A centrifugal cartridge or disk used for extraction of light supernatant fractions from fluid samples is described, particularly for non-homogenous fluid biological samples such as whole blood. The device may be used to collect cell-free blood plasma or a fraction of whole blood containing target cells such as leukocytes. Single or multiple channels are described, including channels with passive valves, gaskets, receiving cavities, inlet holes, capillary wicking ridges, distal cavities for cell retention, separator gel, and density medium. Centrifugal action causes whole blood in a receiving cavity to pass into one or more channels where it separates into blood cells, plasma and optionally fractions of an intermediate density. After spin, the plasma returns to the receiving cavity by way of the one or more channels for extraction through the inlet hole or other inwardly located hole. Disposable cartridges are constructed of monolithic top and bottom plates, which may be joined together by an elastomeric outer seal.