How does a fistula work to dialysis patient?

It acts: As a means by which you can take blood out, run it through the machine (acting as the kidneys) and returning it to the body. It connects blood in the artery to the vein in a way that can be repeatedly punctured with a needle but is also low enough pressure that it won't bleed.

Arterial Diversion: When an artery is connected to a vein, the vein walls thicken and become muscular (due to the new high arterial pressure). This makes finding the vein easier. More importantly, the large bore needles can be removed and pressure held, with clot forming in the muscular walls, to stop the bleeding. If the veins weren't muscular, putting the needle in and taking it out safely, aren't possible.