Re: How is an isotonic solution in a dialysis machine able to work?

Dear cj: Having two solutions with the same tonicity means that both solutions have the same osmotic pressure. This indicates that there will be no net movement of water across the semipermeable membrane that separates the two solutions. The fact that solution S1, for example, is isotonic to a second solution, say S2, does not tell us anything about which solutes are present or in what concentration any one solute will be present.

To give a trivial example, suppose we look at S1 and S2 with only one compound in each solution. Let’s say S1 has 300 mOsm of NaCl (sodium chloride) and S2 has 300 mOsm of urea. If you put them on opposite sides of a semipermeable membrane there will be no net movement of water but the urea will go from S2 to S1 and the NaCl will go from S1 to S2 until equilibrium is reached. This is exactly what happens in dialysis. The metabolic wastes go out because they are more concentrated in the body and this is what cleans the blood.

Ask again if this isn’t clear.

Dr. Swanson