A low trauma, reversible flow pumping system is disclosed which is useful for transfer of biological fluids containing fragile components such as cells. The pumping system comprises a length of collapsible and flexible tubing in fluid communication at each end with a two-way, gravity actuated check valve means having a self-centering, vertically slidable weight member for directional regulation of fluid flow. Said tubing is sealingly enclosed within a hydraulic fluid containing chamber which is in fluid communication with oscillatory pressure providing means to cause alternate expansion and contraction of said tubing.