Abstract: An apparatus for extracting oxygen from sea water in which a plurality of hollow gill members carry on oxygen extracting heme and are positioned within a diffuser. A flow of oxygen bearing liquid, such as sea water, passes through the diffuser and around the gill membranes. The gill membranes are angled with respect to the flow of sea water. The diffuser has a reduced section intake and outlet, and is divided into a number of channels, with the result that the drag force is reduced while maintaining adequate oxygen extraction. Propellers compensate for drag force losses.