A blood filtering device has a tubular housing with a tubular section and tubular housing volume. A filtration section with a filter medium separating a raw side from a clean side is arranged in the tubular housing. The filtration section is movable along the tubular section and movably sealed relative to the tubular housing. The filtration section separates the tubular housing volume into variable first and second tubular housing volumes. A first communication path extends between raw side and first tubular housing volume. A second communication path exits between raw side and second tubular housing volume. When moving the filtration section, blood in the first tubular housing volume flows through the first communication path to the raw side； blood plasma passes through the filter medium to the clean side； residual blood flows through the second communication path into the variable second tubular housing volume.
