A system for cooling or heating a body part, the system comprising: at least one bladder configured to be placed on a body part; a pump ; a heat exchanger in fluid communication with the pump ; a first fluid line in fluid communication with the heat exchanger and the at least one bladder , the first fluid line being located downstream from the heat exchanger when the system is operating to cool or heat the body part; a second fluid line in fluid communication with the at least one bladder and the heat exchanger , the second fluid line being located downstream from the at least one bladder when the system is operating to cool or heat the body part; a main reservoir ; and a separator in fluid communication with the main reservoir and the pump , the separator being located below the main reservoir to receive fluid under the influence of gravity from the main reservoir and to deliver fluid to the pump via gravity, characterized in that a total volume of fluid circulating among the at least one bladder , the pump , the first fluid line , the second fluid line and the heat exchanger is a circuit volume, wherein the pump is positioned in a casing as part of a unit having a unit inlet in fluid communication with the pump , and the pump , the unit inlet , and the separator are configured and positioned in the casing such that the pump primarily draws fluid from the unit inlet and fluid is drawn from the separator by the pump only when fluid is lost in the circuit volume, such that only fluid passing through the relatively closed circuit volume between the unit and the bladder is cooled or heated by the heat exchanger . at least one bladder configured to be placed on a body part; a pump ; a heat exchanger in fluid communication with the pump ; a first fluid line in fluid communication with the heat exchanger and the at least one bladder , the first fluid line being located downstream from the heat exchanger when the system is operating to cool or heat the body part; a second fluid line in fluid communication with the at least one bladder and the heat exchanger , the second fluid line being located downstream from the at least one bladder when the system is operating to cool or heat the body part; a main reservoir ; and a separator in fluid communication with the main reservoir and the pump , the separator being located below the main reservoir to receive fluid under the influence of gravity from the main reservoir and to deliver fluid to the pump via gravity, characterized in that a total volume of fluid circulating among the at least one bladder , the pump , the first fluid line , the second fluid line and the heat exchanger is a circuit volume, wherein the pump is positioned in a casing as part of a unit having a unit inlet in fluid communication with the pump , and the pump , the unit inlet , and the separator are configured and positioned in the casing such that the pump primarily draws fluid from the unit inlet and fluid is drawn from the separator by the pump only when fluid is lost in the circuit volume, such that only fluid passing through the relatively closed circuit volume between the unit and the bladder is cooled or heated by the heat exchanger . <EOS>