During kidney dialysis, a person's blood is passed through a bath that contains several ions and molecules. The blood is separated from the dialysis fluid by a membrane that allows water, small ions, and small molecules to pass, but does not allow large
proteins or blood cells to pass. What should the composition of dialysis fluid be for it to remove urea (a small molecule) without changing the blood volume (removing water from the blood)?
What will be an ideal response?
For the dialysis fluid to remove urea without removing water, it should not contain urea. Because urea is a small molecule, it will diffuse through the dialysis membrane from an area of high concentration (the blood) to an area of low concentration (the dialysis fluid). To prevent an associated osmotic water movement, the dialysis fluid should have an osmotic concentration similar to that of blood plasma, but with higher concentrations of solutes such as bicarbonate ions or glucose, blood solutes that also are small enough to diffuse. As urea diffuses into the dialysis fluid, glucose and bicarbonate diffuse into the blood; as a result, the solute concentrations remain in balance and no osmotic water movement occurs.
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