The apparatus such as patient module (1) comprises a support structure, and a blood circuit having a blood pump (6), an oxygenator (3), a filter (5), and a reservoir (2). The upper side of the oxygenator is below or in the same level as the upper side of the filter, in the operating position of the apparatus in the horizontal position. The liquid-carrying portions of the blood pump are located under the horizontal position of an upper level sensor (10) and below the maximum fill level of the reservoir. The upper side of the filter is higher than the upper level sensor of the reservoir. Independent claims are included for the following: (1) system for extracorporeal blood treatment of patient； and (2) computer program for controlling filling and venting of apparatus for extracorporeal blood treatment.
