(378d) Convective-Diffusive Transport in Krogh Cylinders

The dynamics of drug delivery has been considered here. Previously and particularly for directed delivery, a stirred tank model has been used which appears to have worked well when compared to experiments. The stirred tank models do not include all the mass transfer resistances and towards that end we have used a distributed system called Krogh cylinder. The capillary network is broken down into cylindrical cells each containing a capillary and appropriate amount of extravascular tissue. An organ is made of several of these connected in parallel. The flow is considered to be two dimensional in the capillary and mainly one dimensional, radially outward, in the tissue, as obtained earlier from the solution of continuity and momentum balance. The first case studied is that of a reasonably large solute which is nonreactive. Significant amount of solute is lost, along with plasma, to the lymphatic system. This loss is mainly due to convection in the tissue. The second case is that of a reactive and adsorbing solute. The solution of the conservation of species shows that in the tissue where the reactive sites are made available, the penetration is very low and all due to convection even though the diffusivity stays above 10^-6 cm²/s. The result emphasizes convective mass transfer, in major disagreement with Krogh cylinder models available so far. It also makes it important in future to study the flow more closely to obtain higher amount of convection in keeping with non-quantitative discussions in physiology.