A cooling system for a turbine airfoil of a gas turbine engine is disclosed, whereby the cooling system includes spanwise extending midflow blockers positioned within one or more cooling channels to maintain an internal through flow channel Mach number. One or more cooling channels may have a larger cross-sectional area proximate to an outer end of the airfoil than at an inner end. One or more cooling channels include midflow blockers extending into the cooling channel. In at least one embodiment, the midflow blocker may extend radially inward from the outer end of the airfoil. The midflow blocker may limit movement of cooling fluid from the pressure side to the suction side or vice versa. The midflow blocker may increase in size moving radially outward as the cross-sectional area of the cooling channel increases as well. Such configuration keeps the internal through flow channel Mach number within design limits.