Abstract

Hemodialysis is the most commonly-employed process for cleaning the blood of a patient with kidney failure. In hemodialysis, blood flows on one side of a semipermeable membrane and dialysate solution flows on the other side of the membrane. Today, the typical artificial kidney, or dialyzer, consists of approximately ten thousand cylindrical polymer membranes (“hollow fibers”) through which blood flows. The hollow fibers contain pores which are large enough to let toxins pass, but small enough to retain plasma proteins. Dialysate, a solution containing physiological concentrations of several key solutes, flows around the hollow fibers in a counter-current direction. Hemodialysis cleans the blood by a combination of diffusion, in which toxins move across the hollow fiber wall from high concentration (blood side) to low concentration (dialysate side), and convection, in which solutes are swept along with the fluid that moves through the membrane pores (from blood to dialysate) as a result of a pressure gradient. Small molecular weight waste products (<500 kDa) normally excreted in urine are transferred across the membrane primarily by diffusion, and flow down the drain with the dialysate. Excess water that accumulates between treatments is also removed, leading to convective removal of molecules in the so-called “middle molecule” range of 500 to ~15,000 Da. A typical hemodialysis regimen calls for three treatments per week, with each treatment lasting 3.5–4.0 hours. Approximately one million patients worldwide receive chronic hemodialysis today.

In this article, the process of hemodialysis, design requirements for dialyzers, and properties of currently available hemodialysis membranes, introducing terminology peculiar to the dialysis industry that may be unfamiliar to scientists in other fields are reviewed.