A dialysis membrane is selectively permeable, and substances smaller than proteins are able to pass through. The dialysis machine allows blood to flow past one side of the dialysis membrane and the dialysis fluid flows on the other side of the membrane. If you wanted to use a dialysis machine to remove only urea (a small molecule) from blood, what could you use for the dialysis fluid?
A) a solution that is isotonic and contains only protein
B) a solution that is isotonic and contains the same concentration of substances as blood, except for having no urea in it
C) distilled water
D) whole blood
E) hypertonic salt water

Question

Quizplus · Accepted Answer

Option A is incorrect because a solution containing only protein would not be effective in removing urea from the blood.

Option B is correct because a solution that is isotonic and contains the same concentration of substances as blood, except for having no urea in it, would create a concentration gradient that would allow urea to diffuse out of the blood and into the dialysis fluid.

Option C is incorrect because distilled water would cause the blood to become hypotonic, which could lead to cell lysis.

Option D is incorrect because using whole blood as the dialysis fluid would not remove any substances from the blood.

Option E is incorrect because hypertonic salt water would cause the blood to become hypertonic, which could lead to cell crenation.

A Dialysis Membrane Is Selectively Permeable, and Substances Smaller Than