General review of tracheobronchial clearance.

The human tracheobronchial tree possesses several mechanisms for keeping itself clean and sterile. Mucociliary clearance results from the beating action of cilia lining the conducting airways and propelling the overlying mucus cephalad. Locally produced biological debris and inhaled, insoluble material are swept with the mucus and removed from healthy lungs within one day. Cough augments the often impaired mucociliary clearance of patients with excessive secretions. Cough is limited in is efficacy to the proximal airways. Energy transfer from airflow to mucus transport in airways lined with excessive secretions has been put forward as a third mechanism (two-phase gas-liquid flow) for the removal of lung secretions. Other mechanisms that have been proposed for clearance of lung secretions are: 'milking', 'squeezing' and peristalsis.