Transit time, residence time, and the rate of approach to steady state for solute transport during peritoneal dialysis

Mean transit time (MTT) and mean residence time (MRT) for solutes of low and middle molecular mass passing a system by convective-diffusive transport were calculated for the system of blood capillaries and lymphatic vessels within a tissue layer and dialysis fluid surrounding the tissue, i.e., the system with possible inflow and outflow of solutes through its surface and through the source/sink distributed within it. The general formulas were derived for MTT and MRT for various directions of the net solute flow (e.g., absorption of the solute from dialysis fluid to blood or removal of the solute from blood to dialysis fluid). MRT was calculated for diffusive transport of small and middle molecular weight solutes, and was 20 min for absorption of small molecules from dialysis fluid; however, for middle molecules MRT was increased up to 50 min. MRT for transport from blood to dialysis fluid was considerably shorter than in the opposite direction, especially for thin tissue layers. Furthermore, numerical simulations showed that the rates at which the system approaches the steady state were shorter than their theoretical estimations by MRT for absorption from dialysis fluid. The analysis was possible due to the application of the distributed model of peritoneal dialysis, which represents the real geometric characteristics of the transport system.