Abstract: A heat exchanger enables an efficient and uniform cooling and/or heating of liquids, in particular of blood. The heat exchanger includes an elongated, substantially cylindrical, housing, a plurality of capillaries, which are fastened inside the housing by means of partitions so that they are substantially parallel with the longitudinal axis of the housing and mutually placed in defined distances from, while the housing contains an inlet and an outlet of the cooling/heating liquid and a finishing element at each end. The element contains an input, or output of the cooled/heated liquid and includes inside the housing at least one laminarizer in the form of a partition with a plurality of openings, always one opening for each capillary, the diameter of the opening being bigger than the outer diameter of the capillary, and laminarizer is placed so that capillaries go through the centre of the opening in the laminarizer.