Dialysis

Dialysis is a treatment for kidney failure, meaning a kidney that has stopped filtering. It does from the outside what the kidney no longer does: pull the waste products and the extra fluid out of the blood.

The principle is simple. The blood sits on one side of a semipermeable membrane, a sheet with pores that lets some things through and holds others back. Because dissolved particles move by diffusion, that is, from where they are more concentrated toward where they are less concentrated, the waste crosses to the other side on its own. A fine mesh separates two fluids and lets particles pass from the side with more to the side with less.

There are two ways to set up that membrane. In hemodialysis, the membrane separates the blood from the dialysis fluid. In peritoneal dialysis, the membrane is the peritoneum, the lining that covers the inside of the abdomen, and the waste diffuses across it.