A body cooling system (10a, 10b, 10c, 10d) comprising: a body cooling device through which a heat exchange medium comprising liquid phase matter and frozen solid phase matter is recirculated, said body cooling device being selected from i) a heat exchange catheter that is insertable into the body of a human or animal subject and ii) a body surface heat exchange device that is positionable on the body of a human or animal subject, said body cooling device having an inlet, an outlet and at least one lumen; (70, 72); and an extracorporeal system which includes an extracorporeal heat exchanger which connects to the inlet and outlet, said extracorporeal system and at least one pump (54, 56, 88) configured to recirculate the heat exchange medium such that it flows from the extracorporeal heat exchanger, through the inlet of the body cooling device, through said at least one lumen, out of the outlet of the body cooling device and back into the extracorporeal heat exchanger, the extracorporeal system further comprising a portion (A, B, C, D, E) configured to add to the recirculating heat exchange fluid either i) a quantity of a previously prepared slurry concentrate or ii) a quantity of newly-formed frozen solid phase matter; the body cooling system further comprising: a sensor for sensing the concentration of frozen solid particles in the heat exchange medium; and a microprocessor, computer or other controller which receives signals from the sensor and is programmed to issue control signals to the portion (A, B, C, D or E) of the extracorporeal system to thereby control the concentration of frozen solid particles in the heat exchange medium as sensed by the sensor. a body cooling device through which a heat exchange medium comprising liquid phase matter and frozen solid phase matter is recirculated, said body cooling device being selected from i) a heat exchange catheter that is insertable into the body of a human or animal subject and ii) a body surface heat exchange device that is positionable on the body of a human or animal subject, said body cooling device having an inlet, an outlet and at least one lumen; (70, 72); and an extracorporeal system which includes an extracorporeal heat exchanger which connects to the inlet and outlet, said extracorporeal system and at least one pump (54, 56, 88) configured to recirculate the heat exchange medium such that it flows from the extracorporeal heat exchanger, through the inlet of the body cooling device, through said at least one lumen, out of the outlet of the body cooling device and back into the extracorporeal heat exchanger, the extracorporeal system further comprising a portion (A, B, C, D, E) configured to add to the recirculating heat exchange fluid either i) a quantity of a previously prepared slurry concentrate or ii) a quantity of newly-formed frozen solid phase matter; the body cooling system further comprising: a sensor for sensing the concentration of frozen solid particles in the heat exchange medium; and a microprocessor, computer or other controller which receives signals from the sensor and is programmed to issue control signals to the portion (A, B, C, D or E) of the extracorporeal system to thereby control the concentration of frozen solid particles in the heat exchange medium as sensed by the sensor. a sensor for sensing the concentration of frozen solid particles in the heat exchange medium; and a microprocessor, computer or other controller which receives signals from the sensor and is programmed to issue control signals to the portion (A, B, C, D or E) of the extracorporeal system to thereby control the concentration of frozen solid particles in the heat exchange medium as sensed by the sensor.