A vehicle's engine cooling system is used not only to keep the engine cool, but also to keep it warm enough to ensure efficient, clean operation. System components include a radiator to dissipate heat, a fan to ensure the radiator receives adequate airflow, a thermostatic valve that opens when the desired operating temperature is reached, and a water pump (or coolant pump) to circulate coolant through the engine, hoses, and other components. Most vehicles today use an expansion tank, which expands the coolant and flows out of the cooling circuit when the coolant temperature is high, and then flows back into the cooling circuit when the vehicle is turned off and the engine has cooled. The cooling system also integrates elements of the cabin ventilation system, as engine heat can be used to heat the interior of the vehicle.