Cleaning the Blood: How does dialysis work?

The kidneys have lots of jobs, but one of the most important is keeping the blood clean. Packed inside these bean-shaped organs are hundreds of microscopic filter systems. Blood passes in through the renal arteries at high pressure, rushing into balls of leaky blood vessels. Fluid and waste squeeze out of the bloodstream and into tiny tubes called nephrons. As the fluid passes through these tubes the body reabsorbs useful molecules. The rest forms urine and is removed via the bladder.

This system is sensitive to damage. High blood pressure, diabetes and recurrent infection can stop the filters working. Should this happen then dialysis can take over the job, allowing the kidneys to heal or keeping the blood clean until a transplant becomes available.

First, doctors widen the blood vessels in the arm by creating an arteriovenous fistula, or by implanting a graft. This joins an artery to a vein, allowing blood to flow quickly in and out of a haemodialysis machine. These machines are essentially artificial kidneys, and they clean the blood in almost the same way.

Blood enters the machine and passes over a semi-permeable membrane. Dialysis fluid passes in the opposite direction on the other side. The concentration of waste is higher in the blood than it is in the fluid, so the molecules diffuse across. However, the holes in the membrane are too small for blood cells, so they remain in the bloodstream and return to the arm. The whole process takes around four hours and patients need to repeat it three times a week.