Mucociliary clearance rates at various levels in dog lungs.

Mucociliary transport rates were studied in fresh dog lungs and tracheas in vitro. In 8 lungs the maximal mucous transport rates averaged 1.6 mm per min in the distal bronchi, 4.0 mm per min in segmental bronchi, 8.3 mm per min in lobar bronchi, and 12.6 mm per min in the trachea. Drying, excessive moisture, contamination with blood, and stroking of the mucous membrane stopped mucociliary clearance. Transport of mucus was slowed on a longitudinal strip of trachea cut to one-tenth as wide as the freshly opened trachea. Clearance was slowed by punching holes at irregular intervals in the trachea to simulate bronchial orifices. Transport, therefore, might be slower in small bronchi because segments are narrow and interrupted by the many bronchial orifices that alter the geometry of the clearance path. Furthermore, despite a small increase in the depth of the mucous layer, an eightfold increase in velocity, and perhaps slight evaporation, the transport system could not cope with the fluid load resulting from ...