A thin sheet, or layer, of tissue that lines a body cavity or separates two parts of the body. A semipermeable membrane can act as a filter, allowing some particles to pass from one part of the body to another while keeping other particles in place. In hemodialysis, the artificial membrane in a dialyzer acts as the semipermeable membrane filtering waste products from the blood. In peritoneal dialysis, the peritoneum acts as the semipermeable membrane.