An adapter includes several connecting nozzles. Free nozzle ends of the connecting nozzles are adapted to be connected to line ends of fluid lines. The adapter includes, for guiding flowing fluid in and then out, two mutually separated, tubular flow channels. Moreover, the adapter includes a projection, which extends from the nozzle end with a length to a free projection end remote therefrom. A fluid line system formed by means of the adapter comprises, furthermore, a fluid line with, enveloped by a wall, a lumen. The fluid line can be connected with its line end to the connecting nozzle of the adapter in such a manner that the projection protrudes inwardly into the lumen of the fluid line to form two tubular chambers of the fluid line mutually separated by the projection and adapted for guiding through flowing fluid.