  • -> branch off from the internal iliac arteries and become the umbilical arteries when they enter the umbilical cord

Question 2

Describe the course of fetal circulation

Answer 2

• oxygenated blood from the placenta travels to the fetus in the umbilical vein
  - -> the umbilical vein divides into two branches
  - -> one that supplies the portal vein in the liver
  - -> the other anastomosing with the ductus venosus an joining the inferior vena cava
• most of the oxygenated blood that enters the right atrium passes across the foreman ovale to the left atrium which mixes with a very small amount of blood returning from the lungs from where it passes into the left ventricle via the bicuspid valve and then the aorta
• the head and other extremities receive approx 50% of this blood via the coronary and carotid arteries and the subclavian arteries respectively
• the rest of the blood travels down the descending aorta mixing with deoxygenated blood from the right ventricle via the ductus arteriosus
• deoxygenated blood collected from the head and upper parts of the body returns to the right atrium via the superior vena cava
• blood that has entered the right atrium from the superior vena cava enters at a different angle to the blood tat enters from the inferior vena cava and heads towards the foramen ovale
  - -> hence there are two distinct blood flows entering the right atrium
  - -> most of the lesser oxygenated blood entering the right atrium from the superior vena cava passes behind the flow of highly oxygenated blood going to the left atrium and enters the right ventricle via the tricuspid valve
  - -> there is a small amount of mixing where the two blood flows meet in the atrium
• from the right ventricle a little blood travels to the lungs in the pulmonary artery for their development
• however, most blood passes from the pulmonary artery through the ductus arteriosus into the descending aorta
• this blood although low in oxygen and nutrients is sufficient to supply the lower body of the fetus
• it is also by this means that deoxygenated blood travels back to the placenta via the internal iliac arteries which lead to the hypogastric arteries and ultimately to the umbilical arteries
• this circulation means that the fetus has a well-oxygenated an perfused head, brain and upper body compared to its lower extremities

Question 3

Describe the adaptation to extrauterine life

Answer 3

• the cessation of umbilical blood flow causes a cessation of flow in the ductus venosus and a fall in pressure in the right atrium
• as the baby takes its first breath, blood is drawn along the pulmonary artery and as a consequence pressure increases in the left atrium
  
  • -> this is due to the increased blood supply returning to it via the pulmonary veins
• the alteration of pressure between the two atria causes a mechanical closure of the foramen ovale
• in addition as the baby takes its first breath the lungs inflate, and there is a rapid fall in pulmonary vascular resistance of approx 80%, a slight reverse flow of oxygenated aortic blood along the ductus arteriosus and a rise in oxygen tension
  
  • -> this causes the smooth muscle in the walls of the ductus arteriosus to contract and constrict usually within 24 hours following birth

Question 4

What happens to the temporary structures as the structural changes in extrauterine life become permanent?

Answer 4

• umbilical vein -> ligamentum teres
• ductus venosus -> ligamentum venosum
• ductus arteriosus -> ligamentum arteriosum
• foramen ovale -> fossa ovalis
• hypogastric arteries -> obliterated hypogastric arteries except for the first few centimetres which remain open and are known as superior vesical arteries

Question 5

What does adaptation to extrauterine life also involve?

Answer 5

• maintenance of a nutritional state through the establishment of breastfeeding
• elimination of waste via the kidney and gastrointestinal system
• establishment of the portal and liver circulation
• temperature control
• communication developed through parent-baby interactions