A thin layer of tissue or plastic with many tiny holes in it, through which the process of dialysis takes place. In peritoneal dialysis, the patient’s peritoneum provides the dialysis membrane. For haemodialysis, the dialysis membrane is made of plastic. In each case, the membrane keeps the dialysis fluid separate from the blood (essential because dialysis fluid is toxic if it flows directly into the blood). However, the tiny holes in the membrane make it semi-permeable, allowing water and various substances to pass through it.