Method and apparatus for ultrafiltration of blood

1. An ultrafiltration filter for an extracorporeal blood circuit having an input for blood withdrawn from a human patient and a blood output for filtered blood to be infused into the patient, said ultrafiltration filter comprising:

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an input at a first end of the body to receive the withdrawn blood;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 (m 2 ) 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.

2. The filter as in claim 1 wherein the active filter membrane surface area is no greater than 0.1 m 2 .

3. The filter as in claim 1 wherein the filter membrane surface is an interior surface of a bundle of filter fibers.

4. The filter as in claim 1 wherein the filter fibers have a length of at least 20 centimeters.

5. The filter as in claim 1 wherein the bundle of filter fibers has at least 620 fibers.

6. The filter as in claim 1 wherein the filter body is substantially straight.

7. The filter as in claim 1 wherein the filter membrane comprises hollow fibers and said fibers are arranged in a substantially straight bundle no greater than 1.5 centimeters in diameter.

8. An ultrafiltration filter for an extracorporeal blood circuit having an input for blood withdrawn from a human patient and a blood output for filtered blood to be infused into the patient, said ultrafiltration filter comprising:

a filter body having a length of at least 20 centimeters (cm) and an interior cross-sectional area of no greater than 1.77 cm 2 ;

an input at a first end of the body to receive the withdrawn blood;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 (m 2 ) 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.

9. The filter as in claim 8 wherein the active filter membrane surface area is no greater than 0.1 m 2 .

10. The filter as in claim 8 wherein the filter membrane surface is an interior surface of a bundle of filter fibers.

11. The filter as in claim 8 wherein the filter fibers have a length of at least 20 centimeters.

12. The filter as in claim 8 wherein the bundle of filter fibers has at least 620 fibers.

13. The filter as in claim 8 wherein the filter body is substantially straight.

14. The filter as in claim 8 wherein the filter membrane comprises hollow fibers and said fibers are arranged in a substantially straight bundle no greater than 1.5 centimeters in diameter.

15. The filter as in claim 8 wherein the cross-sectional area of the filter body is circular an has a diameter no greater than 1.5 cm.