Circulation system for a vehicle, at least comprising: - a first circuit which carries a first fluid and in which at least one compressor, at least one heat exchanger (7, 9) as a condenser or gas cooler, at least one expansion element and at least one further heat exchanger (17, 21) as an evaporator are arranged in the intended flow direction of the first fluid, and - a second circuit which carries a second fluid and in which at least one conveying device for the second fluid is arranged, wherein at least one first heat exchanger to which the first fluid can be supplied in order to realize a heat exchange connection with the second circuit is arranged downstream of the at least one compressor and upstream of the at least one expansion element in the intended flow direction of the first fluid, and it is possible to switch between at least two circulation modes for the second circuit by way of a switching device which is comprised by the circulation system, wherein, in the first circulation mode, at least one external heat exchanger, through which the second fluid can flow and which is preferably designed as a radiator, is connected to the second circuit next to the at least one conveying device for the second fluid and to the one first heat exchanger in such a way that the second fluid, apart from the conveying device, the first heat exchanger and the at least one external heat exchanger, flows through no further heat exchanger (39, 40), no convector and no heating device (43, 45) which can additionally heat the second fluid, and, in the second circulation mode, this at least one external heat exchanger is not connected to the second circuit, which contains at least the conveying device and also contains the first heat exchanger, which is preferably a heating circuit in this case, characterized - in that the circulation system for a vehicle comprises a third circuit which carries a third fluid, preferably a cooling circuit, and - in that at least one heating device (43, 45), which can additionally heat the second fluid, is connected or can be connected to the second circuit in the second circulation mode, and - in that at least one further heat exchanger, to which the first fluid can be supplied in order to realize a heat exchange connection with the third circuit, is arranged downstream of the at least one expansion element and upstream of the at least one compressor in the intended flow direction of the first fluid in the first circuit. - a first circuit which carries a first fluid and in which at least one compressor, at least one heat exchanger (7, 9) as a condenser or gas cooler, at least one expansion element and at least one further heat exchanger (17, 21) as an evaporator are arranged in the intended flow direction of the first fluid, and - a second circuit which carries a second fluid and in which at least one conveying device for the second fluid is arranged, - in that the circulation system for a vehicle comprises a third circuit which carries a third fluid, preferably a cooling circuit, and - in that at least one heating device (43, 45), which can additionally heat the second fluid, is connected or can be connected to the second circuit in the second circulation mode, and - in that at least one further heat exchanger, to which the first fluid can be supplied in order to realize a heat exchange connection with the third circuit, is arranged downstream of the at least one expansion element and upstream of the at least one compressor in the intended flow direction of the first fluid in the first circuit.