The dialyzer has two main parts: one for the blood and one for the dialysate (the dialysis solution that allows the blood to be cleaned). Both parts are separated from each other by millions of thin membranes that the dialyzer contains. As blood passes through these fibers, excess fluids, and toxins are eliminated. These fibers, being very thin, allow blood cells and proteins to remain in the blood. Minor toxins such as urea, creatinine, potassium, phosphorus, and extra fluids are eliminated. During each dialysis treatment, only about 250 ml of blood remains outside the body.