A flushable catheter assembly having features to enable selective activation of fluid flow through the catheter assembly is disclosed herein. A septum is placed within the catheter adapter of the catheter assembly and includes a pathway that is closed prior to being biased open via a septum activator also positioned within the catheter adapter. A plurality of air vent channels is interposed between the septum and the inner surface of the catheter adapter to permit “flashback” of blood during insertion of the catheter into a patient. The septum activator is advanced through the pathway of the septum as a coupler is attached to a proximal opening of the catheter adapter.