A wound therapy and tissue management system utilizes fluid differentiation. Fluid is differentiated by establishing a gradient within the system. A gradient can be established with matter or energy. Patient interfaces for establishing, maintaining and varying one or more gradients include transfer elements with first and second zones having different flow coefficients. The transfer elements exchange fluid with a patient, generally through a wound site, and with external components of the system. Osmotic solution gradients are controlled by a methodology involving the present invention for extracting solutions, which can include toxins, from patients and for introducing fluids and sumping air to wound sites.