Apparatus for the treatment of extracorporeal blood comprising: a console (42) for treating extracorporeal blood including a controller (18)； a blood circuit that includes a blood passage having a blood return line (20) adapted to transport the treated blood to be infused into a mammalian patient, a blood pump line (17) coupled to a chamber ( 34) of blood having an area (48) filled with gas and a blood access line (16) adapted to receive the blood drawn from the patient； a peristaltic pump (32) associated with the extracorporeal blood treatment console (42) and adapted to connect to the blood pump line (17) of the blood passage； a source (12) of priming fluid； a pressure sensor (50b) that generates pressure data indicative of a pressure in the blood chamber (34)； the controller (18) controlling the peristaltic pump (32) and receiving the pressure data, in which the controller (18) is configured to perform the following stage: a. control the pump (32) to pump the priming fluid into the blood passage； characterized in that the apparatus comprises an interruption device (14) and that the controller (18) is configured to perform the following steps: b. stop the pump and close the blood passage downstream of the blood chamber by means of the interruption device (14) or cause the blood passage to be closed downstream of the blood chamber； C. receive first pressure data while the pump (32) is stopped and after the blood passage is closed； d. determine and store a first pressure value based on the first pressure data； and. While the interruption device (14) is closed, control the pump (32) to perform a predetermined pumping operation to pump an additional amount of the priming fluid into the blood passage or to pump an additional amount of the priming fluid to that the pressure detected by the pressure sensor reaches a predetermined pressure value； F. collect second pressure data generated by the pressure sensor (50b) while the interruption device (14) remains closed and th
