Volume-pressure relationships of the thorax and lung in the newborn

The volume-pressure relationships of the respiratory apparatus have been studied in the dog fetus and in newborn and adult dogs. The mechanical changes produced when air first enters the lung have been described. In the newborn the lung-thorax compliance per unit vital capacity, taken over an extended range, is larger than in the adult: it is smaller, over the tidal volume range, during the first few days of life. The compliance of the thorax, particularly high at birth, decreases then progressively. The compliance of the lung after some days of life is similar to that of the adult. The relaxation volume and the functional residual capacity per total lung capacity increase significantly during growth. The lung retraction force, at relaxation volume, is smaller in the newborn than in the adult.