A method for filtering blood including: withdrawing blood from an adult patient; performing ultrafiltration by filtering the withdrawn blood in a filter having an active filter membrane surface of no greater than 0.2 meters squared (m2) to remove ultrafiltrate from the blood, wherein the filter membrane blocks the passage of blood molecules having a molecular weight of at least 60,000 Daltons, wherein an amount of the removed ultrafiltrate is an effective therapeutic amount for treating a fluid overload condition of the patient; infusing the ultrafiltrated blood into the adult patient, and pumping the ultrafiltrate from the filter a rate no greater than one liter per hour.