When kidneys fail, waste products such as urea and creatinine build up in the blood. The walls of the abdominal cavity are lined with a membrane called the peritoneum. The peritoneal membrane enclosing the digestive organs allows waste products and extra fluid to pass from the blood into the dialysate without letting the dialysate seep out of the abdominal area into the body. These wastes then leave the body when the dialysate is drained from the abdomen. Each cycle of filling and draining is called an exchange.
Many factors affect how much waste and extra fluid are removed from the blood.
Some factors cannot be controlled, such as the patient's capacity for dialysate and the permeability, or speed of diffusion, of the peritoneum. The dialysis dose can be increased by using a larger bag, but only within the limits of the person's abdominal capacity. Everyone's peritoneum filters wastes at a different rate. In some people, the peritoneum does not allow wastes to enter the dialysate efficiently enough to make PD feasible. Other factors that determine how efficiently a person's blood is filtered can be controlled.