A 'Fistula' is like a surfaced artery, except it immediately goes back to your heart.  Like a highway loop with no stops in between.

Apparently, this can happen naturally: some people have an artery that connects to a vein directly instead of going through capillaries first.  It can cause several problems due to the abnormal blood flow and pressures.  Large fistulas can be surgically fixed and removed.

Somehow nature inspired a doctor to do the opposite: create a fistula where none existed before.  Usually this is done in your arm, near your wrist or elbow.  The surgeon shunts an artery to the returning vein, so some of the blood flow immediately loops back through the shunt instead of continuing to your hand.  Since veins are on the surface and the blood is headed back to the heart, this creates an unusually good blood access point.  A vein on steroids.

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