A fluid-flow restricting apparatus for mixing fluids employs a body member housing elongate flow restrictors (typically capillary tubes) which restrict the flow of the fluid therethrough. Demountable fluid-tight seals are provided between the capillary tubes and passages in the body in which the tubes are housed. An inlet header is demountably attached to the body so as to distribute fluid to the capillary tubes. There is a common outlet passage communicating with the outlet ends of the capillary tubes. The inlet header may be fed with the fluids to be mixed from a pressure regulation system and has several inlets. Valves are operable to select which header inlets are placed in communication with the respective fluids to be mixed.