$(6)4.$ Dificien theaugh ar Oriendrical Sheil

Consider a bollow. fiber membrane twod as an imtificial kidncy to remove urea and other contuminants froen many uren. The diffusivity of uren within the membrane wall $($which is mosthy waver within a sporge$-$like polymer$)$ is $D_{AB}.$ The blood is noc prossurved, sa convetive flow through the Gber wall may be neglectod