Abstract: In a fluid transmission line, a valve comprising a housing that establishes a lumen for transmission of a fluid through said valve; a drive mechanism and a drive gear mounted in said housing to be selectively driven in a first or second rotational direction by said drive mechanism. The drive gear has a central throughhole and a plurality of pins around the central throughhole. A plurality of leaves are pivotally mounted on the pins, and oriented to extend radially inward into said central throughhole. A fixed extension has an annular aspect disposed in the drive gear, and has a plurality of engagement members disposed to operatively engage one of said leaves. The engagement members bias the leaves to close an orifice when said drive gear rotates in said first direction and to open the orifice when said drive gear rotates in said second direction. Each of the leaves maintains a substantially sealing engagement with each adjacent leaf throughout a range of motion of the leaves.