A hemodialysis catheter assembly with a flexible catheter lead that can control blood flow at a hemodialysis access site even under high blood pressures encountered at such sites. Flexible tubing extends through the hollow interior of a housing having opposite, open ends. The assembly includes components to alternately occlude or not occlude the lumen of the flexible tubing. In a first embodiment, spring clips press against the tubing, but insertion of a hollow sleeve into the lumen thereof retracts the clips and provides a flow path. In a second embodiment, a pinch bar can be manually moved into and out of engagement with the flexible tubing to occlude or open a flow path. In a third embodiment, the housing has a screw cap. A spring-loaded pinch bar presses against, and occludes the lumen of, the collapsible tubing when the screw cap is screwed into a threaded cutout in the housing, and provides a flow path through the tubing when the screw cap is removed. In a fourth embodiment, the tang portion of a tongue when in an extended position presses against, and occludes the lumen of, the compressible tubing; advancement of a capture ring attached to a sliding button retracts the tang portion and opens a flow path.