An extracorporeal blood cooling or heating circuit includes an intravenous catheter for withdrawing a patient's blood coupled to a combined pump/heat exchanger device. One or more sensors are provided upstream and/or downstream of the pump/heat exchanger device for measuring pressure, temperature, fluid flow, blood oxygenation, and other parameters. A controller is operatively coupled to the pump/heat exchanger device and the one or more sensors to control the speed of the pump inside the pump/heat exchanger device and regulate the blood temperature by controlling the operation of the heat exchanger. The combined pump/heat exchanger device includes a housing having at least one inlet and at least one outlet, a pump portion defining a blood circuit inside the housing, and a heat exchanger portion contained within the housing for selectively heating or cooling the blood.