This filter is an artificial kidney called a dialyser. One needle withdraws blood for cleansing. The other needle returns filtered blood to the body. A pump pushes blood through the dialyser. Blood passes on one side of the filter.
Solution made by the dialysis machine passes on the other side.
The solution draws excess fluid and waste out of the blood. A filter is used with pores large enough to allow waste to leave. Larger molecules like blood cells cannot pass through the filter. The average person receives three treatments per week. Each treatment lasts three to four hours.