Lungs and circulation

Adaptation to air breathing takes place over several hours in a term baby.

At birth, air replaces lung fluid and the oxygen levels within the alveoli begin to rise, this causes the capillaries surrounding the alveoli to relax, dilate and fill with blood. The blood begins to flow to the lungs and the blood pressure within the pulmonary circulation gradually falls.

In the term baby during the first day the pressure in the right atrium (one of the four heart chambers) falls below that of the left, this causes the foramen ovale (flap-like valve connecting the right and left atrium) to functionally close. The permanent closure of the foramen ovale takes several weeks in a term baby and however may be delayed in the preterm baby.

The ductus arteriosus constricts and functionally closes generally within the first 12 hours in the term baby permanent closure again takes longer. Closure of the ductus arteriosus may take several weeks in the preterm baby.