Haemodialysis versus peritoneal dialysis: how each works

Waste products and excess fluid build up when the kidneys fail, so dialysis steps in to remove them artificially.

In haemodialysis ("haemo" means blood), blood is pumped out of the body into a machine that contains a dialyser, an artificial filter. Inside it, blood runs along one side of a semipermeable membrane (a thin barrier that only some substances can cross), while dialysis fluid (dialysate) runs along the other side. Waste such as urea moves across by diffusion (movement from a higher to a lower concentration), and ultrafiltration (pressure-driven removal of water) takes off the excess fluid. The cleaned blood then goes back to the patient. Blood has to leave and return during every session, so the patient needs vascular access (a connection to a blood vessel, such as a fistula), usually an arteriovenous fistula, a surgical join of an artery to a vein, often in the arm. A typical pattern is about four hours, three times a week, usually in hospital, although home haemodialysis is also an option.

In peritoneal dialysis the filter is inside the patient. The peritoneum is the thin lining of the peritoneal cavity, the space inside the abdomen. Because it contains many small blood vessels, it works as a natural semipermeable membrane. A catheter (a soft tube) is placed through the abdominal wall, and dialysis fluid runs in from a bag. After a few hours, waste and excess water have crossed from the blood into the fluid, which is then drained away. The blood never leaves the body, so no vascular access is needed.

Peritoneal dialysis can be run overnight by a machine called a cycler, yet the machine does not change the principle: the blood stays inside the body, whereas in haemodialysis the blood is diverted out of it.