ater molecules. Local differences in concentration disappear as the solution reaches diffusion equilibrium. Required time for mixing depends on solute molecular size, temperature, and length of diffusion path or container size. The dialyzer can be viewed as a triple laminated solution with flowing blood and dialysate with stationary membrane water interposed between the two streams. In the perfect dialyzer, diffusion equilibrium would result in the blood and dialysate streams during passage through the device; single pass blood and dialysate flows at equal velocity, urea concentration equilibrium, would be reached between the blood inlet and dialysate outlet streams and all urea contained in the inflowing bloodstream would be transferred to the outflowing dialysate stream. This efficiency level in not reached; efficiency is about 50 percent (5).
The artificial kidney is to provide the controllable transfer of solutes and water across a semipermeable membrane separating the flowing blood and dialysate streams. These transfer processes are dialysis and ultrafiltration. The device is a dialyzer. Structural elements of all dialyzers are the blood compartment, the membrane, and the dialysate compartment. Design and material requirements must be met in the construction of dialyzers. Blood-contacting surfaces and flow geometry need to be relatively nonthrombogenic. Materials need to be nontoxic and free of leachable toxic substances. Blood and dialysate flow paths need to result in the distribution of blood and dialysate in thin films uniformly flowing across the membrane; the ratio of membrane surface area to contained volume needs to be high. Hydraulic resistance to blood and dialysate flow must be low and predictable. Designs attempting to achieve these goals include the coil, parallel plate, and hollow fiber configurations (5).
Care issues within the process include facility staffing patterns, numbers of staff physici...