Blood is withdrawn from a vein by a double lumen catheter that has an elongated tube with unitary outer walls. A longitudinal planar septum divides the interior of the tube into an inlet lumen and a longer return lumen. The inlet lumen extends from a proximal end of the tube to an end terminating in a distally forward facing aperture. The return lumen extends contiguously with the inlet lumen from the proximal end of the tube to an end terminating in a distally forward facing aperture spaced in the longitudinal direction distally forward of the inlet lumen aperture. A diverting structure extends from an outer wall of the return lumen distally forward of the aperture of the inlet lumen. The diverting structure diverts flow of treated fluid discharged from the return lumen away from the distally forward facing aperture of the inlet lumen. The blood withdrawn via the catheter is circulated for treatment in a blood purification system.