| Abstract: | Widdicombe, John. Airway and alveolar permeability andsurface liquid thickness: theory. J. Appl.Physiol. 82(1): 3-12, 1997. The thickness ofairway surface liquid (ASL) can be calculated as the ratio of thepermeability coefficient of an absorbed inert tracer to the percentagerate in which it decreases in content in the airway lumen. Thepercentage clearance of radiolabeled diethylenetriaminepentaacetic acid(DTPA) from human airways or lungs has been measured many times, with amean value of 1.04 ± 0.25 (SD) %/min. Rates of clearance fromanimal lungs of most species give values of the same order, althoughthey are lower in the sheep and higher in the dog. Permeabilitycoefficients have not been measured simultaneously with percentageclearances and not at all for human tissues. Values for mannitol andsucrose, of which the former gives a permeability coefficient ~25%greater than that for sucrose and DTPA in airway tubes and isolatedmucosal sheets from experimental animals, give a mean of ~7.1 × 10 7 cm/s. This correspondsto thicknesses of ASL of ~20-150 µm for various species. Theassumptions underlying this estimate are discussed. It is concludedthat ASL thickness in vivo may be considerably greater than in vitromeasurements involving rapid freezing of the airway wall. Estimates ofalveolar permeability suggest that either it is very considerably lowerthan that of the airway epithelium, that methods to measure alveolarpermeability mainly reflect airway permeability, or both. |
