A rotary valve unit is provided for blending fluid sources to regulate a temperature of fluid flowing to a heat exchange device in an extracorporeal blood perfusion circuit. A valve body includes a valve chamber having a first inlet port, a second inlet port, a first outlet port, and a second outlet port. A wedged-shaped rotary valve is disposed within the valve chamber and is rotatable within the valve chamber for controlling a blending of the cold fluid entering the fluid chamber from the first inlet port and the return fluid entering the fluid chamber from the second inlet port to regulate the desired temperature of fluid provided to the heat exchanger. The wedge-shaped rotary valve is positionable to provide either only cold fluid to the first outlet port, only return fluid to the first outlet port, or blending of the cold fluid and return fluid to the first outlet port.