The Fate of Particles Deposited in the Intrapulmonary Conducting Airways

Abstract

ABSTRACT

Inhaled particles deposited in the intrapulmonary conducting airways may be retained considerably longer than 24 hours. We studied structure and function of the 2–phase coating (gel, sol) of the wall of these airways with respect to retention and clearance of such particles. The airway surface tension was determined In vivo in sheep and horse tracheas from the spreading properties of test fluid droplets, using video bronchoscopy. Stereological analyses by light and electron microscopy were performed on hamster lungs. Immediately following inhalation, we found a number of 6 μm polystyrene particles submerged in the sol-phase and coated with surfactant. Of these particles 14% were still retained 24 hours later. The gel and sol phases were separated by a surfactant bilayer. We measured a surface tension of 31 dyn/cm in vivo, at the gel–air surface, in the trachea of sheep and horse. In vitro experiments showed that surfactant promotes the immersion of particles into the aqueous subphase. Based on our findings, we postulate, (1) that after deposition the particles are displaced into the gel–phase (initial retention) from where they may be cleared by mucociliary activity (fast clearance phase) and (2) that some particles may be further displaced into the sol–phase (prolonged retention), from where they will be cleared by airway macrophages (slow clearance phase).