An in-vivo plasmapheresis or in-vivo ultrafiltration membrane comprises a plurality of elongated hollow microporous fibers each fiber having an outer wall, an inner wall and an interior lumen extending along the length thereof and defined by an inner wall surface, and wherein the fiber wall structure is a substantially continuous change in mass density from the outer wall to the inner wall and comprises a continuum of voids bounded by solid frames, the fiber wall having an asymmetrical pore size and asymmetrical mass density between said inner wall surface and the outer wall surface with a higher mass density adjacent to the outer wall and a lower mass density adjacent to said inner wall, and characterized by a nitric oxide donor composition capable of producing or releasing nitric oxide or inducing nitric oxide production or release in-vivo with a blood vessel, distributed on or in the fiber wall.