Diffusion in blood is of little consequence in the microcirculatory exchange of metabolites because of the extremely small dimensions and consequent high surface-to-volume ratio of blood capillaries. Diffusion is important in larger vessels where pathological transport of cells and molecules to the vessel wall can either result in transformation of the wall or release of materials that modify homogeneous reactions in surrounding blood. In artificial organs, diffusion is a major factor limiting removal of catabolites and other molecules from blood. Few measurements of diffusion in blood have been attempted, a notable exception being those of Colton in the 1980's. Blood is an opaque, complex fluid, the disposition of whose components is profoundly affected by flow. Meaningful diffusion data can be obtained only by measurement under conditions of flow similar to those of an intended application. Migration of cells across streamlines (forced diffusion) and ordinary diffusion of cells, augmented by cell-cell collision, are of interest, as is also the effect of these movements on the diffusion of continuous phase molecules. In the simplest case, cells can either impede molecular movement by blocking diffusive pathways or enhance it by rotating and thus carrying molecules from one place to another faster than would occur in single-phase diffusion.

We have previously described a microfluidic cell in which diffusion can be measured between coaxial streams, one or more of which can be blood. Preliminary measurements of albumin self-diffusion in a simple fluid using a dye label were reported, found to be close to previously reported values, and thus were claimed as proof that the cell was functioning as expected. In this paper we will report the apparent diffusivity of albumin measured from albumin transport between adjacently flowing layers of whole blood. Results may also be reported from measurements of bilirubin diffusion in blood. Bilirubin is a putative toxin in advanced liver disease. It binds to albumin and its removal by dialysis and plasma filtration has been reported. Studies of phosphate diffusion in blood will be outline. Phosphate exists in several forms, in both intra and excellular fluid, separated a substantial transport resistance. Phosphate transport in dialysis has never been fully explained.