How does dialysis work?

In order to understand what diffusion is, it may help to picture a simple experiment from science class: An empty glass box is separated into two halves by a thin wall. The wall has fine pores (tiny holes) in it. A clear fluid is then poured into the right half, while a colored fluid is poured into the left half. What happens next? The colored particles in the left half will gradually move (diffuse) through the pores in the permeable wall into the clear fluid in the right half. Eventually, both fluids will be the same color. The number of colored particles in the water (the concentration) is then the same on both sides of the wall.

The next step is to empty the fluid from the right half and replace it with fresh, clear fluid. Again, colored particles will move (diffuse) from the left half to right the right half and spread evenly throughout the entire box. If you repeat this procedure several times, eventually almost no color will be left in the half that was originally colored.

Dialysis works in a similar way: Blood is like the colored fluid. It contains many substances that are dissolved into particles. In dialysis, it is brought very close to a clear liquid (dialysis fluid). The blood and dialysis fluid are separated only by a thin wall, called a semipermeable membrane. This membrane allows particles that the body needs to get rid of to pass through it, but doesn’t let important parts of the blood (e.g. blood cells) pass through.

In hemodialysis, the blood is cleaned outside the body. The treatment is typically carried out in a dialysis center. The procedure involves removing blood from a blood vessel, usually in a forearm, passing it through the dialysis device and then transporting it back into the body.

Inside the dialysis device, the blood flows through small tubes. These are made of semipermeable membranes and are surrounded by dialysis fluid. The dialysis fluid flows in the opposite direction to the blood. This is the best way to remove harmful substances, waste products and excess water from the blood and get rid of them together with the dialysis fluid.

Some dialysis methods use pressure to "push" water out of the blood. When the water is removed, any harmful substances dissolved in the water are removed at the same time. The technical term for this procedure is convection. Some hemodialysis devices can use convection as well as diffusion.

Each session lasts about four to five hours. During this time, all the blood in the body is pumped through the dialysis device several times. Afterwards, the blood is clean enough. In Germany, hemodialysis is typically carried out three to four times per week. It is best to talk to your doctor if you would rather come more often but for shorter individual sessions. It is possible that you could try out an individualized schedule.

In peritoneal dialysis the cleaning of the blood takes place inside the abdominal cavity, so it is done inside the body instead of outside.

You use a catheter to put the dialysis fluid into your abdominal cavity yourself. The blood and dialysis fluid are separated only by a thin layer of skin, called the peritoneum. The peritoneum is well-supplied with blood and covers the organs like the small and large intestines. The peritoneum is used as a semipermeable membrane in peritoneal dialysis: Harmful substances can then move from the blood vessels and into the dialysis fluid.

The dialysis fluid contains sugar to also draw water out of the blood. That is done through osmosis. The principle of osmosis is an effect you may know from everyday life, for example if you've ever sprinkled sugar on top of a bowl of freshly cut fruit. The sugar "draws" water out of the fruit, and eventually the bits of fruit are swimming in fruit juice. This same principle is used in peritoneal dialysis to remove excess water from the body.

After a few hours, you drain the dialysis fluid from your abdominal cavity again and then usually replace it with fresh dialysis fluid right away so that the blood is constantly being cleaned. Alternatively, some people use a device that they remained connected to overnight called a cycler to replace the dialysis fluid.