Fetal Circulation

• During pregnancy, the fetal circulatory system works differently than after birth:

• The fetus is connected by the umbilical cord to the placenta, the organ that develops and implants in the mother's uterus during pregnancy.


• Through the blood vessels in the umbilical cord, the fetus receives all the necessary nutrition, oxygen, and life support from the mother through the placenta.


• Waste products and carbon dioxide from the fetus are sent back through the umbilical cord and placenta to the mother's circulation to be eliminated

• Blood from the mother enters the fetus through the vein in the umbilical cord. It goes to the liver and splits into three branches. The blood then reaches the inferior vena cava, a major vein connected to the heart.

Inside the fetal heart

• Blood enters the right atrium, the chamber on the upper right side of the heart. Most of the blood flows to the left side through a special fetal opening between the left and right atria, called the foramen ovale.


• Blood then passes into the left ventricle (lower chamber of the heart) and then to the aorta, (the large artery coming from the heart).


• From the aorta, blood is sent to the head and upper extremities. After circulating there, the blood returns to the right atrium of the heart through the superior vena cava.


• About one-third of the blood entering the right atrium does not flow through the foramen ovale, but, instead, stays in the right side of the heart, eventually flowing into the pulmonary artery.  


• Because the placenta does the work of exchanging oxygen (O₂) and carbon dioxide (CO₂) through the mother's circulation, the fetal lungs are not used for breathing. 


• Instead of blood flowing to the lungs to pick up oxygen and then flowing to the rest of the body, the fetal circulation shunts (bypasses) most of the blood away from the lungs. 


• In the fetus, blood is shunted from the pulmonary artery to the aorta through a connecting blood vessel called the ductus arteriosus.

Fetal versus adult hemoglobin

• Fetus has high percentage (75%) of haemoglobin F (HbF). 


• HbF has lower affinity 2,3-diphosphoglycerate (2,3-DPG).


• Fetus has a higher haemoglobin concentration (18g/dl-1) at birth.


• The oxyhaemaglobin dissociation curve for HbF is shifted to the left compared with adult haemoglobin due to a lower affinity for 2,3-DPG. This favours oxygen uptake in the placenta.
  
  

Changes at birth 

• Several cardiopulmonary adaptations occur which result in gas exchange being transferred from placenta to the lungs. 


• The umbilical vessels are obliterated when the cord is clamped externally.


• There is therefore a fall in blood flow through the IVC and the ductus venosus, the latter subsequently closes passively over the next 3-10 days.


• A dramatic fall in the pulmonary vascular resistance (PVR) occurs with lung expansion (opening up the pulmonary vessels).


• A reduction in hypoxic pulmonary vasoconstriction and stimulation of pulmonary stretch receptors contribute to this process.


• The increase in pulmonary blood flow leads to a large rise in pulmonary venous return to the left atrium.


• The left atrial pressure therefore exceeds the right atrial pressure. This reversal of pressure gradient across the atria allows the flap of the foramen ovale to push against the atrial septum and the atrial shunt is effectively closed. 


• Although the initial closure of the foramen ovale occurs within minutes to hours of birth, anatomical closure by tissue proliferation takes several days. As a result, all blood from the right atrium now passes into the right ventricle. 


• The ductus arteriosus constricts due to the high partial pressure of oxygen. The process is usually complete within 2 days after birth.


• Other changes over several weeks include a reduction in the thickness of the walls of right ventricle and the muscle layer of the pulmonary arterioles; and an increase in the left ventricular wall.