A system for controlling fluid flow in a catheter assembly (101), comprising: an intravenous catheter assembly having a catheter adapter (14) and a needle connector, wherein the catheter adapter has a lumen (16 ) inside； a partition (50, 356) disposed inside a part of the inner lumen, wherein said partition has a slit (56, 66, 366)； an introducer needle (350) having a first end and a second end, in which the first end is coupled to the needle connector and the second extends through the inner lumen of the catheter adapter, in which a middle part of the introducer needle is positioned inside a part of the partition； and a partition activator (80, 180, 380, 344) disposed inside a part of the inner lumen (16) adjacent to the partition (50, 365), a distal end of the partition activator in contact with a proximal surface of the partition and adapted to open, by displacement, the slit of the partition, and in which a proximal end of the partition activator is positioned adjacent to an opening (26) of the catheter adapter, in which the proximal end of the partition activator is accessed by inserting an external device into the opening (26) of the catheter adapter； characterized by a groove (60) formed on an inner surface (24) of the inner lumen of the catheter adapter, in which the partition is completely arranged inside the groove； a ventilation channel (70) formed in the groove so that the ventilation channel is interposed between the partition wall and the inner surface of the inner lumen of the catheter adapter, in which the ventilation channel has a selected surface area and perimeter to allow the passage of at least one of air and blood at a desired flow rate.Un sistema para controlar el flujo de fluido en un conjunto (101) catéter, que comprende: un conjunto catéter intravenoso que tiene un adaptador (14) de catéter y un conector de aguja, en el que el adaptador de catéter tiene un lumen (16) interior； un tabique (50, 356) dispuesto en el interior de una parte del l
