In the fetal circulation all the blood flows through the placenta which supplies the fetus with food and oxygen. Only five to ten percent of the fetal blood moves through the nonfunctioning lungs, just enough to keep them alive.
The diagram on the next page shows the fetal circulation:
THE CIRCULATORY SYSTEM:
Adult and Fetal
(figure available in print form)
Blood containing oxygen and dissolved food returns from the placenta through the umbilical vein.
It enters the right atrium via the inferior vena cava. Instead of then being pumped into the right ventricle, it moves through a hole between the two atria called the
and ends up in the aorta without going to the lungs.
Blood coming from the head in the superior venacava enters the right atrium also and then the right ventricle but on its way out the pulmonary artery it is shunted over a bypass called the
into the aorta so it also avoids the lungs.
What happens at birth? As soon as the neonate draws its first breath and the cord is cut, pulmonary blood flow increases by two hundred percent (Crelin, 1973) and the pressure of the blood returning from the lungs to the left atrium closes the valve of the foramen ovale.
With the increased oxygen pressure the ductus arteriosus also closes functionally and eventually is converted into a ligament. Occasionally the ductus arteriosus and foramen ovale remain open in premature babies who are suffering from hypoxia.