A catheter assembly for use in accessing a vasculature of a patient is disclosed. In one embodiment, the catheter assembly includes a catheter body that includes a flattened oval outer surface and defines first and second lumens. The catheter body defines a distal tip region that includes a venous lateral opening that is in fluid communication with the first lumen and includes a distal-facing portion. The distal tip region further includes an arterial lateral opening that is in fluid communication with the second lumen, includes a distal-facing portion, and is substantially un-staggered with respect to the venous lateral opening. A distal end opening is in fluid communication with a power injectable third lumen. In another embodiment, the first and second lumens each generally include a reniform cross-sectional shape. In yet another embodiment, a dual-lumen catheter includes first and second lumens that each define a modified ellipse cross-sectional shape.