Dialysis is used to treat patients with kidney failure - it can also be referred to as renal replacement therapy (RRT). This means that the treatment acts to replace the function of normal kidneys - to filter the blood. Dialysis machines use the process of simple diffusion to filter waste from the blood. Blood passes from the patient, through a dialysis machine where it is filtered, and then back into the patient. The membranes within the machine are partially permeable; erythrocytes and large proteins cannot pass through, but urea, small ions and molecules (including glucose) can pass through the membrane.
Answer to the question:Since simple diffusion is used to filter blood through a partially permeable membrane in the dialysis machine, it is important to control concentrations of substances in the dialysis fluid. There is no urea in dialysis fluid so that there is a net movement of urea from the blood into the fluid. The concentration of mineral ions in dialysis fluid is the same as in normal plasma so that any excess ions move out of the blood into the fluid - restoring the correct balance of ions in the blood. The concentration of glucose in dialysis fluid is the same as normal blood plasma so that there is no net loss of glucose from the blood into the dialysis fluid.
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