you are referring to the 0.9% NaCl(Saline) solution, used during the process of dialysis.

The reason is simple, the solution is isotonic with the concentration of our blood.

We can’t use normal water(less concentrated than blood), or more concentrated solution than blood, as it may result in process of osmosis and destroy RBC’s and other blood cells(shrink/swell).

So, for flushing of blood in and out of dialysis machine(pipes) and all other purposes, 0.9% NaCl solution is used so as to maintain same concentration, as that of blood.

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