Airway closure
A low lung volume may potentially
give rise to a problem. Locally lung volume may have diminished
to the extent that lung recoil pressure is nil. Further exhalation
will not lead to regional lung emptying, while lung compartments
higher up will still be able to empty. As expiration continues
the number of compartments that have reached minimum volume
increases in a vertical direction. Their weight will compress
lower lung compartments, so that the pleural pressure in compressed
areas will exceed atmospheric pressure. Small intrapulmonary
airways will no lunger be distended by their environment,
but instead be compressed. We are dealing with airway closure.
At low lung volumes, particularly in the case of a flaccid lung, local airway closure may bring alveolar ventilation to a halt. Blood that perfuses such alveoli will pass blood with a low oxygen and a high carbon dioxide content to the main circulation (shunt-like effect); due to the shape of the oxyhemoglobin dissociation curve it is impossible to compensate in well-ventilated compartments for the deficiency in the oxygen supply to the main circulation.
Conclusion
The neonate has a lung which is both flaccid and which functions at a low lung volume, at which airway closure occurs. This is thought to explain the low partial oxygen pressure observed in neonates. As the lungs and thorax become stiffer during normal growth, the shunt-like effect diminishes and oxygen pressures gradually attain adult values.