A catheter assembly used for extracorporeal treatment, such as dialysis, of blood or other body fluid, includes an outer and inner catheter in a coaxial relationship. A fluid path is provided by a passageway defined between the catheters and a distal end of the outer catheter, and another fluid path is provided through a lumen and a distal end of the inner catheter. A sealing member is coupled to one of the catheters. The member in a first position is configured to permit the distal ends of the catheters to transport fluid, while in a second position the member is configured to inhibit at least one of the distal ends from transporting fluid. The sealing member may include a flared portion for enhance sealing. The sealing member may also have a portion extending outward from the catheters to position the distal ends away from vessel contact.
