An ultrafiltration filter for an extracorporeal blood circuit having an input for blood withdrawn from a human patient and a blood output for filtered blood to be infused into the patient including: a filter body having a length of at least 20 centimeters (cm) and an interior diameter of no greater than 1.5 cm; an input at a first end of the body to receive the withdrawn blood; an output at a second end of the body to discharge the filtered blood; a filter membrane in the body defining a blood passage through the body, wherein the membrane has an active filter membrane surface area of no greater than 0.2 meters squared (m2) and the filter membrane blocks passage of blood molecules having a molecular weight cut of greater than 60,000 Daltons and a volume of the blood passage in the filter being less than two percent of a cardiac output of the patient, and an ultrafiltrate output to the body and open to a side of the filter surface area opposite to the blood passage.