We used aerosol boluses to study convective gas mixing in the lung of four healthy subjects on the ground (1 G) and during short periods of microgravity (µG) and hypergravity (~1.6 G). Boluses of 0.5-, 1-, and 2-µm-diameter particles were inhaled at different points in an inspiration from residual volume to 1 liter above functional residual capacity. The volume of air inhaled after the bolus [the penetration volume (V_p)] ranged from 150 to 1,500 ml. Aerosol concentration and flow rate were continuously measured at the mouth. The dispersion, deposition, and position of the bolus in the expired gas were calculated from these data. For each particle size, both bolus dispersion and deposition increased with V_p and were gravity dependent, with the largest dispersion and deposition occurring for the largest G level. Whereas intrinsic particle motions (diffusion, sedimentation, inertia) did not influence dispersion at shallow depths, we found that sedimentation significantly affected dispersion in the distal part of the lung (V_p >500 ml). For 0.5-µm-diameter particles for which sedimentation velocity is low, the differences between dispersion in µG and 1 G likely reflect the differences in gravitational convective inhomogeneity of ventilation between µG and 1 G.

gravity; convective mixing; aerosol bolus; ventilation inhomogeneity

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