Fractal character of pulmonary microvascular permeability.

The pulmonary microvasculature offers a heterogeneous barrier to the motion of large solutes as they pass between blood and lymph. While this barrier has been approximated by a few discrete pathways or by statistical ensembles of many pathways, these descriptions only partly capture the structural and functional properties of the pulmonary microcirculation. The concept that this barrier may be a fractal object is explored. Endothelial cleft geometry displays scaling in junctional path length and self-similarity in its spatial organization. It is shown that a fractal cleft produces heterogeneous spaces capable of transporting water and macromolecules. Cleft location, size, and depth are characterized, in part, by a fractal dimension of approximately 0.8. The consequences for transport through a fractal barrier are then determined. Predicted sieving of macromolecules by a fractal barrier is found to be consistent with lung microvascular transport data. Nonlinear transport phenomena are one consequence of a barrier having a fractional dimension.