Promising microfluidic devices are proposed herein to continuously and passively extract plasma from whole human blood. These designs are based on the red cells lateral migration and the resulting cell-free layer locally expanded by geometric singularities, such as an abrupt enlargement of the channel or a cavity adjacent to the channel. After an explanation of flow patterns, devices are experimentally and biologically validated for plasma extraction. They are also successively optimized with extraction yields up to 17.8% for a 1:20 blood injected at 100 microL/min.