Disappearance of tracheobronchial fluid at birth in the lamb.

Bronchography was performed in lambs before, during and after the first breath. Swallowing of the contrast medium containing tracheobronchial fluid was observed before the first breath. Intratracheal pressure changes ranging up to 20 mmHg were observed during respiratory movements which did not open the alveoli. The lamb did not start breathing as long as the snout was covered although there was a direct connection between the trachea and atmospheric air through a tracheostomy. When the snout was uncovered a lamb in good condition drew its first breath and the spreading of the contrast material into the peripheral parts of the lungs was almost explosive. The diameter of the tracheobronchial tree widened as a result of the initiation of respiration. It is suggested that both swallowing in the pharynx and absorption into the blood circulation below the larynx play an essential role in the disappearance of the tracheobronchial fluid. Intrauterine aspiration seems less probable in the light of these observations.