Dialysis is a scientific term for the movement of substances across a membrane by a process called diffusion.
If a great deal of a substance is in one place, some of that substance will tend to move out to where the substance is not present.
If we gently add a teaspoon of milk to a cup of tea, the milk will diffuse out with time to all areas of the cup.
In dialysis, we add a barrier or membrane between two liquids.
On one side of the membrane is a liquid with a high concentration of a substance.
On the other side of the membrane is a liquid without that substance.
If the membrane has small holes or pores, the substance on one side of the membrane will travel or diffuse across the membrane to the other side until the amount on each side is equal.
Dialysis membranes are said to be semipermeable.
They allow some small substances to pass through while large substances cannot fit through the holes. In medical dialysis treatments, one liquid is a patients blood.
On the other side of the semipermeable membrane, is another liquid called dialysate.
Substances in the blood, if they are small enough, will move through the membrane into the dialysate.
If the dialysate is then discarded and new dialysate is added, more of the substance can be removed.
Removal of substances depends on the size of the substance, the electrical charge of the substance, and also the difference in the amount of the substance on one side of the membrane compared to the other.