The breathing of half micron aerosols-III dispersion of particles in the respiratory tract.

Experiments on the inhalation of 0·5 μm dia. particles are described. For a single inhalation of aerosol the effects of separately varying tidal volume and expiratory reserve volume were studied. The results are consistent with the idea that mechanical mixing within the tidal air and between the tidal and reserve air takes place only in the anatomical dead space of the lungs. Experiments with constant reserve volume on the inhalation of a bolus or slug of aerosol, followed by clean air to complete the tidal inhalation, showed that the depth of penetration of the aerosol into the reserve air was not much less than when aerosol was inhaled for the entire tidal inhalation. This indicates that the first part of tidal air, which penetrates most deeply, is not much affected by the volume subsequently inhaled, the reason being that it is in the alveolated airways where there is no mechanical mixing of air. This supports the 'first in, last out' theory of breathing, subject to some spreading in volume. A compartment theory of the dispersion of particles in the lungs, based on tidal, reserve and residual volumes, enables six transfer coefficients of aerosol particles to be evaluated from wash-in and wash-out experiments. With these coefficients it is possible to predict the behaviour of aerosols during various breathing manoeuvres. The theory is independent of assumptions about the mechanisms of particle exchange. Evaluation of the transfer coefficients would identify differences between subjects and could be of clinical interest