Arrangement for extracorporeal circulation of blood having a genereally uniformflow diameter where the risk for stagnation of blood is avoided or reduced. Thearrangement comprises a generally circular cylindrical means for temporaryexpanding the flow diameter, a means for flow expansion (9). The means for flowexpansion (9) is arranged connecting the bloodline (5) and a component (6, 8)for circulation of the blood. The means for flow expansion (9) comprises a firstpart with a first flow diameter, dl and a second part downstream the first partwith a second flow diameter d2 where d2/dl≥l, 5. The first part has a lengthextension Ll and the second part has a length extension L2 where L2/Ll≥0,5.
