ABSTRACT
The alveolar epithelium and the capillary endothelium have a very high permeability to water, most gases and lipophilic substances. There is an effective barrier however for many hydrophilic substances of large molecular size and for ionic species. The alveolar type 1 cells have tight junctions, effectively limiting the penetration of molecules to those with a radius of less than 0.6 nm. Endothelial junctions are much larger, with gaps of the order of 4 to 6 nm. Clearance from the alveoli by passage across the epithelium bears an approximate inverse relationship to the molecular weight. The normal alveolar epithelium is almost totally impermeable to proteins and small solutes, for example the half-time for turnover of albumin between plasma and the alveolar compartment is of the order of 36 hours1. The microvascular endothelium, with its larger intercellular gaps, is far more permeable for all molecular sizes and there is normally an appreciable leak of protein into the systemic circulation.
