A working fluid treatment device for mass transfer between a working fluid and two fluid exchange media

An extracorporeal blood treatment device comprises a single housing defining an internal blood flow cavity. The housing accommodates an oxygenator, a heat exchanger and an additional mass transfer assembly, each having an array of fluid conduits. The arrays are co-located within the internal blood flow cavity such that blood flowing through the internal blood flow cavity flows substantially homogeneously around all the conduits. The arrays are arranged relative to one another within the internal blood flow cavity such that they together define a continuous blood flow path through the internal blood flow cavity along which blood can flow. The continuous blood flow path has a blood entry surface at one end and a blood exit surface at the opposite end. The overall blood flow direction from the blood entry surface along the blood flow path to the blood exit surface follows substantially a straight line.