The kidneys are responsible for filtering out small waste molecules and maintaining the fluid balance in the bloodstream. When the kidneys no longer function properly, dialysis is a method of artificially cleaning the blood through the use of machinery instead. The blood is transferred from the body through a dialyser for cleaning and then back in to the body. The main principle involved for removing small waste products like urea is diffusion. In the dialyser, the blood is passed by a semipermeable membrane with a clean, dialysis solution on the other side of the membrane. The membrane allows urea and ions to diffuse through the membrane into the solution (from an area of high concentration to low concentration) while larger molecules like proteins and red blood cells cannot pass through the membrane. But excess water content also is removed from the blood and the diffusion of water (a solvent) through a membrane is osmosis. So both diffusion and osmosis are involved with dialysis but diffusion is what specifically removes urea from the blood.