Abstract: A blood oxygenator includes a heat exchanger wherein heat transfer fluid flows through a tube which is ribbed along its length. The tube is positioned within a chamber connected in an extracorporeal blood circuit such that the blood is caused to flow over the exterior surface of the ribbed tube. In the preferred embodiment, the blood flows through a plurality of continuous, restricted area flow paths offering substantially uniform flow impedance to the blood, these restricted flow paths being provided by constructing the tube with an integral, substantially continuous, hollow, helical rib, and by forming the helically ribbed tube in a helical configuration mounted between an inner cylindrical column and an outer cylindrical shell such that the blood is caused to flow through the plural paths of restricted cross-sectional area provided by the helical flute.