The relative compliance of the lung of the newborn is similar to that of the adult. Measurement of lung and chest wall compliance suggest that the newborn should have a lower per cent RV and a lower per cent FRC than the adult. In fact, the per cent FRC in the newborn is equal to the adult's and the infant's per cent RV is slightly greater. FRC decreases when intercostal muscle activity is inhibited during active sleep. The reason for elevated RV is not entirely clear, but it could result from airway closure during an active expiration as part of crying vital capacity maneuver.
Resistance is inversely proportional to lung volume.
Thus, airway resistance is lower during inspiration than during expiration because of the effects of changes in intrapleural pressure on airway diameter. During inspiration, pleural pressure become negative, a distending pressure is applied across the lung. This distending pressure increases airway diameter as well as alveolar diameter and decreases the resistance to gas flow. During expiration, pleural pressure increases and airways are compressed. When intrapleural pressure is high during active expiration, airways may collapse and gas may be trapped in the lung.