Extrauterine adaptations in the newborn

The transition from fetal to neonatal life necessitates major physiological adjustments. In a matter of a few minutes, newborns who had previously been dependent on the placenta for exchange of gases must be able to perform the same task independently. Changes in circulation occur when the fetal bypass shunts close and blood begins to circulate normally. Lungs that had been filled with alveolar fluid must clear the fluid and adjust to the mechanics of breathing air. The pressure and concentration of blood gases and the binding properties of haemoglobin must also adjust, as well as the acid-base balance established by those factors. In addition, the newborn must gain the feedback mechanisms for respiratory control. Adaptations to each of these aspects of respiration occur during prenatal, natal and postnatal periods. Each stage is characterized by specific pulmonary physiology to which certain adjustments are made. This article discusses these adjustments from the concept of our understanding of both fetal animal research and clinical observations.