Which statement correctly describes the relation of blood and dialysate compartments in a dialyzer during operation?

• A. They are connected by a shared passage
• B. They are separated by a semipermeable membrane
• C. The dialysate passes through the blood
• D. The blood passes through the dialysate

Answer: (B)

The key idea is that blood and dialysate are kept in separate channels by a semipermeable membrane. This membrane lets small solutes and water move between the blood and dialysate, enabling diffusion of waste products from the blood into the dialysate and removal of extra fluid, while preventing blood cells and larger proteins from crossing over. There’s no direct mixing of blood with dialysate because the membrane acts as a barrier. Saying the dialysate passes through the blood or the blood passes through the dialysate would imply mixing, which isn’t how a dialyzer works.