An exchanger for oxygenating blood includes a casing having walls and a cavity, sheet material with first and second ends covering distal edges of the walls, and a cover extending over the cavity to sandwich the first and second ends of the sheet material between the cover and the edges of the walls. The cover is melted into the sheet material and the distal edges of the walls. In another embodiment, a blood oxygenator includes a first compartment having a closed casing divided by a folded sheet of membrane material forming a first blood chamber and a first gas chamber. The casing has first and second manifolds in fluid communication with the first blood chamber. A second compartment is interconnected with the first compartment and has a closed casing divided by a folded sheet of membrane material forming a second blood chamber and a second gas chamber. The casing has third and fourth spaced apart manifolds in fluid communication with the second blood chamber. A first conduit connects an outlet of the first manifold to the an inlet of the third manifold, and a second conduit connects an outlet of the fourth manifold to an inlet of the second manifold.