The investigators aim to study the structure of the vein that is used in making fistulas with a microscope and also to test it in an engineering laboratory to see how much it will stretch. The investigators hope that gaining information about the structure of the vein and its ability to stretch will help determine what it is about the vein that affects how well it works as part of a fistula. This information may help surgeons select the best possible vein in a given patient to give the best chance of a working fistula in the future.