Abstract: A header unit designed for use in a blood dialysis device, providing laminar flow conditions with no regions or stagnation or turbulence which could cause blood hemolysis. The specification discloses a diffuser adapted to provide uniform and nontraumatic diversion and diffusion of blood from a blood supply tube having a given cross-section area to a treatment face of a dialysis device in which said treatment face is of larger surface area than said supply tube. The diffuser comprises a first diffuser section of pipe continuous with the supply tube, flow turning vanes, and a second diffuser section leading to the liquid treatment face.