A cannula assembly, which can be used in a catheterization process, has an elongate cannula tube with a proximal inlet opening, a distal outlet opening, a lumen extending continuously between the inlet opening and outlet opening, and a sharpened cannula tip delimiting the outlet opening. The cannula tube is configured to receive and guide a flexible medical functional section, which is insertable into the lumen and movable along the lumen. A blocking device with a blocking element is assigned to the lumen. The blocking element is configured to interact with the flexible medical functional section and, depending on a movement direction of the flexible medical functional section, is automatically transferred between a blocking position, in which the blocking element blocks a withdrawal movement of the flexible medical functional section by acting on the functional section, and a release position, in which the blocking element releases an advancing movement.