A ring device is integrated externally onto and around a regular hemodialysis arterio-venous graft conduit. The ring device is adjustable to reduce or to normalize (by rebounding) the diameter of the graft, and eventually to change blood flow through the graft. This ring device is designed such that it can be scaled up or down to change its diameter, and simultaneously the diameter of the associated graft prior to and even after its implantation into a patient's arm or leg. Since a hemodialysis arterio-venous graft is usually implanted superficially underneath skin, the device and its adjustable parts can be electively or urgently located and operated from outside by touching and pushing the skin, subcutaneous tissues and the device without a surgical incision. The ring device can be located anywhere within the graft and can function with a single ring or as a set of multiple rings.