Apparatus and methods are described including a mechanical support element that is placed inside a first vein of a subject. At least one electrode disposed on the mechanical support element is placed inside the first vein, in a vicinity of a site upstream of a bifurcation with a second vein of the subject. A control unit enhances downstream blood flow from the first vein by driving the at least one electrode to divert blood downstream into the second vein by constricting the first vein at the upstream site, by driving the at least one electrode to apply a current to the vicinity of the site. The mechanical support element prevents the first vein from collapsing by providing mechanical support to the vein. Other embodiments are also described.