A medical device which defines a lumen for flowing a bodily fluid from an upstream end of the device to a downstream end thereof is disclosed. The device has a luminal wall (14) that extends between the upstream and downstream ends and defines the lumen within which the fluid flows. The wall exhibits a succession of protuberances spaced from each other along the length of the device. Each protuberance has a flank facing upstream (54) and a flank facing downstream (64), the flank facing upstream extending into the fluid flow so that a radially outermost part of the flow of fluid from the upstream to the downstream end of the device impinges on the upstream flank and is thereby caused to reverse its flow, and flow upstream from the upstream flank to the downstream flank of the next adjacent protuberance upstream, creating micro-vortices between two adjacent protuberances.