A extracorporeal system for lung assist includes a housing, a blood flow inlet in fluid connection with the housing; a blood flow outlet in fluid connection with the housing; a plurality of hollow gas permeable fibers adapted to permit diffusion of gas between blood and an interior of the hollow gas permeable fibers, the plurality of hollow gas permeable fibers being positioned between the blood flow inlet and the blood flow outlet such that blood flows around the plurality of hollow gas permeable fibers when flowing from the blood flow inlet to the blood flow outlet; a gas inlet in fluid connection with the housing and in fluid connection with inlets of the plurality of hollow gas permeable fibers; a gas outlet in fluid connection with the housing and in fluid connection with outlets of the plurality of hollow gas permeable fibers; and at least one moving element to create velocity fields in blood flow contacting the plurality of hollow gas permeable fibers. The plurality of hollow gas permeable fibers may extend generally perpendicular to the direction of bulk flow of blood through the housing.