A. Circulation in the foetus:
One of the most astonishing events in our body is what happens when a baby is born and starts to breathe!
As you may know/remember, during foetal life, the baby cannot breathe inside his mother’s pregnant uterus. That is not because he/she has no lungs (it has!) but there is no air inside the uterus and therefore not in the lungs.
The oxygen required for the development of the baby does not come from the lungs but from the placenta (i.e., from the mother).
In the first place, the oxygen is transported through the umbilical veins from the placenta to the foetal hart. This is the opposite of what happens in an adult body; here, oxygen is transported in a vein!
The oxygen-rich venous blood has to flow into the arterial system of the foetus and does this through two channels:
- the foramen ovale, an opening between the right and the left atrium
- the ductus arteriosus; a connection between the pulmonary artery and the aorta
In this way, oxygen-rich blood flows from the inferior vena cava, through the aorta, to all the organs in the foetal body.
When the baby is born, someone cuts the umbilical cord thereby stopping the blood flow from and to the placenta.
If this does not occur for some reason, no problem, as the placenta will also loosen from the uterus wall and be expelled from the uterus.
But this relatively sudden decrease in oxygen-rich venous blood will cause a decrease in oxygen in the born baby which will also decrease the oxygen concentration in the medulla oblongata in the baby brain.
Therefore, much more blood will now flow, from the right heart to the lungs, thereby decreasing the pressure in the right atrium.
And now you understand why the foramen ovale consists of actually two flaps overlapping each other; the left flap will now be pushed against the right septum, thereby occluding the hole.
In the case of the ductus arteriosus, something similar occurs; after birth, blood starts to flow in the opposite direction, from aorta to the pulmonary artery.
After a few hours, the artery will start to constrict, limiting this reverse flow until all flow stops after several hours. This is followed by gradual growth of fibrous tissue, thereby permanently closing this vessel.
