Radial bulk diffusion into heterogeneous tissue

An expression is derived which describes the transient distribution of solute diffusing into heterogeneous tissue from a fully-stirred cylindrical region in which there has been a step change in solute concentration. This is envisaged as a model for the uptake of drugs by vessel walls, although the same approach has been extended for estimating the mode of total uptake of substances by annular specimens of tissue. Tissue is regarded as effectively composed of two phases—an extracellular (continuous) phase, similar to water, and a dispersed phase comprising cells of irregular profile. In each phase the relevant mode of uptake is taken as bulk diffusion rather than the permeation of a surface membrane.