Osmosis is the diffusion of water across a selectively permeable membrane, which is a barrier that has holes large enough to allow water molecules to pass but small enough to block certain molecules like salt or glucose. Water with solutes (substance dissolved in water) diffuses from an area of lower concentration of solutes to an area of higher concentration until the concentrations on both sides of the membrane are equal.
Dialysis is the separation of smaller molecules from larger molecules by using a differential permeable membrane (like the semi-permeable membrane for osmosis). This membrane has holes that allow smaller molecules to pass while blocking larger molecules in a solution.
Osmosis deals with how much solvent (substance that dissolves another substance when combined) passes through the membrane, while dialysis deals with what type of solute (i.e., sodium, protein, glucose) passes through.
For example, when examining the equilibrium concentration between water and solutes in an animal cell vs. its extracellular environment, osmosis is used to determine the rate of how much water is going in and out of the cell.
In dialysis, such as in removing toxins from diseased kidneys, small molecules, like urea, are filtered by the differential permeable membrane while larger molecules, like protein and starch, are not.
Although the concepts of osmosis and dialysis are similar, they have different applications in the fields of nutrition, biology and medicine. Osmosis is used to explain natural phenomenons such as why does a freshwater fish die if it is put in the ocean or how waste products are removed in the bloodstream. Water treatment plants use osmosis to filter unwanted solvents from water as part of their purification and sanitation process.
Dialysis is a process often used in medicine to help patients with failed kidneys remove wastes from the bloodstream. Arterial blood flows from the patient to the dialysis machine that contains the dialysis tubing (a long, coiled cellophane tube) that acts as a differentially permeable membrane. Around this tube is a solution called dialysate, a solution that contains isotonic concentrations of all the components that are to remain in the blood without its waste products. And so, the wastes flow from the blood into the dialysate faster than they return. Then, the isotonic solution returns to the blood to maintain a proper solute concentration.
However, dialysis does not fully replace the function of kidneys because the process does not produce erythropoietin, a hormone that regulates red blood cell production, and calcitrol (a form of vitamin D).