Individuals with kidney failure can undergo a dialysis procedure in which the blood is pumped through a machine containing a semipermeable mem- brane. As blood flows along one side of the membrane, a fluid called the dialysate flows in the opposite direction on the other side. This countercur- rent arrangement maximizes the concentration differences between the two so- lutions so that waste materials such as urea and creatinine (present at high concentration in the blood) will efficiently diffuse through the membrane into the dialysate (where their concentrations are low). Excess water can also be eliminated, as it moves into the dialysate by osmosis. The “cleansed” blood is then returned to the patient.