A heat exchanger for cooling the exhaust gas of an internal combustion engine of a motor vehicle, having: a first partial heat exchanger with at least one first flow channel through which a medium to be cooled is to flow and at least one third flow channel through which a first coolant is to flow, at least one second partial heat exchanger with at least one second flow channel through which the medium to be cooled is to flow and at least one fourth flow channel through which a second coolant is to flow, wherein the at least one first flow channel is arranged upstream of the at least one second flow channel in the flow direction of the exhaust gas, wherein the at least one first flow channel and the at least one second flow channel are fluidly connected and the at least one first flow channel and the at least one second flow channel have at least one respective first specific heat transfer surface and at least one respective second specific heat transfer surface, wherein the second specific heat transfer surface, divided by the first specific heat transfer surface, yields a quotient (ψ), and the at least one first and the at least one second flow channel (21, 22) are tubular and has a first tube interior surface that forms the at least one first heat transfer surface and the at least one first flow channel has first turbulence elements and the at least one second flow channel has second turbulence elements, respectively as second specific heat transfer surfaces, characterised in that the at least one first flow channel has a larger quotient (ψ) than the at least one second flow channel, wherein the quotient (ψ) of the at least one first flow channel has values of 1.0 to 2.5 and the quotient (ψ) of the at least one second flow channel has values of 0 to 1.5. a first partial heat exchanger with at least one first flow channel through which a medium to be cooled is to flow and at least one third flow channel through which a first coolant is to flow, at least one second partial heat exchanger with at least one second flow channel through which the medium to be cooled is to flow and at least one fourth flow channel through which a second coolant is to flow, wherein the at least one first flow channel is arranged upstream of the at least one second flow channel in the flow direction of the exhaust gas, wherein the at least one first flow channel and the at least one second flow channel are fluidly connected and the at least one first flow channel and the at least one second flow channel have at least one respective first specific heat transfer surface and at least one respective second specific heat transfer surface, wherein the second specific heat transfer surface, divided by the first specific heat transfer surface, yields a quotient (ψ), and the at least one first and the at least one second flow channel (21, 22) are tubular and has a first tube interior surface that forms the at least one first heat transfer surface and the at least one first flow channel has first turbulence elements and the at least one second flow channel has second turbulence elements, respectively as second specific heat transfer surfaces, characterised in that the at least one first flow channel has a larger quotient (ψ) than the at least one second flow channel, wherein the quotient (ψ) of the at least one first flow channel has values of 1.0 to 2.5 and the quotient (ψ) of the at least one second flow channel has values of 0 to 1.5.