Final answer:
An osmosis chamber uses a semipermeable membrane to allow selective passage of water, which defines osmosis. In hemodialysis, the semipermeable membrane permits urea to pass but blocks red blood cells, aligning with option d. urea, red blood cells.
Step-by-step explanation:
The tub of water referred to in the student's question is an osmosis chamber, because it involves a membrane that allows only water to pass through when needed. This is the defining characteristic of osmosis, which is the movement of water through a semipermeable membrane from a region of high water concentration to a region of low water concentration.
In dialysis, particularly hemodialysis, the semipermeable membrane is likely permeable to substances like urea and is impermeable to larger molecules like red blood cells. This allows waste molecules such as urea to pass through from the blood into the dialysate, while red blood cells remain in the blood to be returned to the body.
Therefore, the semipermeable membrane is likely permeable to urea and impermeable to red blood cells, making d. urea, red blood cells the correct answer to the original question.