Devices and methods to cool a target tissue region inside the body are described. Fluid cooled below normal body temperature and blood at a normal body temperature are provided to the tissue region in proportions to cool the tissue region. A system for controlling the temperature of the target tissue region is disclosed. The system includes a catheter and a control system that controls the amount of cool fluid and blood provided to the tissue region. A catheter for providing cool fluid to the tissue region is also provided. The catheter includes a temperature sensor that extends to a location distal to the distal end of the catheter to sense the temperature of the tissue region.