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.
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.