Fluid-filled bag following Continuous Ambulatory Peritoneal Dialysis (CAPD), an alternative form of dialysis to that performed on a kidney machine. In CAPD, fluid is infused directly into the peritoneal cavity via a plastic catheter. The purpose of both peritoneal dialysis and haemodialysis (performed on a kidney machine) are to remove waste products from body fluids of a person whose kidneys' function insufficiently. The same physical principle applies in both cases: waste solutes in the blood diffuse across a semipermeable membrane (in CAPD it is the peritoneal membrane) towards the lower solute concentrations of the dialysis fluid.