UPSC Civil Services Model Paper 4

Dialysis works on the principle of the diffusion of solutes and ultra-filtration of fluid across a semi- permeable membrane.
Diffusion is a property of substances in water; substances in water tend to move from an area of high concentration to an area of low concentration.
Blood flows by one side of a semi-permeable membrane, and a dialysate, or special dialysis fluid, flows by the opposite side. So, option 2 is correct.