A blood filtering device has a filtration section with filter medium separating raw side and clean side. A first communication path connects raw side and a first variable blood reservoir volume； a second communication path connects raw side and a second variable blood reservoir volume coupled to a receptacle. When varying the first variable blood reservoir volume, blood contained therein flows through first communication path to raw side, plasma/serum passes through the filter medium from raw side to clean side, and residual blood flows from raw side through second communication path into the second variable blood reservoir volume. When varying the second variable blood reservoir volume, blood contained therein flows through second communication path to raw side, plasma/serum passes through the filter medium from raw side to clean side, and residual blood flows from raw side through first communication path into first variable blood reservoir volume.
