Abstract: A blood oxygenator wherein blood and oxygen are admixed by flowing blood, into which has been introduced bubbles of oxygen, through a three dimensional, open cellular mixing material having a substantial void volume. In one embodiment, the admixing chamber is provided by an upright cylinder having an open interior. In a second embodiment, the chamber has an annular interior configuration. In both embodiments, the open cellular mixing material completely fills the cross-sectional interior of the chamber. The resulting arterialized blood and blood foam rise to the top of the admixing chamber and are contained in a channel located at the top of the chamber and directed through this channel to the input of a defoamer chamber. A defoamer filter of annular configuration is retained within the defoamer chamber such that the defoamer inlet is located at the top of the defoamer filter within the interior annular space thereof.