A device for oxygenating blood during extracorporeal circulation includes a box-like body having a first end and a second end opposite thereto and defining therebetween a treatment chamber with an inner surface. The device further includes, in the treatment chamber, at least one exchange unit having at least one flat diaphragm that is impervious to blood and has a major axis and a minor axis. The diaphragm has a winding of hollow microporous fibers. The device further includes an inlet port and an outlet port axially aligned with one another in an alignment direction. A compartment is provided for accumulation and delivery of blood to be treated, another compartment for collecting treated blood, the compartments being located downstream of the inlet port and upstream of the outlet port, between the exchange unit and the inner surface of the treatment chamber. The diaphragm is disposed with the major axis parallel to the alignment direction.
