A cooling unit for a heat exchanger (W), integrated in an oxygenator (O), for the purpose of controlling the temperature of blood conveyed in an extracorporeal blood circuit, comprising: - a reservoir in which a liquid is stored, - a reaction vessel comprising a reactant which, in conjunction with the liquid, can initiate an endothermal reaction, - functional means for providing a fluidic access between the reservoir and the reaction vessel , and - a fluid line extending at least in regions inside the reaction vessel and which has an inlet line and an outlet line , which are each fluid-tightly connectable or connected to a hose system of the heat exchanger (W), and which, together with the hose system of the heat exchanger (W), forms at least part of a fluid circuit; characterised in that an expansion tank having an overflow into which the liquid from the reservoir flows after fluidic access is generated between the reservoir and the reaction vessel , in that the overflow provides a fluid connection to the reaction vessel through which a proportion of the stored liquid passes into the reaction vessel to initiate an endothermal reaction with the reactant , in that the fluid line is fluidically connected to the expansion tank , into which a remaining proportion of the stored liquid from the reservoir passes, and in that a fluid pump is located along the fluid line , by means of which liquid from the expansion tank passes into the fluid line which extends at least in regions inside the reaction vessel . comprising: - a reservoir in which a liquid is stored, - a reaction vessel comprising a reactant which, in conjunction with the liquid, can initiate an endothermal reaction, - functional means for providing a fluidic access between the reservoir and the reaction vessel , and - a fluid line extending at least in regions inside the reaction vessel and which has an inlet line and an outlet line , which are each fluid-tightly connectable or connected to a hose system of the heat exchanger (W), and which, together with the hose system of the heat exchanger (W), forms at least part of a fluid circuit; - a reservoir in which a liquid is stored, - a reaction vessel comprising a reactant which, in conjunction with the liquid, can initiate an endothermal reaction, - functional means for providing a fluidic access between the reservoir and the reaction vessel , and - a fluid line extending at least in regions inside the reaction vessel and which has an inlet line and an outlet line , which are each fluid-tightly connectable or connected to a hose system of the heat exchanger (W), and which, together with the hose system of the heat exchanger (W), forms at least part of a fluid circuit; characterised in that an expansion tank having an overflow into which the liquid from the reservoir flows after fluidic access is generated between the reservoir and the reaction vessel , in that the overflow provides a fluid connection to the reaction vessel through which a proportion of the stored liquid passes into the reaction vessel to initiate an endothermal reaction with the reactant , in that the fluid line is fluidically connected to the expansion tank , into which a remaining proportion of the stored liquid from the reservoir passes, and in that a fluid pump is located along the fluid line , by means of which liquid from the expansion tank passes into the fluid line which extends at least in regions inside the reaction vessel . <EOS>