Particle Clearance in the Intrathoracic Airways

Inhaled particles deposited in the intrathoracic airways are eleared by several mechanisms, resulting in two major clearance pathways partiele transport and absorption of dissolved material. Particle transport is directed either toward the larynx, from which they are swallowed into the gastrointestinal tract (GIT) or toward the lymphatic system. Material cleared by absorption is eliminated mainly by the blood. 

The rates and amounts of material cleared by each mechanism depend primarily on the site of particle deposition and the physieochemical properties of the material (1,2). The following discussion addresses the elearance of particles that do not dissolve readily. 

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The cilia are essential to move the layer of mucus. They have a diameter of about 0.25 pm and variable lengths between 5 and 50 pm, depending on their function and location. Cilia length is greatest in the eentral airways (traehea and main bronchi) and decreases toward the terminal bronehioli (12). The density of ciliated and seeretoiy cells per unit surface area also deereases toward the peripheral airways. 

Mucus behaves as a nonnewtonian fluid that is, the viseosity decreases with increased applied force. The measurement of mucus s viscosity and elasticity, therefore, depends on the given shear rate and stress. The viscoelastic properties of mucus in normal humans showed no great differences from those observed in normal dogs, but high variability within individuals (13). Yeates (14) measured 

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