Disclosed is a filling device of a fluid conducting system of an extracorporeal blood treatment device that includes a spike for connecting to the single fluid connector of a medical fluid container and a manually operable fluid blocking mechanism arranged directly downstream of the spike that fluidly connects 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 such as the arterial line section of a blood purification device can be connected to it in a detachable manner while the filling device is in operation.