Comparison of Mucociliary Clearance Velocities in Human and Rat Lungs for Extrapolation Modeling

Mucociliary clearance velocities in bronchial airway generations were calculated in asymmetric multiple-path models of the bronchial tree by solving mass transport equations based on the assumption of conservation of mucus volume and normalized to measured tracheal mucus velocities of 5.5 mm/min for humans and 1.9 mm/min for rats. Average mucus velocities decreased in a roughly exponential fashion with increasing airway generation number in the human lung and displayed a distinct two-exponential relationship in the rat lung due to its asymmetric branching. In contrast to the human lung, mucus velocities in single airways of the rat lung exhibited a significant statistical relationship with airway diameters but not with generation numbers, illustrating the monopodial nature of the branching pattern in the rat lung. Retention curves reflecting the combined effects of deposition and clearance were computed for particle diameters of 0.1, 1 and 2 μm for resting breathing conditions. These retention curves indicated that ~10–15% of the particles initially deposited in the bronchial tree were still retained in the human lung after 24 h, while all particles in the rat bronchial tree were cleared after ~6–8 h.