A filling device of a fluid conducting system of an extracorporeal blood treatment device is disclosed. The filling device includes a spike that is adapted for connection to a single fluid connector of a medical fluid container of the fluid system and a manually operable fluid blocking mechanism that is arranged directly downstream of the spike and is adapted or provided in such a way so as to remain fluidly connected with the spike while the filling device is in operation. The fluid blocking mechanism has at least one fluid outlet connector that is adapted so that a line section or hose of the fluid conducting system, preferably an arterial line section of a blood purification device, can be connected to it in a detachable manner while the filling device is in operation.
