How does dialysis work?

When the kidneys fail, dialysis can do their job of removing harmful substances and excess water from the body. This is done with the help of technology that makes use of the physical principles of diffusion, convection and osmotic pressure. The two main types of dialysis are hemodialysis and peritoneal dialysis.

What is diffusion?

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.

Illustration: Principle of diffusion - as described in the articlePrinciple of diffusion

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, or dialysate) that contains few particles. 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.

What effect do osmotic pressure and convection have?

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. Here, the dialysis fluid contains sugar to draw water out of the blood.

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.

How does hemodialysis work?

In hemodialysis, the blood is cleaned outside of the body (extracorporeal). The treatment usually takes place in a dialysis center (a clinical facility that specializes in dialysis). 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. The technical term for this is counter-current flow. 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.

Hemodialysis usually takes around 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 usually done three times a week.


Illustration: In hemodialysis, the blood is cleaned outside the body – as described in the articleIn hemodialysis, the blood is cleaned outside the body

How does peritoneal dialysis work?

In peritoneal dialysis, the blood isn't cleaned outside the body, but inside the body, in the abdominal cavity (the hollow space surrounding the organs in the abdomen). The lining of the abdominal cavity (the peritoneum) serves as a semipermeable membrane. The peritoneum is well-supplied with blood and covers the organs like the small and large intestines.

Before starting peritoneal dialysis, a catheter is inserted into the abdominal cavity. The catheter allows you to put the dialysis fluid into your abdomen yourself. Any harmful substances will then diffuse into the dialysis fluid from the blood vessels in the peritoneum. Because the dialysis fluid contains sugar or substances similar to sugar, excess water is also removed from the blood by osmosis.

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. At night, you can use a device called a cycler to automatically drain and replace the dialysis fluid.


Illustration: In peritoneal dialysis, dialysis fluid is passed into the abdominal cavity – as described in the informationIn peritoneal dialysis, dialysis fluid is passed into the abdominal cavity