A turbine airfoil used in a gas turbine engine, the airfoil including an internal cooling air circuit that includes two nested and parallel serpentine flow circuits that both flow in the airfoil aft direction. The first and second legs of the two nested serpentine circuits are separated by ribs so that the cooling air flows do not mix. The last leg of the two nested serpentine circuits is separated by a rib which includes a plurality of cross-over holes so that the flows can mix. The aft-most serpentine flow circuit of the two nested circuits includes a last leg that is connected to cooling slots spaced along the trailing edge of the airfoil such that cooling air from the last leg flows out the ducts. The forward-most serpentine circuit of the two nested circuits includes a blade tip channel extending along the blade tip and connecting the first leg with the second leg, the blade tip channel including a plurality of blade tip cooling holes. A leading edge cooling supply channel is connected to at least one impingement cavity by leading edge impingement holes to provide cooling to the leading edge region. The leading edge cooling supply channel is separated from the forward-most serpentine flow circuit by a rib such that the flows do not mix.