The invention is directed to a fluid mixing device with gas separation, comprising a containing body having an internal active surface and having at least one first inlet aperture for a physiological fluid, and at least one fluid outlet aperture, spaced apart from the said first inlet aperture wherein the containing body has at least one second inlet aperture located above the said first inlet aperture and designed to convey a second fluid into the containing body to form a layer of the said second fluid above the said physiological fluid.
