a As water passes in bulk across a porous membrane, any dissolved solute molecularly small enough to traverse the pores passes in bv filtration Aqueous pores vary in size from membrane to membrane and thus in their individual permeability characteristics for certain drugs and other substances◼ As water passes in bulk across a porous membrane, any dissolved solute molecularly small enough to traverse the pores passes in by filtration. ◼ Aqueous pores vary in size from membrane to membrane and thus in their individual permeability characteristics for certain drugs and other substances