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The human kidney can be damaged from a number of causes resulting in a patient’s inability to filter toxins (i.e. urea) from the body which could result in death. Complete kidney failure is usually first treated with dialysis which “cleans” the blood.
Figure 1: The “dialyzer” is a glass container that has 3 main components: (1) blood percolating through to be cleaned; (2) a dividing membrane; and (3) the dialysate. The latter is liquid containing chemicals used to draw fluids and toxins out of the bloodstream and supply electrolytes and other chemicals to the bloodstream
(image: kidney.niddk.nih.gov).
All of the following are consistent with the process of dialysis EXCEPT:
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Correct Answer: D. Excessive minerals and toxins cross the membrane in the dialyzer by osmosis.
On the Surface: Dialysis only refers to the transfer of the solute (diffusion), while the transfer of the solvent is called osmosis.
Going Deeper:
On average, there is one question regarding diffusion or osmosis on the MCAT. Diffusion is the movement of a molecule across a semi-permeable barrier down its concentration gradient (from a region of high concentration to one of lower concentration). This is the process that must be occurring in the dialyser to remove toxins from the blood (because the dialysate, logically, would have no toxins so the toxins defuse out of the blood into the dialysate). Note that vital substances in the blood should be kept in the blood: options would include having a dialysate with the same concentration of those substances (so they don’t diffuse out of the blood) or by adjusting the permeability of the membrane.
Osmosis is the movement of water – specifically water – across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration.
So in summary, osmosis deals with how much water passes through the membrane, while dialysis deals with what type of solute (i.e., sodium, protein, glucose) passes through by simple diffusion.
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