A waterless blood heater/cooler device directly controlling temperature of blood flowing through an extracorporeal blood circuit. A thermoelectric module is coupled to a supply voltage to generate a temperature difference. A heat exchanger cassette comprising a core and first and second laminar flow guides is in thermal contact (directly or indirectly) with the thermoelectric module. The cassette has a plurality of tubes for carrying parallel channels of the blood. The first and second laminar flow guides provide an inlet and an outlet for coupling to the extracorporeal blood circuit and respective intermediate chambers for receiving respective ends of the tubes in order to guide the blood to and from respective tubes in a substantially laminar flow.