A catheter for use in the extracorporeal treatment of bodily fluids comprises a catheter body having a withdrawal port, an infusion port, and a plurality of lumens therein. The lumens comprise two or more withdrawal lumens that merge to form a single lumen proximal to withdrawal ports for transport of fluids withdrawn from a body vessel through the withdrawal port to an extracorporeal treatment unit, such as a dialyzer. The lumens also comprise an infusion lumen for infusion of treated fluids from the extracorporeal treatment unit through the infusion port into the vessel. The infusion lumen is disposed substantially at the center of the catheter body, while the withdrawal lumens extend longitudinally adjacent the infusion lumen along the catheter body length. A manifold may be provided at the proximal end of the catheter body to include the merger of the withdrawal lumens.