Patients whose kidneys have failed need to receive dialysis treatment, most commonly with a dialysis machine. In order to be connected to the machine an operation is often performed to join an artery to a vein in the arm. This forms what is known as an arteriovenous fistula. The fistula causes an increase in the flow of blood through the vein and the vein reacts to this by becoming bigger and thicker, making it easier to connect the patient to the machine.