At time of birth, two events are responsible for the functional adaptation of the body to postnatal life:
- Disruption of the placental circulation system
- Unfolding of the lungs with the first breaths
The supply of blood from the placenta via the venous duct into the baby's body is disrupted with the cutting through of the umbilical cord. Thereby the blood supply in the right cardiac atrium is decreased and the pressure in the right atrium is reduced. At the same time, through the first couple of breaths of the newborn, the pressure in the small circulatory system is massively attenuated (compare Dilation of the muscular arteries of the small circulation system). The result of these pressure changes in the body is a reduction of the blood flow via the arterial duct and through the foramen ovale and an increase of the blood flow through the lungs. The reflex closure of the arterial duct after the first breaths of the newborn and the elevation of the pressure in the large circulation system are subsidiary mechanisms.
With the cutting of the umbilical cord, the placental low pressure area falls away.