A cooling unit for a heat exchanger, integrated in an oxygenator, 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 (14, 19), which are each fluid-tightly connectable or connected to a hose system of the heat exchanger, and which, together with the hose system of the heat exchanger, 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 (14, 19), which are each fluid-tightly connectable or connected to a hose system of the heat exchanger, and which, together with the hose system of the heat exchanger, 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 (14, 19), which are each fluid-tightly connectable or connected to a hose system of the heat exchanger, and which, together with the hose system of the heat exchanger, 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.