Previous heat transmission units have only low cooling efficiencies in case of small fluid mass flows. According to the invention, it is proposed to configure a heat transmission unit (1) in such a manner that a channel (4) conducting the fluid to be cooled is separated, by a partition wall (14;23,24;29,30), into at least two separated partial channels (15,16), a first one of these channels being adapted to be shut off by a shut-off means (21;27;31) arranged at a first partial fluid inlet (17) of this channel. Preferably, in spite of the closed condition of this inlet cross section, full use is made of the existing cooler surface in that there is effected, by suitable arrangement of the partition walls (14;23,24;29, 30) and by further shut-off means (28;32), a deflection of the fluid mass flow in the heat transmission unit (1). A heat transmission unit of the above configuration is particularly suited for exhaust-gas recirculation in internal combustion engines; thus, an optimum cooling performance can be obtained throughout various exhaust-gas recirculation rates.