Cooling system of a vehicle, comprising a coolant circuit which can be operated as a cooling circuit (2.1) for an AC operation and as a heat pump circuit (2.2) for a heating operation, comprising: - an evaporator, - a coolant compressor, - a heat exchanger as coolant condenser or gas cooler for the coolant circuit (2.1) or as heat pump evaporator for the heat pump circuit (2.2), - a first expansion element (6.1) associated with the evaporator, - a second expansion element (6.2) associated with the heat exchanger when functioning as a heat pump evaporator, - an internal heat exchanger having a high-pressure section (8.1) and a low pressure section (8.2), wherein the low pressure section is fluidically connected to the downstream-side coolant compressor, and - a heat pump pipe section (2.21) having a heat register (5.1) for forming the heat pump circuit (2.2), which can be connected to the cooling circuit (2.1) via a valve element and can be disconnected from the coolant circuit (2.1) for AC operation, characterised in that - for performing an air-heat pump function by means of the heat exchanger the heat pump pipe section (2.21) can be fluidically connected via the second expansion element (6.2) to the high-pressure section (8.1) of the internal heat exchanger, - the high-pressure section (8.1) of the internal heat exchanger is arranged in a coolant circuit section (2.4) connecting the second expansion element (6.2) to the heat exchanger, and - for performing the AC operation the high-pressure section (8.1) of the inner heat exchanger can be fluidically connected by means of the second expansion element (6.2) to the series connection of the evaporator and the associated first expansion element (6.1). - an evaporator, - a coolant compressor, - a heat exchanger as coolant condenser or gas cooler for the coolant circuit (2.1) or as heat pump evaporator for the heat pump circuit (2.2), - a first expansion element (6.1) associated with the evaporator, - a second expansion element (6.2) associated with the heat exchanger when functioning as a heat pump evaporator, - an internal heat exchanger having a high-pressure section (8.1) and a low pressure section (8.2), wherein the low pressure section is fluidically connected to the downstream-side coolant compressor, and - a heat pump pipe section (2.21) having a heat register (5.1) for forming the heat pump circuit (2.2), which can be connected to the cooling circuit (2.1) via a valve element and can be disconnected from the coolant circuit (2.1) for AC operation, characterised in that - for performing an air-heat pump function by means of the heat exchanger the heat pump pipe section (2.21) can be fluidically connected via the second expansion element (6.2) to the high-pressure section (8.1) of the internal heat exchanger, - the high-pressure section (8.1) of the internal heat exchanger is arranged in a coolant circuit section (2.4) connecting the second expansion element (6.2) to the heat exchanger, and - for performing the AC operation the high-pressure section (8.1) of the inner heat exchanger can be fluidically connected by means of the second expansion element (6.2) to the series connection of the evaporator and the associated first expansion element (6.1).